Sunday, August 06, 2017
Issue #546: Two Down, One to Go...
One more then, my friends, one more, a CGEM mount. I have absolutely loved using this Celestron German equatorial, but with my back twinging today, I know I have to let it go the way of the Atlas. If you've been following this saga on Facebook and here on blog, you know that after an accident, a fall, I had three years ago, I began having intermittent back problems. They come and they go, and one thing I have learned is not to aggravate them. Especially by lifting 40-pound GEM heads onto tripods.
Case in point? Last week, I was testing my Atlas EQ-G, which would be the first piece of "too heavy" gear to go up for sale. I was careful while putting it on its tripod and all seemed well. I had a lot of fun taking pictures with the mount, and had almost convinced myself to hang onto it. Until, when I was disassembling the Atlas after three days in the backyard, I--yep--aggravated my poor back again despite my caution. I knew then that both the Atlas and the CGEM just had to go.
As for the current item, my CGEM, I've had a lot of fun with it in the couple of years I had it and will miss it. The mount never failed me, never prevented me from doing what I wanted to do in video, visual observing and long exposure deep sky imaging. You can read about some of our exploits here and here. Certainly I'll be sorry to see the mount go, but I plan to, as I mentioned a while back, replace it with a lighter GEM with a similar payload capacity.
Anyway, the CGEM is just over two years old and includes the standard NexStar hand controller, a DC power cord, AND the optional (and not exactly inexpensive) 5-amp Celestron AC power supply. The counterweight is the single 17-pound job normally included with the mount, but any CG5/Synta type counterweight will work on this mount.
This is priced to MOVE at $800.00, little more than half its price two years ago. Like the Atlas, this is a PICKUP ONLY item, but, as with them, I am willing to drive a reasonable distance to meet somebody--New Orleans, Montgomery, Pensacola, Panama City, etc.
Please note that the TPI spreader shown in some of the pictures is NOT included. I have moved that to my AVX.
Oh, one last thing to add to the pile of loot. I've realized that after I sell the CGEM, I'll have no further use for my ADM Vixen to Losmandy adapter, so that goes with the mount too.
If you're interested, the best way to contact me is via Facebook Messenger. I do still monitor my old email addy at rmollise@bellsouth.net, however... If you think the CGEM might be just the thing for your observing program, I urge you to contact me without delay. It won't, I presume, last at this price.
Sunday, July 30, 2017
Issue #545: Good, Old EQ-6…
Actually, I could have called this issue “Lo, There Shall be an Ending”—Part II. If you’ve
read the linked post, you know that for a number of reasons I been thinning out
my telescope and mount herd. I got rid of a bunch of stuff in the weeks
following that article’s posting, and I thought I was in a good spot, finally,
with astronomy gear I could and would
use. Well, you know what they say about “Best Laid Plans,” doncha?
I liked my Celestron CGEM mount
a lot. It had been a good performer, guiding well and not exhibiting any of the
problems some of these Celestron GEMs—mostly earlier examples—have been heir to.
Then trouble began. Actually, this
trouble started a year before I bought the CGEM.
I took a fall at Chaos Manor South one afternoon not long after
we moved out three years ago. The details? Let’s just say it was a boneheaded
stunt. I fell on my back and right onto the concrete front steps of the Old
Manse. Ouch! right? But while I was bruised and sore, I thought I’d dodged a
bullet—until last summer.
Summer of 2016 bought a spate of back problems that just
wouldn’t quit. Until they did rather abruptly a month or so after
their inception. I was again fine until the beginning of this summer. The latest installment of My Aching Back began following an afternoon when I set up the CGEM
in the backyard, lifting the 40-pound plus mount head onto its tripod.
The next morning my back pain returned—big-time—and I was pretty sure what had caused it. I'd
been careful lifting the CGEM, I thought, but apparently not careful enough,
and again went through weeks of suffering. End result? I won’t say I’m scared
of setting up the CGEM…but… OK, I’ll say it: I am scared to do that. I haven’t used the mount in months and don’t
believe I’m going to use it much ever again without some outstandingly good
reason—something far beyond just wanting to take deep sky snapshots.
It seemed to me, then, that it was time for, yes, “Lo There
Shall be an Ending: Part 2.”. The CGEM will have to be sold, I’m afraid. I also
still have my nine-year-old Atlas EQ-6, which I’ve been holding onto as a
backup for the CGEM. It is only a little lighter than the Celestron mount, so
I’ve reluctantly decided it must depart as well. Alas, the same goes for my beloved
carbon fiber tube C11, which I am also hesitant to wrestle with now.
Don’t despair for me, however. I intend to replace the two
mounts with a single lighter one with comparable or higher payload capacity—maybe
a Losmandy GM-811G or an iOptron CEM60. Mounts that will accommodate my Edge
800 better for imaging than my Celestron Advanced VX mount can, but which I won’t be afraid
of lifting. It does require me to spend more money on the replacement than I
spent on either the CGEM or Atlas, but even penny-pinching me is ready to dole
out money for a mount that will accommodate my heaviest scopes, but that I need
not fear using now or in the coming years.
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| M15 with C8 + Atlas EQ-6... |
Lighter weight but as much (or usually more) payload
capacity is the payoff when you go to the next mount price tier above the CGEM
and EQ-6. I’m not saying those Synta-made mounts aren’t an incredible
value—they are—but increased weight is the penalty for both respectable payload
capacities and low prices.
While I’d used the CGEM early this summer, I hadn’t done
anything with the Atlas for a couple of years,
not since the Peach State Star Gaze of 2015.
I was their Keynote Speaker that year, and because of the event’s relative
closeness in Georgia I was willing to drive up rather than have them fly me in.
So, I was able to take the EQ-6 and a couple of telescopes with me. The mount
performed well, but that was then. I didn’t want to sell Atlas to someone
without giving him a through checkout, which I began doing one recent and
somewhat cloud-free night.
I decided to recount my process of setting up, aligning, and
interfacing the Atlas EQ-6 here, since I thought that might be instructive for
those of you considering buying one or who are new to the SynScan mounts—the
Atlas EQ-6, the Sirius HEQ-5, and their sisters—sold under the Orion and
SkyWatcher brand names.
The first thing you gotta do if you wanna play telescopes
with an Atlas is get the big equatorial head onto the tripod. Following my
debacle with the CGEM earlier this year, you can bet your freaking bippy I was cautious.
I carried the head to the tripod, which I’d already assembled, leveled, and
oriented with its azimuth alignment peg north, in a plastic case with two good
handles (from Walmart, natch). I was awfully, awfully careful to lift with my
legs, not my back, when I pulled the mount out of the shallow box.
How bad was it? I didn’t like doing it, but I didn’t strain
anything. The mount is actually a little easier to get on the tripod than the CGEM
in my opinion. Something about its shape seems easier to hold onto. Also, its
counterweight bar, which is slightly slimmer than that of the CGEM, can be
retracted into the mount, and I found doing so made the head less awkward to
lift.
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| M33 with William Optics 80 Fluorite + Atlas EQ-6... |
With the R.A. lock locked securely to keep the mount from
flopping around, I hoisted the head onto the tripod, aligning the peg on the
tripod with the azimuth adjuster assembly on the mount, lowered the GEM head onto the
tripod and secured it with the tripod’s threaded bolt (I leave that slightly
loose till polar alignment is done).
Mount safe on tripod, I proceeded to do the usual set up
things: Extend counterweight bar and load one Synta 11-pound “pancake” counterweight
on it—all that is needed for my 5-inch refractor. Place telescope in the Atlas’
Vixen style saddle and secure it with two lock bolts. Attach hand control and
power cords, taking care to thread the power cord through the mount’s strain
relief widget.
Taking care to dress and secure the power cord is important.
The Synta power cables are notorious for losing their connections, the earlier cables,
anyway. As on the CGEM, the power receptacle rotates with the Atlas’ RA axis,
and the cord tends to become loose or even disconnected. That is the reason
Celestron (Synta) used a power receptacle with a threaded collar on the CGEM
when that re-design of the EQ-6 was undertaken. The latest EQ-6es, the EQ-6 Rs, also have that
feature.
After balancing the telescope in RA and declination (it’s
best to have the EQ-6 very slightly east heavy in RA for best tracking during
photography, but that is not as critical as it is with mounts in the CG5
class), comes a fairly important operation, setting the mount to home position.
The EQ-6 has neither position switches nor alignment marks,
so it is up to you to place the mount accurately in “home” position. That is necessary
to allow the mount (which has no encoders; it just counts stepper motor steps)
to know where it starts from. Technically, I suppose, after you accurately goto
align the mount, how good or bad your home position setting was should no
longer matter. It should just help the mount land near the initial alignment
star. Nevertheless, at times it sure seems that the more care I take with
setting home position, the more accurate my gotos are. Go figure.
Home position for the EQ-6 is with the telescope pointed
north and the counterweight bar straight down. It’s easy to achieve this
accurately using a small carpenter’s level. Set the mount to 90-degrees in RA
with the counterweight bar on the left and the scope on the right as viewed
standing behind the mount. Use the level to get the counterweight bar as level
as possible. When that is done, lock the RA lock, loosen the RA circle, and set
it to “6” using the scale appropriate for your hemisphere; the upper one is for
the Northern Hemisphere.
 |
| Ready for testing! |
Next take care of declination. With the mount still positioned
with the counterweight bar level, undo the declination lock and level the tube.
Then, set the declination setting circle to the value shown on the mount’s
latitude (elevation scale). I am at 30 degrees latitude, so my elevation scale
is on 30, and I set the declination circle to thirty degrees.
Now to actually set home position. Undo the RA lock and move
the mount in that axis until the RA setting circle reads zero. Then, do the
same for declination: unlock it and move the scope in declination until it
reads zero, too. If you did everything correctly, the mount should be in
accurate home position with the counterweight bar down and the tube pointing
due north. After a couple of times, this procedure will become second nature.
Next up is polar alignment. Unlike with the Celestron
branded mounts, the accuracy of polar alignment affects the accuracy of gotos,
so try to do a good job. I use the Sharpcap program’s polar alignment tool to
get a dead-on polar alignment, but the EQ-6’s included polar scope can do OK.
You should go beyond the simple “match the constellations” polar alignment
outlined in the manual, however. See this article
for a simple to do but more effective method of polar borescope alignment.
Can’t see Polaris? The SynScan hand control now includes an
AllStar Polar Alignment Procedure in the Align menu (it will not show up until
you complete the goto alignment). See the manual for details. I understand this
procedure can yield an alignment at least as good as a careful polar scope
alignment, just like ASPA on the Celestron branded mounts. I have not used it
enough to be able to testify to its accuracy, however.
With my mount in home position and polar aligned, it was
time to do the goto alignment. Once you get past time, date, location, etc. in
the hand control, it will ask if you want to proceed to alignment. You do, but
the question then becomes “Which alignment?” since you have three main options,
One Star, Two Star, and Three Star.
One Star: You line up one measly star and hit enter.
Choose this option if your mount is well polar aligned and you’ll be working in
a relatively small area of the sky. Near the alignment star, you’ll get good
gotos, and they should be OK, at least, on the same side of the Meridian as the
alignment star. On the other side of the Meridian, your goto quality will
likely decrease. It may also suffer toward the horizons and at large distances
from the alignment star, even on the same side of the Meridian as that star.
Two Star: Use a two-star alignment, centering two alignment stars, if you want to
range a little more widely afield in the sky. Gotos should be good everywhere, assuming
the telescope doesn’t display a lot of cone error, that is, its optical axis is
pretty much in line with the mount’s polar axis.
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| EQMOD with settings screen... |
If your scope does have some cone error? Well, you can try
shimming it in the saddle to eliminate that, but a Three Star alignment is an easier go, I guess. In this method, you
center an additional star, a third star, which will be on the opposite side of
the Meridian from the other two.
So, there I was out in the backyard wanting to give old
Atlas a clean bill of health. Since I have often used a Schmidt Cassegrain on
this mount, I am accustomed to doing a Three-Star. Even if the tube itself
doesn’t display much cone error, mirror flop due to the SCTs moving mirror
focusing system can introduce some error anyway. However… I wasn’t really in
the mood for a Three Star on this evening.
The day had started out hot, humid, and partly cloudy. By
nightfall it was just about as hot, even more cloudy, and seemed stickier and
more humid than ever. Despite the presence of my Thermacell bug repeller, the
mosquitoes were threatening to carry me off. I wanted to be done and done
quick. A One-Star it would be. Frankly, I often use this alignment method
anyway. I most often employ the Atlas for imaging, and usually only do one or
two targets a night—typically targets in the same general area of the sky. A One Star alignment on a nearby bright star is all I need.
Alrighty then. I told the SynScan I wanted to align, and
selected One Star. I then scrolled through the available stars until I got to
Vega, selected it, and, after the slew stopped (Vega was in the finder but not
the main eyepiece), I centered the star using the up and right keys—just like
with a Celestron—which is what you’re instructed to do with current SynScan
firmware.
My results after the HC declared “Alignment Successful”?
What was in the immediate area? There was M13. I punched that in, hit enter a
couple of times, and the mount slewed that way. When it stopped and beeped, there
was a little fuzz spot dead center in the field. Now, this was a 40mm (Plössl) eyepiece,
mind you, but one with a fairly narrow AFOV, so there wasn’t a whole lot of
true field. Also, I’d done the One Star with this eyepiece rather than with a high-power
reticle ocular (recommended) because I was lazy. All things considered, that
was pretty impressive goto-accuracy, I thought.
After M13, I decided to see what the mount would do on the other
side of the sky. Arcturus was in the eyepiece, but off toward the field edge.
So was Mizar. That was just what I’d
expected. Back in the eastern half of the heavens, M57 was dead center. So was
M13 once more, when I decided to take one last look at it before adjourning to
the cool den.
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| EQMOD connection with USB EQDIR cable... |
Any other goto alignment tips? Try to adhere to the “rules”
for alignment star choice given in the manual. Especially the one that says
that stars one and two in a two or three-star alignment should be at least 3
hours of right ascension apart (that is, separated by 45 angular degrees
east-west if at all possible). The current SynScan firmware does a better job
of picking alignment stars than it used to, but keep these rules in mind.
Try not to use a star near the horizon or the zenith, for sure.
Still getting gotos that are “off”? Try PAE, “Pointing
Accuracy Enhancement.” See the SynScan controller manual for details, but this
allow you to enter multiple additional alignment points all across the sky. I
don’t often use PAE, since my telescope and camera combos give wide enough
fields that the mount doesn’t usually miss if I’ve been careful with setup and
alignment, but I have found it to come in handy a time or two.
Anyhow, fairly assured the Atlas was still in good working
order, I parked it, covered mount and scope with my Telegizmos cover, and
headed for the blessed coolness of the house. I wasn’t completely done, though.
Next, I wanted to test the mount with a laptop, sending it on gotos with
Stellarium and StellariumScope. But that was a task for another evening. I was
covered in sweat, suffering from a summer cold, and despite my success with the
Atlas was just this side of “out of sorts.”
As the Sun sank on evening two of the EQ-6 check-ride, the
sky was not looking good, not good at all. Not completely cloudy, no, but hazy
with large swathes of thin clouds slowly drifting through. Still, I figured it
would be good enough for stage 2, making sure the mount would still goto its
gotos under control by a laptop computer.
One thing I wanted to try in that regard was the new
SkyWatcher ASCOM driver. Previously, I’d used a Celestron driver for the EQ-6.
That worked fine, no problem, but recently, with the advent of the new
Celestron unified driver, support for the Celestron scopes had been
discontinued, I had been told. I could simply have used an older Celestron
driver, but I wanted to see how the SkyWatcher one worked.
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| EQMOD's normal display... |
With my Scopestuff SynScan serial cable plugged into the
base of the hand control and the other into my KeySpan USB-Serial converter, I proceeded
to fire up the Stellarium/StellariumScope combo, which is about all I use to
control my goto scopes these days. Hokay, selected the new SkyWatcher driver,
hit Connect, and immediately got a warning about my hand control. Said text
informed me that the driver wouldn’t work with a version 2 HC, needing at least
a Version 3 or 4. Rut-roh.
I was puzzled since I do
have a version 3 HC. It doesn’t have the very latest firmware loaded, no. It is
at v3.37 instead of the current 3.38, but that is still pretty recent. After I
dismissed the warning window, however, everything seemed normal. I was sitting
on Vega, and the onscreen scope crosshairs were on Vega as well. I clicked on
M13, hit the CTRL + 1 key combo Stellarium uses to initiate gotos, and
the mount responded immediately, moving the scope right to M13. The big
star-ball, nearly centered in a 13mm Plössl at 75x, actually looked better
than I thought it would in the yucky sky.
The same was true of any object I requested. I even let the
scope track unattended for a half hour or so to see if the driver would crash,
but it didn’t. Verdict? Warning or no warning, the driver worked well.
I took another gander at M13 and a peep at M92, and, as I was
pondering whether there was anything else that would look good on such a putrid
night, the sky well and truly closed down with a big thud. I covered scope and
mount and left the mosquitoes to fend for themselves.
The next morning, I investigated the driver issue further.
It turned out that what it was trying to tell me was that I did indeed need
SynScan firmware version 3.38 for full operability. I’m not sure which features
of the driver might not work with 3.37—goto was fine which is all I care about.
At any rate, I am a big fan of “if it ain’t broke, don’t fix it,” and believe I
will leave it to the mount’s next owner to decide whether to upgrade the HC to
3.38.
Only one major thing remained on my testing agenda. There’s
computer control, and then there is EQMOD.
If you’ve got or have been considering buying a SynScan mount I’m sure you’ve
heard of that, but what it is is a special ASCOM driver. It doesn’t just send
goto commands to the mount, it replaces
the SynScan hand controller—much the same as the NexRemote program replaces
Celestron’s NexStar HC. I began using EQMOD with the mount not long after I purchased the Atlas in November of 2007, and its capabilities have always impressed me.
Why would you want to do eliminate the SynScan controller? EQMOD,
which was developed by the UK’s Chris Shillito and other talented programmers,
adds features the HC is missing. As the years have gone by, features have been added to the SynScan firmware,
but it still falls behind the NexStar HC, iOptron’s Gotonova controller, and
Meade’s Autostar. But above and beyond adding extra stuff, EQMOD does one very
important thing: it fixes the SynScan
mounts’ somewhat lackluster goto performance.
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| A game pad is a perfect solution for EQMOD scope control... |
While the SynScan HC is usually more than adequate for
imagers going after one or two targets a night with a fairly wide-field set up, for people cruising to many celestial destinations over the course of an evening—video
observers or visual users covering a lot of ground for whatever reason—the
SynScan HC’s goto precision or lack thereof can sometimes be frustrating.
Its
shortcomings in this area are mostly the result of its simple goto alignment
system. The 1-2-3-star alignment of the HC is comparable to what Celestron GEMs
were using almost a decade ago. In contrast, EQMOD features sophisticated alignment
algorithms and a system that allows as many alignment points as desired to be
added to the alignment model—one, two, or three, is OK, but you can do ten if
you want—or fifty.
In order to get the mount working with EQMOD again—I hadn’t used
the driver in quite a while, largely because I hadn't used the mount much in a long time—I first of all
needed to round up my EQDIR cable. While you can run EQMOD using a serial cable
connected to the HC (after enabling the SynScan controller’s “PC Direct” mode, which bypasses the HC), EQMOD is more stable and reliable using an EQDIR
cable.
My EQDIR cable, the Shoestring Astronomy USB2EQ6, plugs into the mount’s hand control port on one
end, and one of the laptop’s USB ports on the other. That’s possible because it
has a built in USB-Serial converter (recommended), but you can
get models that plug into an outboard USB – serial converter cable instead. One
thing NOT to do? Never connect a standard serial cable to the mount’s HC port.
The voltage level will be wrong. That’s the major purpose of the EQDIR cable, converting
serial voltage levels to the TTL levels used by the mount’s hand control port. EQDIR
cables come in two flavors: one with a DB9 connector for the HC ports of EQ-6 (Atlas)
mounts, and one with an RJ connector as on the HEQ-5 and EQ-8 (Sirius/HDX)
GEMs.
I wanted to load the latest version of EQMOD, which I
obtained from the EQMOD Yahoogroup, which tends to have later versions as compared
to the EQMOD Sourceforge page. I also needed
to fix EQMOD, which (thanks to
me no doubt) had been a little squirrelly the last time I'd used it, I recalled. I suspected the problem lay in EQMOD's .ini file, which is carried over unchanged when you install a new version of the driver.
So, I loaded the new version of EQMOD and then, using the EQMOD Toolbox app that accompanies the driver, I deleted the EQMOD.ini file (if the .ini is deleted, the next time EQMOD is used a new one will be automatically created). Testing with the (included) EQMOD simulator, which is a godsend, showed my weird problems had been banished.
So, I loaded the new version of EQMOD and then, using the EQMOD Toolbox app that accompanies the driver, I deleted the EQMOD.ini file (if the .ini is deleted, the next time EQMOD is used a new one will be automatically created). Testing with the (included) EQMOD simulator, which is a godsend, showed my weird problems had been banished.
EQMOD is not a standalone program, it is a driver, and must be used in conjunction
with a planetarium program. Most people using EQMOD pair it with either Cartes
du Ciel or Stellarium, both are good choices, but EQMOD can be used with any ASCOM
compatible program.
When the sky finally began to get dark, I plugged the EQDIR
cable into Atlas and laptop, turned on the EQ-6, and started StellariumScope and
Stellarium. I selected “EQASCOM” in the ASCOM Chooser window, and then pushed the
“Properties” button to configure the driver (there’s a separate EQMOD Setup app
included with the driver if you want to use that instead). I configured the
usual things: com port, baud rate, etc., etc. See the EQMOD Wiki for details.
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| Assigning gamepad functions... |
Ready to go, I checked the “Connect” box in StellariumScope,
which brought up the EQMOD control panel. Since I’d already done some
configuring inside using the Simulator, all I had to do was unpark the mount
which, looking at the Stellarium sky display, was sitting on the North
Celestial Pole just as it should have been with the mount in home position
(where I’d parked it the previous evening).
Now comes the cool part. I began aligning Atlas, building an
alignment model. How do you do that? It couldn’t be simpler: goto a star (since I was using Stellarium, I did that with the usual CTRL + 1 key combo), center it in the eyepiece, and
press Sync in the planetarium program. I did that, choosing six bright stars scattered around
the sky. Given the haze and passing clouds, I was pretty lucky to see six
bright stars, so that was as many as I did.
What do you do then?
That’s it. You goto objects. When you are done for the evening, you park the
mount to home and shut everything down. Oh, if you want, you can add a new
alignment point at any time over the course of the observing run by going to
an object and syncing on it. No special procedure is required.
“OK, Unk, but how do you center a star or other object in
the eyepiece? You told us the computer takes the place of the HC. Do you
have to have the laptop next to the telescope?” You could do that, centering the
alignment target with EQMOD’s onscreen direction buttons, but it is far easier (and more fun) to use a wireless gamepad, just like we used to do with
NexRemote.
Almost any PC gamepad will work with EQMOD, and setting up
and calibrating one is a simple procedure. In addition to the use of a joystick
for scope movement (way better than any telescope hand control’s buttons), you
can map gamepad buttons to other EQMOD functions. I, for example, have a button
on the gamepad that does the sync, one that unparks the scope, one that parks
it, and four that allow me to choose mount slew rates.
So, to sum up, what I did was, start EQMOD, unpark the
mount, slew to a bright star, center it with the joystick, double-click the
sync button on the gamepad (a double-click is required to prevent you from
accidentally syncing when you don’t want to). I did the same for five more
bright stars. And that was it.
How was goto performance? Stellar. Anything I asked for from
horizon to horizon was in or near the center of a 12mm Plössl. That’s impressive
considering the fact that my choice of alignment stars was quite limited. I was
pretty good in the east, but, thanks to clouds, in the west all I had was
Arcturus and Dubhe.
After alignment, I went to as many targets as I could, given
the clouds—maybe twenty or so deep sky objects and stars. I let the rig track
unattended for half an hour. I parked the scope, shut down EQMOD, and started everything
from the beginning. Never any glitches or problems. Rock solid.
The weather soon degraded to the point where even Vega and
Arcturus were invisible, so I somewhat reluctantly shut down. How was I feeling
about the Atlas? A little blue. It was like the day I drove my 1996 Toyota
Camry (with 250,000 miles on it) to the dealer to trade it in on a new one. When I
pulled into Springhill Toyota, the car seemed to whisper, “Daddy, I don’t like this
place. Why don’t we go for a nice, long drive instead?” It sure was hard to let
go, since the Camry still ran just as well as she had the day I’d driven her off the lot.
Sunday, July 16, 2017
Issue #544: To PEC or not to PEC
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| Me and my trusty Ultima 8 PEC circa 1995... |
If you read the last installment of the good old Astro Blog,
you know I am a proponent of shorter (60 – 120-second) sub-frame exposures for
astrophotography. When appropriate.
Like from a light polluted site. There are times when you want to go longer, to
300-second or 600-second or longer sub-frames, however. Say when you are at your
dark site and want to pick up as many details in the target object as possible.
Some of you, especially cheapskates like me who use inexpensive
mounts like my Celestron Advanced VX German equatorial mount (GEM), are afraid of longer exposures. How can you
break the 300-second barrier without getting trailed stars? There are ways to
do that fairly easily. Good polar alignment is one. Spending time tweaking the
settings in your auto-guide program is another. One other thing that is often overlooked
and unused, but which can maybe get you the last step on the road to longer
subs, is PEC.
Yeah, PEC, aka “PPEC.” You know what that is doncha? You don’t? Well, PEC, “periodic error correction,”
was an idea that came out of the early 1990s, when microprocessors and memory
chips not only came down in price, but began to appear in telescopes. I don’t
know who originally came up with the idea, but it was a good one.
In those days, we amateurs were still guiding manually.
You’d monitor a guide star in the guide scope or off axis guider with a cross-hair reticle eyepiece. When the star
wandered away from the center due to the inevitable “periodic” errors in the mount’s
gears, or drifted north or south due to polar alignment error, you pushed a
button on the hand control (which we still called a hand “paddle”) to move that
pesky star back to the cross-hairs. You did this over the duration of a long
deep sky photographic exposure, which in the days of film was likely at least
half an hour.
The good idea represented by PEC was this: What if there were a way to record your button pushes? Record your
corrections for that periodic error and play them back? That would, if nothing
else, make guiding easier. Thanks to the recurring—periodic—nature of the gear
error in worm gear sets, which is what most of us were using by then, it sounded
like PEC could indeed work.
Again, I am not sure who came up with the PEC idea, but the
first people to implement it in a commercial telescope were Celestron, the old
American based (though Swiss owned) Celestron out of Torrance, California. They brought PEC to the
market initially with their top of the line C8 Schmidt Cassegrain, the Ultima
8, in a new version of the telescope, the Ultima 8 PEC.
It just so happened that not long after the final and best
version of that telescope was released, the 9-volt battery powered Ultima 8
(PEC), I was in the market for a slightly upscale SCT. Well, as upscale as
penny-pinching me ever goes. By the spring of 1995, I finally had an Ultima of
my own, and after buying a few rather expensive (I thought) accessories like a
declination drive motor (optional in those days) and a counterweight and rail
for this fork mount scope, I undertook to do some deep sky imaging. I was
curious to see if PEC really took the pain out of guiding.
 |
| Phd2 guiding graph: VX with auto-guiding + PPEC... |
What I found was that it helped.
You certainly could not expect to go unguided with the C8 at 1500mm (at f/6.3) for
over a minute or two—not long enough to accomplish much with film—but it did
make guiding less arduous. You still had to watch the guide star, but if you
did a good PEC recording, you would usually be OK if you looked away from the
reticle for a moment or your attention wandered. The periodic error was still
there, but its magnitude was lessened.
Celestron’s PEC implementation was certainly not the end of
manual guiding. Naturally, it only recorded your east-west corrections. Any
errors in declination are not periodic; they are due to polar alignment error
or seeing, not periodic error. Too bad I was always rather lazy about polar
alignment in those days. That meant I still had to monitor the guide star
attentively.
Also, Celestron’s version of PEC left a little to be
desired. Mainly because you had to do a brand-new recording every single night.
It was a shame you couldn’t save an especially good PEC track for future use.
Turn off the scope at the end of the evening, and your PEC recording disappeared
into the ozone.
Nevertheless, I used PEC to the end of the film days, and it
did improve my photos, no doubt about that. But when electronic cameras and
auto-guiding came in, I forgot all about PEC. Yes, there had been improvements
in it—Meade and, finally, Celestron had equipped their mounts with PPEC, permanent periodic error
correction. With PPEC, your recording was preserved through power cycles. But why worry with PEC since you had a guide
camera and a computer watching that guide star now?
There was also an old wives’ tale making the rounds. That
PEC and an auto-guider would FIGHT each other. That the auto guider and PEC
would conflict, one wanting to
correct this much and the other that much; one wanting to go in this direction,
and the other in that. Some of those old wives were pretty sharp, but this particular tale
doesn’t really make a heck of a lot of sense when you think about it.
Nevertheless, for a while it was the conventional wisdom astrophotographers
embraced. I just sort of accepted it—when I thought about PPEC at all, which
was seldom. I was auto-guiding and that always seemed to be enough.
 |
| M13: 300-seconds at 900mm f/l... |
Well, I thought it
was enough, anyway. Auto-guiding with my Advanced VX GEM with my QHY-5L II
camera and PHD2 resulted in an RMS error around 2” usually. That was good enough for my most used
telescopes, my 80mm and 120mm f/7 ED refractors and my DSLRs. There were some
occasional R.A. spikes, though, and at times the error would climb above 2”.
What could I do to improve on that error figure? One thing
I’d avoided doing since buying the VX:
spending some time fine-tuning those blasted PHD2 brain settings, the settings in the program that modify the guiding
algorithm. I’d pretty much left them on their defaults other than just
increasing the guide-step size to accommodate my fast 50mm guide scope.
So, I finally buckled down and took one whole night where I
did absolutely nothing but tweak PHD settings. The result? My guiding was now
smoother. The spikes were gone, and I rarely had an error above 2”; usually it
was 1.5” or just a bit more. The elimination of the spikes meant I could now do
longer exposures without having to throw out more than a few subs.
And then I got to thinking. Why should I stop there? If I
could get that error just a little lower, it would make imaging with my Edge
800 (at f/7) easier. But how to do that? I’d worked hard on those PHD settings,
and had also begun using Sharpcap’s polar alignment tool in lieu of the less
accurate ASPA polar alignment in the NexStar HC—that really brought the
declination error down.
What else could I do, though? Well, how about PPEC? If it
helped A-P’s fancy mounts, which come with factory recorded PPEC to lower their
already impressive error figures, why couldn’t it help my plebian (like me) mount?
I decided to find out. Luckily, we were undergoing a
strangely cloud-free pause in the usual summer evening thunderstorms. My VX
along with my 120mm SkyWatcher ED refractor, Hermione, was still set up in the
backyard, polar aligned, goto aligned (hibernated), and ready to rock following
the night of my Yearly M13. Since the evening didn’t look that good for
imaging—haze and also unsteady seeing in advance of the next storm front—why
not devote it to PEC? After all, I’d spent a whole evening getting my PHD2
settings tuned to the VX.
OK, so how exactly would I make a latter-day PEC recording? I’d
taken a brief look at the short set of instructions in the VX manual and
concluded there really wasn’t much to it. The process wasn’t much different
from back in the day except for the fact that PHD2 Guiding would be doing the
“button pushing” and not me (thank God). Since I guide via the mount’s
auto-guide port with an ST-4 cable, the set up was simple. No need to worry
about ASCOM or anything like that.
While the book instructs you to use a bright star for
guiding when PEC recording, there’s no reason for that with today’s sensitive
guide cameras. I pointed the scope at the field of M57 and there were dozens of
good guide star candidates. Some folks will also tell you that for best PEC
results you need to make the recording using a star near the Celestial Equator.
That is also untrue. It really doesn’t make any difference.
Step one, it appeared, was “indexing,” allowing the
mount’s PEC routine to find the worm gear “index,” a marked point on the worm
that is picked up by a sensor. For PEC to work, the hand control has to match
the gear to the recording every time you use PEC, the gear and the recording
must be synchronized. Indexing does that. Anyhow, I selected PEC in the Utility
menu, and hit enter. The mount then indexed, which only took a second or two
(if the worm has to rotate far to bring the index mark to the sensor, the mount
will move slightly). Time to record.
Over at the PC, I started PHD2 guiding on my pre-selected
guide star. I gave it a little while to settle down, returned to the mount, and
hit “record.” That began the ten-minute process of making the actual PPEC
recording. Unlike the old days, there wasn’t anything for me to do. Assured
PHD2 was guiding with its usual alacrity, I headed for the den to cool off from
the hot and muggy backyard.
 |
| M57: 600-seconds... |
The rest was rather anti-climactic. I returned outside ten
minutes later, and could see from the HC that the recording was done. Just like
with a tape recorder, you don’t just record, you play back. I selected the
playback function and let her rip, beginning a 600 second exposure of M57 with
the main camera, my Canon 400D.
What were the results? Error wise, they weren’t like night
and day, but there was a difference.
As above, my normal error level with good PHD2 settings and a good polar
alignment had been around 1.5”. With PEC playback on, the RMS R.A. and declination error
declined to around 1” to about 1.10” with the actual R.A. error almost always
well under 1”. While I was slightly out of focus with the main telescope, the
stars in the ten-minute exposure were decently round, as were the stars in a
300” exposure of M13 despite the fact that it had crossed the Meridian and the
mount was not balanced properly in R.A. for imaging in the west. My verdict?
Recording PEC had been nearly effortless and certainly worth it for a noticeable
improvement.
Over the last several months, my easy improvements: PHD settings, polar alignment, and PEC have
taken my AVX mount from an average total guiding error of 2 – 2.5” to 1”. While
I was getting by before, I am certainly doing better now. And the fact that my
guiding is now consistently smooth with no excursions mean I am much, much more
able to undertake longer exposures, 300-seconds and above, when
appropriate. My sense is that as long as
balance is reasonable, the mount will guide at the above error level for as
long as I want to go.
So, am I at the end of my improvements? Maybe, and maybe
not. I could certainly leave the AVX alone now. But there is one further “easy”
improvement I could essay. My single PEC recording improved the error figure.
But averaging several runs and uploading that resulting smoothed curve to the
mount might make it even better.
Celestron still offers a free program to do that, “Pec Tool”
(even though it hasn’t been updated or publicized in a long time). I may be
reaching the point of diminishing returns with the VX—1” RMS error is pretty
good for a mount in this class—but it might be worthwhile to take this one last
step. I’ll let you know how it goes if I decide to do that (ain’t broke/don’t
fix it) and if clear skies ever return this summer. It is, yes, raining hard
now.
Sunday, July 09, 2017
Issue #543: My Yearly M13 (from the backyard…)
One of my traditions is that each year, sometime over the
course of the summer, I take a picture of star cluster Messier 13. Why? Well, it’s
tradition as Tevye said. But it also ensures I get out at least once during the hot, humid, hazy, and usually stormy Gulf Coast summer and take
a few deep sky pictures.
I’ll admit these days I am not sanguine about braving sweat
and mosquito bites trying to get images from skies that look like milk. If
I lay off until fall, however, I get out of practice. And as complex an
endeavor as deep sky astrophotography is, you do not want to get out of
practice.
Usually, I do my portrait of the Great Globular in Hercules from
my dark site in the wilds of northwestern Mobile County. Not this year. With
June already segueing into July and hurricane season threatening to get started in earnest, I
thought I’d better get my M as soon as possible. The conditions were just lousy,
though. So lousy that I had no intention of loading a ton of gear and driving
half an hour to the dark site only to sit under clouds hoping for sucker holes
while providing dinner for hordes of six-legged fiends. The good, old, backyard
it would be.
Can you get decent photos of deep sky objects from the
backyard? Yes, you can, and not just of the brighter objects, either. You’ll
notice in the shots here that M13’s little “companion,” the near 12th
magnitude galaxy NGC 6207, shows up readily and even gives up its nebulous
disk. M13 itself and similar bright
clusters are really no challenge. But whether you’re trying easy or hard from
the back forty, what will lead to success is the understanding that imaging the
deep sky from brighter skies is a battle.
This battle is between the
target object and the bright
background. While it is much easier to pull a washed-out object out of the
light pollution today with electronic cameras and digital processing, it’s
still best to minimize light pollution induced background brightness to the
extent you can.
 |
| Pac Man Nebula with "Imaging" LPR filter... |
One thing you can do to accomplish that is use a relatively slow telescope.
Why? Have you ever tried a wide-field image from light pollution? If you have,
you know it’s pretty hopeless. After little more than a minute—or maybe even
less—the image appears to be of the daytime sky. Most (fixed focal length) camera
lenses are so fast, f/2 or faster, that the background blows out in a hurry,
before many details in the object you are wanting to image are recorded. So,
slow it down. I like f/6 or, better, f/7 from the backyard.
How about filters? I’ve tried them, mild “imaging” LPR
(light pollution reduction) filters, and it’s a mixed bag. I do find them
helpful in capturing fainter nebulae. A filter allowed me to get a respectable
image of the Pac Man Nebula from my yard on a not so good evening. There is a
penalty, however—color shift. While the nebula was easy enough to color
balance, when it was just right the stars were a distinct reddish hue due to
the presence of the filter. On the other hand, I was able to get a better
picture of the Pacmeister by far with than without the filter. I use a filter only when there is no
alternative.
In the interest of keeping the background glow a little
lower and not burning out—overexposing—the cores of globulars and similar
objects with bright centers, I generally set my DSLR’s ISO no higher than 800.
That is more than adequate to bring home faint nebulosity, and in addition to
keeping the background less overpowering, it reduces the noise in my frames.
Stacked ISO 800 frames are visibly less noisy than stacked ISO 1600 ones.
The big question, though? How long should your subframe exposures be and how many should you
take? The latter is easy to answer: “As many as possible.” Each additional
subframe added to a stack decreases noise and makes processing easier. Certainly,
you shouldn’t keep exposing when the object reaches problem areas like the
Meridian (for some mounts) and the horizon (for all mounts). But the more good
subs you can get the better the results will be. Don’t be shy about throwing
out poor subframes, of course—ones with trailed stars or aircraft or satellite
intrusions. If you take lots of subs, it won’t be as painful if you have to
delete a few.
How long should the
individual exposures be? That’s harder. Longer exposures pick up more details
and are less noisy than shorter ones. Remember, no matter how many frames you
stack, no details not present in a single subframe will be visible in the
finished, stacked image. So, the basic requisite is that you must expose long
enough for desired details to be visible in individual frames.
 |
| In a 1-minute exposure the background is brown... |
At a dark site, go as long as necessary, or as long as you
and your mount can stand it exposure wise. In the backyard, though, you will be
limited. Expose for much over a minute or two and the sky background will become
incredibly bright and color shifted as in the picture below, a two-sub
300 second exposure with my f/7 120mm ED refractor, Celestron AVX mount, and
Canon 400D. Processing can bring back a passable final result, especially when
it comes to darkening the background, but fixing the light pollution caused color
shift is a more serious and difficult problem.
As you can see in my final 300-second x two subs picture in the comparison shot below, M13
is noticeably (too) blue. I got the background unreddened using the
“background color offset” function in Nebulosity, but that left M13 with a cool tinge. That can be fixed as well, but it takes more work and
more skill.
While the 300-second sub picture shows more stars, frankly I
think the 60-second x 10 image actually looks better. 60-seconds isn’t long,
no, but NGC 6207 is just as visible in the shorter sub-stack. It was also much
easier to process with a less bright background and not as much color shift (the background was more on the order of
brown than red).
Conclusion? In a light polluted backyard, shorter, more
numerous subs are often better, or at least easier to process, than longer subs
no matter how many longer subs you take. What your exposure limit should be
depends on the degree of light pollution and the current sky conditions.
For me, 300-seconds is a good subframe exposure on a
dark(er), dry winter night when I have a zenith limiting magnitude of 5.0 or so.
On a spring or summer evening when humidity scatters light pollution, 1 –
2-minute subs are what I do. On this summer’s night, ten 60-second subs were
definitely preferable two two 300-second subs. And more 60-second subs would
have been better still. So why did I stop with ten? Ah, on that hangs the short tale of this annum’s M13…
 |
| 300-seconds and the background is a bright pink-red... |
As July came in, the question became not “When will I get
M13?” but “Will I get M13 at all?” There had been precious few opportunities to take deep sky pictures all spring long. And
not that many this past winter, either. Summer was thus far shaping up to be as bad if
not worse. So, when Accuweather’s Astronomy Forecast on the web and my Scope
Nights and Clear Sky Chart apps on the iPhone began to look slightly favorable,
I got my rig set up in the backyard tout
suite despite temperatures climbing well past 90 (try “feels like 101F”) and
high humidity.
Said rig? My SkyWatcher 120ED refractor, Miss Hermione
Granger, Celestron AVX GEM, and old Canon 400D. Why was I using the lighter
mount rather than the Celestron CGEM? I
was a wimp. An astro-wimp. I couldn’t
face the prospect of lugging the 40-pound plus CGEM head out into the backyard in the heat.
By the time I finished cabling up everything—camera to
computer, mount to computer, guide scope to computer, shutter control cable to
camera, dew heater, mount power cord, hand control, etc., etc. etc.—I was wet
with sweat and just this side of being overheated. Seeing as how it doesn’t get
dark till way past 8:30 in these days of daylight savings time, however, I had
sufficient time to cool off before starting the run.
When the stars finally began to wink on, I got the VX polar
aligned. As I mentioned some time ago, I no longer use Celestron’s All Star
Polar Alignment routine (in the hand control) to do my polar align. I find Sharpcap’s polar alignment tool, which uses the guide scope and guide camera is
easier and more effective. My declination error with a Sharpcap
polar alignment is noticeably lower than it ever was with ASPA, even
given two ASPA iterations.
 |
| 60x10 (top) and 300 x 2 (bottom)... |
When I was
satisfied with my composition, I switched to PHD2 and got its guiding
calibration out of the way, clicking on a bright, but not too bright field star. PHD2 calibrated readily, and when that was
done began guiding. I always give the auto-guiding a few minutes to settle
down, and, so, walked back inside to enjoy the cool for a few minutes.
Returning outside, looking at PHD2 revealed the RMS guiding was about 1.5” or lower,
more than good enough for my 900mm focal length refractor and APS-C size chip.
That being the case, I returned to Nebulosity, and instructed it to take 25
60-second exposures.
A great
thing about Nebulosity and PHD2? They are rock solid. If I wanted, I could have
just sat inside and let them do their thing without me. I got bored with channel
surfing however, and returned to the laptop on the deck before long. PHD2 was
guiding great, and the frames coming up on Nebulosity looked good. I noted little
NGC 6207 immediately. All was well. Until...
Just as I
began to wonder whether I should go back to the den and see if there were
something good on Netflix, my iPhone just about gave me a heart attack with its
alert tone. The issue? “A line of severe thunderstorms is headed your way.”
Rut-roh, Raggy! Looking to the west, I realized that what I’d thought
was distant fireworks was actually lightning.
Hmmm. Should
I wait and see? I’d only accumulated ten subframes so far. Unfortunately, the
phone insisted the weather would arrive by 11:45, and it was already past 11:30.
Deciding discretion was the better part of valor, I turned off the AVX, covered
Hermione and the mount with my Telegizmos cover (recommended), disconnected the
computer, and scurried inside.
I was a
little miffed, but back in the blessedly cool den, I realized that out in the
heat and humidity I had begun to get dehydrated without realizing it, so
mesmerized by PHD2’s tracking graph I had been. I re-hydrated with a Gatorade
and called it a night. I was tired enough that I didn’t even deign to look at
the year’s M13 on the laptop.
 |
| My yearly M13 2017... |
Next morning,
I stacked and processed my shots—which I thought were pretty pleasing and far
from the worst annual M13 I’ve ever done—and strategized about the coming night.
The storm had come and the storm had gone, so I would be able to get out for a
second summer night in a row (!) it seemed.
What would I
do? I had two things to accomplish. First, I wanted to take some longer subs of
M13, 300-second subs, for the comparison above. I also wanted to do a little
experimenting with the PEC function on the AVX, something I had not previously
gotten around to despite having owned the mount for four freaking years.
And so, I hit
the backyard once again. My experience with PEC and long(er) subs on the AVX?
That, my friends, is a subject for next
week.
Sunday, June 11, 2017
Issue #542: The Curious Case of the Schmidt Cassegrain Telescope
In the last few years, however, I’ve made it no secret that
I’ve somewhat turned away from SCTs and to refractors for a variety of reasons. What gives? Am I now fer ‘em or agin’ ‘em? Answer? It’s complicated.
In fact, just about everything concerning the SCT is
complicated except for its relatively simple design (other than that dratted corrector). Moreso than any other
telescope it raises strong emotions. It doesn’t just have fans and skeptics, it has lovers
and enemies. It’s unusual to see a general discussion of the SCT online on
BBSes like Cloudy Nights that doesn’t degenerate into slings and arrows from
both sides.
That being so, I thought this Sunday we’d go over the
arguments of both the prosecution and defense in the Curious Case of the
SCT: Is it a Good Telescope or Not?
For the Defense:
The SCT is Portable
The Schmidt Cassegrain is just naturally easy to transport and set up given its
folded optics and short tube. The C8 packs two meters of focal length into an
OTA less than two feet long. Carrying around and mounting an 8-inch f/10
Newtonian is something of a nightmare, but an 8-inch f/10 SCT is practically a
grab ‘n go scope, to summon that overused cliché.
A modern fork mount model is virtually a self-contained
observatory. Not only is there goto, computerized pointing, to the tune of tens
of thousands of objects, some models now feature built in auto-guiding, wi-fi,
and can align themselves with little user intervention. An 8-inch SCT, a
battery, maybe a DSLR, a box of eyepieces and you are ready for either visual
or imaging work at the drop of a hat.
These Telescopes are Supremely
Affordable
The most amazing thing about SCTs? They’ve gotten cheaper. Back in the day, in 1995, I
paid over two-thousand dollars for my Ultima 8 SCT. It was a nice enough
telescope with very good—if not perfect—optics and a sturdy fork mount and tripod.
However, there was no goto or other computerization. The mount was powered by
a 9-volt battery. Turn it on and the telescope tracked, turn it off and it
stopped. It did have a Periodic Error Correction (PEC) feature, but when you
turned the scope off at the end of the night, your hard-won PEC “recording” was
lost and you had to re-do PEC all over again the next evening. Otherwise? There
wasn’t even an auto-guide port.
 |
| Me and my new Ultima 8 circa 1995... |
Today, 1500 bucks in our decidedly smaller dollars will get
you a Meade LX90, a very competent SCT with a sturdy-enough mount and tripod
and a hand control with zillions of objects. If you can bump the budget up to 2700
George Washingtons, you can have an LX200 GPS, a telescope that doesn’t just
include just about every conceivable computer feature, but which has a mount at
least comparable to that of my old Ultima’s massive fork. Oh, and the LX has a
much better-looking field edge than the Ultima 8’s thanks to the telescope’s
ACF, “Advanced Coma Free” optics. While I haven’t done the computation, the LX200’s
current price is still likely less in real dollars than what I paid for that manual telescope in '95.
Meade and Celestron’s
Optics are Excellent Now
I’m not just talking about the dreaded Halley-scopes, the
SCTs produced during the comet Halley craze, when I say Schmidt Cassegrain
optics could be variable in the past. There were some excellent ones, but there
were also some dogs. The good news is that both companies are very
consistent today. There may be fewer stand-out scopes, but there are also far
fewer poor ones. Add to that advances like Celestron’s Edge optics which reduce
coma and field curvature, and Meade’s ACF optics, which reduce coma (and which
are available in f/8), and it’s no exaggeration to say that in general terms
SCT optics are better than they ever were.
The SCT is Well-suited
for a Variety of Tasks
Thanks to features like the moving mirror focusing system dreamt
up by the man who invented the commercial SCT in the 1960s, Celestron’s founder
Tom Johnson, few telescopes are so suited to such a wide variety of tasks. You
can take high resolution pictures of Jupiter one night, observe deep sky
objects the next, and do spectroscopy of distant galaxies the evening after
that.
It’s not just that the moving mirror focusing gives the SCT
tremendous a back focus range that will accommodate eyepieces, cameras, and
other sensors of all types, it’s that it has become the PC or telescopes. The SCT has been around in commercial form
for over 50 years, and both companies have more or less retained the standards
Celestron implemented in the 70s. A visual back from the mid-1980s will still
screw on to any modern Meade or Celestron SCT. That means there’s a huge number of
accessories and add-ons for these scopes.
Those many accessories include focal reducers and extenders
(Barlows), and thanks to the Schmidt Cassegrain’s focus range, it’s easy to
make these things work on the telescope for visual or imaging use. Most SCTs are natively f/10 telescopes, but just a
few dollars gets you extenders and reducers that give you an f/20 and an f/6.3
too. It’s like having three telescopes for the price of one.
The Schmidt CAT is
Usually on the Cutting Edge of Technology
When some new innovation is developed, it’s usually
developed for SCTs first. When goto came
to commercial amateur scopes, it came in the form of the Celestron Compustar
and Meade LX200 SCTs first. If you want the latest and the greatest—like
Meade’s Starlock System, which provides integrated guiding and goto—look for it
in Schmidt Cassegrains and especially fork-mount SCTs first. Why? In part
because Meade and Celestron SCTs are still the most popular commercial
telescope. The numbers keep the prices down despite ever increasing (electronic)
features. An impetus to this innovation is the fact that you’ve
got two companies competing for one small market. It’s like Honda and
Toyota—the two keep innovating and adding more features in hopes of pulling
ahead of the competition.
 |
| High-tech observing in the 90s... |
For the Prosecution…
The Schmidt Cassegrain
May be Transportable, but it isn’t always Portable
Certainly, smaller SCTs are quite portable, but not quite as
portable as they’re often said to be. If you don’t mind carrying a telescope
out in at least two pieces and assembling it, a 5 to 8-inch SCT is portable. However, even A C5
stretches the idea of grab ‘n go. Oh, some people might pick one up in one
piece and waltz it into the backyard, but it’s more of a handful than you’d
think. Most Fork mount C8s? Few of us would want to carry one more than a few
feet. A fully assembled C8 on a GEM,
even on one of the relatively light CG5/VX/Exos size mounts? No way, not unless
you are the incredible Hulk.
Get above 8-inches, especially with fork mount models, and
even “transportable” becomes dicey. A 10 – 12-inch fork SCT OTA/fork/drivebase will weigh in
at around 50 to 70 pounds. Even when setting up in alt-azimuth mode, more than
a few people will be challenged. You’ll have to lift that awkward
tube/fork/drivebase combo onto a tripod at least waist high and get it
oriented on and bolted to said tripod. Onto a wedge’s tilt-plate for use in
equatorial mode? At the 10-inch level that is sometimes a job for two, and at 11 and 12-inches it most assuredly is. The biggest CATs, the 14s and 16s, are telescopes for permanent or semi-permanent
installations where you can at least wheel scope out of storage and onto an observing
pad.
How about a GEM, then? A 10-inch isn’t bad. At 11-inches, however, many normal adults will be stressed. It’s not so much the
weight as it is the awkward bulk that has to be held steady as the dovetail is
slid into or tipped into the mount’s saddle. A 12-inch is entering the realm of
scary for most, and a 14-inch—which is like wrestling with a full garbage
can—is a daunting task. Mounting a 16-inch is a serious undertaking for at least two
men.
SCTs in smaller sizes are
transportable and convenient, but as aperture increases, they become
surprisingly less portable than some other designs. Today there are 20-inch
(ultra-light) Dobsonians that are far
more portable than a 12-inch LX200.
These Telescopes are
Affordable, but You Do Get What You Pay for
There is no denying Meade and Celestron Schmidt Cassegrains
are supremely affordable considering their apertures and features, but there is
a reason they are cheap. While both companies have thousands of satisfied
customers, the road to satisfaction is sometimes a rocky one. One of the reasons
SCTs are inexpensive is that they are made and sold in (relatively) large
numbers. Another, more concerning, reason is the Chinese companies’ QA programs
are not exactly robust. Even scarier is that some of the materials and parts used
in these telescopes are of lower quality than they should be.
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| Me and the bigun... |
A case in point was Meade’s much heralded RCX400. The one I
used (loaned me by a Meade rep at a star party) worked well and had some
amazing features. I did note the fit and finish was rather poor even for a mass
produced SCT, especially given its 4000 dollar plus price tag. Internally, it
turned out, the story was even worse. Many of the telescopes arrived DOA, often
thanks to the inexpensive motors Meade used in the focusing and collimation
system—and that was just the tip of the iceberg.
While the RCX is an extreme example, the same sort of thing,
the same low-balling of parts and lax QA, is evident all across Meade's and Celestron’s
product lines. Wise advice? Don’t be an early adopter of one of the companies’
scopes. While both do tend to get
their products right, it often takes a while—that “while” extending even to
“years.” If you get a DOA SCT and have to ship it back to your dealer or the
maker, or if your scope develops continuing problems, it may not seem like such
a bargain after all.
Meade and Celestron
Optics are Pretty Good Now, but are Still a Compromise
Both companies have come a long way since the 1980s and
early 1990s when it comes to optical quality. And, certainly, credit where
credit is due for them introducing improved designs like the Edge and ACF.
However, SCT optics with their 30% range central obstructions will always be a
compromise. Their contrast characteristics are never going to be as good as
those of unobstructed or minimally obstructed scopes. Also, thanks to their
mass-produced nature, these days you will likely get good optics but you will rarely—if ever—get great optics.
Surprisingly, both companies produce great refractor optics and often excellent
MCT optics. It seems SCTs are just a little more difficult to get to that
level, and it appears they always will be.
The Schmidt CAT is a
Jack of All Trades, but Master of None
The SCT is indeed good at many things, but there are often
other designs that are better at any one of those things. If you are an
astro-dilettante like Uncle Rod, that may not matter, but if you have a special
interest area in our avocation, it might. A Newtonian or a refractor, for
example, is a better instrument for planetary observing. A large and portable
Dobsonian is better for visual deep sky work.
A refractor, an APO refractor, is arguably a better choice for deep sky
astrophotography (although an SCT can certainly shine when it comes to imaging small
DSOs or planets).
Sometimes We’d be
Better Off Without All the High-Tech Gimmickry
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| Even "just" a C11 is a handful... |
The Verdict?
That is up to you. I’ll let you make up your minds about
it and would love to hear your decisions in the comments section. Me? These
days I am perhaps not quite the Schmidt Cassegrain evangelical I once was, but I still use them. My stable may have shrunk to a single
C8 (my Edge 800) and a C11 (whose days may be numbered), but, yeah, I still use
‘em.
When I do pull my C8, Emma Peel, out of her case, it’s like coming home. I know, Mr. Wolfe said you can’t go home again, and that is somewhat true. I am more aware of the design’s warts than I used to be (or would admit), but I still believe “right tool for the right job,” and that right tool is still sometimes the good old Schmidt Cassegrain.
When I do pull my C8, Emma Peel, out of her case, it’s like coming home. I know, Mr. Wolfe said you can’t go home again, and that is somewhat true. I am more aware of the design’s warts than I used to be (or would admit), but I still believe “right tool for the right job,” and that right tool is still sometimes the good old Schmidt Cassegrain.
Sunday, May 28, 2017
Issue #541: Taking Pictures with a C8
When I’m speaking about the history of Schmidt Cassegrains
at star parties, club meetings, or cons,
I often get puzzled looks and questions from new amateurs about one of the
things I say: “One of the big reasons for the 8-inch SCTs becoming the most popular
commercial telescope in the 70s was astrophotography.” What? Everybody knows SCTs aren’t good for taking long
exposure deep sky pictures. For that you need a short focal length refractor,
right?
Maybe and maybe not. Firstly, back in the 70s when the
Schmidt Cassegrain began its rise to fame, the other common telescope alternatives
for deep sky astrophotography were the cumbersome, shaky Newtonians practically
everybody owned, and refractors with focal ratios of f/15 or more. Take it from
someone who was there, it was a million times easier to take deep sky
astrophotos with a C8 than one of those telescopes.
Also, while I won’t disagree that for beginners in
astrophotography, a refractor of short focal length is easier to manage in the
beginning, we don’t remain beginners forever. Eventually you may discover more
focal length, aperture, and resolution than what your 80 – 100mm refractor
offers can be a good thing. So what are the problems with using the average Celestron
C8 or Meade 8-inch for deep sky imaging?
The first gremlin is simply all that focal length. With a C8, you start
out with a native focal length of about 2000mm. That is what, more than
anything else, makes long exposures tough with the telescope. At 2000mm, every tracking
faux pas your mount commits will be exaggerated. Not as stable as it ought to
be? A tiny gust of wind will ruin your picture by creating trailed stars no
matter how well you guided. That may make anything but the shortest exposures
problematical in autumn and winter when the winds are wont to blow.
Also, if you’re a plebe like me, you won’t be using a 10 thousand
dollar mount for your telescope and will have to guide it. You’ll use a small
auxiliary camera to keep the telescope precisely centered on the target despite
the inevitable back and forth motion of periodic error caused by less than
perfect gears. At 2000mm, you will have to guide precisely. How precise depends on the pixel size and sensor chip size of the imaging camera,
but you can bet there won’t be much room for error.
Then there are the mirror flop blues. To focus, the primary mirror of a
Celestron or Meade SCT slides up and down on the baffle tube that protrudes
from the main mirror. The mechanical tolerances there are OK for visual use,
but are loose enough that the mirror can move slightly when the attitude of the
telescope changes significantly—as when crossing the Meridian. Result? Those
darned trailed stars if you’re using a separate guide telescope for
auto-guiding the mount. To the guide camera, everything looked fine, but the
image moved in the main camera when the mirror flopped.
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| An imaging rig back in the day! |
None of these things present insuperable difficulties,
though. After all, me and my mates were using C8s to take good pictures—which I
define as pictures that made us happy—thirty and forty years ago. We didn’t
have electronic cameras, either. We manually guided our telescopes and usually exposed
for a minimum of half an hour even on bright objects and with “fast” film in
our SLRs. If we could get decent shots with a Schmidt Cassegrain then,
certainly you can now.
Again, I don’t endorse a C8 or Meade 8 as your first
astrographic telescope. Cut your teeth on the vaunted fast ED refractor—they
are cheap now and come as close to being foolproof for deep sky imaging as you
can get. But when you are ready to move up in focal length and aperture, however,
begin collecting the astro-stuff you will need…
Get a Modern SCT
Get an Edge (Celestron) or an ACF (Meade). Their better
field edge performance is a good thing, no doubt about that, especially if you
also intend to use the scope visually. Admittedly, unless you are employing a
camera with a full frame 35mm sensor, you won’t notice the difference in images,
but you might as well invest for the future so that if/when you move to a
bigger chip you’ll be ready.
The really big deal with modern SCTs for imagers is not
necessarily the field edge, but that they have mirror locks. The Celestron Edges
have them, and so do the 8-inch Meade ACF telescopes. These locks stabilize the
primary mirror and prevent it from flopping if you are guiding with a separate
guide scope.
Get a Focal Reducer
All the Meade and Celestron 8-inch SCTs come in at f/10,
that 2000mm we talked about above. Not only does that many millimeters make
guiding and tracking more difficult, it makes for longer exposures and can be a
challenge for accurate goto pointing. The solution? If you get the Celestron,
buy the Edge f/7 reducer. If a Meade, the standard Meade f/6.3 reducer
corrector (the Celestron 6.3 works fine on Meade scopes too). The Meade and
Celestron 6.3 reducers are reducer correctors, designed to flatten the field
edge of non-ACF telescopes, but they work just fine with ACFs since most of
their effect is to, yes, flatten the field rather than remove coma—which the
ACFs’ optical system does itself.
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| A 66mm f/7 makes a nice guider... |
How about other focal reducers? Like those from Optek? They
can be a good choice if you’ve got a Meade scope, but some can’t be used
visually. Those for the Edge scopes definitely can’t. Only the Celestron f/7
Edge reducer can be used for that. Since you’ll probably want to eyeball the heavens your Edge SCT once in a while, get a reducer that will work
with an eyepiece.
Get a Good Enough
Mount
This is the most important thing if you’re considering SCT
astrophotography: how good is the mount’s tracking? Especially with a payload consisting
of an 8-inch SCT, camera, and guide scope (which may be upwards of 30 pounds).
It doesn’t matter if you image with a fork mount or a German equatorial—both
have their pluses—it just matters that you get good tracking with a tricked
out 8-inch SCT onboard.
Can you get by with the fork mount that came with your
telescope? Maybe, if it’s of fairly recent vintage. Older forks can be a
crapshoot. I once encountered a Meade LX200 GPS with 90” of periodic error
(that’s a lot). Modern forks like the CPC Deluxe from Celestron and the fancy
LX600 from Meade are certainly much better for imaging than the old ones.
HOWEVER, thousands of good long exposure images have been taken with the
minimalist AC driven fork mounts of the 70s and 80s. Use what you have, but a
good mount makes things easier.
For most of us, a good mount is a GEM. A German
equatorial has the advantage of allowing you to use a variety of scopes on the
mount. You can do widefield with a refractor without the hassle of trying to
piggyback it on a fork mount’s SCT OTA. One is also more portable than a
fork mount, though an 8-inch fork SCT isn’t too much of a hassle for most
of us to transport and set up.
How much should you spend on a mount? That’s up to you.
Prices for GEMs usable for imaging with an 8-inch Schmidt Cassegrain range
from about 800 dollars all the way up to 10 thousand dollars and more. Before
spending oodles of cash, though, ask yourself how often you are really going to
be able to or want to take pictures. For most of us that is maybe once or twice a month--IF the weather cooperates.
Me? Thanks to our stormy Gulf Coast, I rarely do
astrophotography even once a month. For me, an inexpensive imported GEM is more realistic than a top of the line AP, Bisque, or 10Micron. Keep the sub-frame exposures down to 5-minutes for less and an
Atlas or a CGEM can work very well with an 8-inch SCT. Given my usual conditions, it’s
not like I’m going to be taking 12-hour exposure sequences anyhow.
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| Off-axis guider... |
Don't scrimp on the mount, though. While I’ve
taken OK images with my C8 and a CG5 or AVX GEM, it was clear these mounts were at their limits with the telescope. And so are the
other GEMs in this class up to and including the HEQ-5 (Sirius). For ease and reasonable consistency of results, consider the next
step up, the EQ-6 (Atlas) or CGEM or CGX mounts. If your skies and your skills
are better than mine, and you are less lazy than me, I wouldn’t criticize you
for bumping the mount choice up to a Losmandy G11 (about 4K), but you don’t
have to do that to shoot good deep sky astrophotos with a C8. An Atlas type
mount will do it.
Get a Sufficient
Guide Scope
Today’s sensitive, high resolution guide cameras don’t
require the crazy long focal length guide-scopes we used in the day of manual
guiding. Still, you need a guide scope (a refractor or a reflector that does not use a moving primary mirror to
focus) with enough resolution so the guide camera can “see” small errors
when imaging with an SCT.
I am lazy and get along with one of those 50mm
finder-guide-scopes that are so popular now, but I suggest a minimum of 400mm of focal length for the
guide telescope when doing C8 astrophotography. A Short Tube 80 or similar will
do as long as you can lock the focuser down securely. And you have a sturdy mounting for the 80. That is incredibly important
when imaging at these focal lengths, since the smallest amount of flexure in the
guide scope rings will show up as trailed stars in the main scope’s images.
Get an Off-axis Guider
Well, maybe. I
suggest you try a guide scope first and only if you find you just cannot get
the gremlins out of your guiding setup no matter how you tighten things down or
tweak the Brain settings in PHD2, should you
consider an off-axis guider.
An “OAG” allows you to both guide
and image through the main scope. One contains a little “pickoff” prism that
diverts a small amount of the light at the edge of the telescope’s field to the
guide camera. Since it is seeing the same images as the main scope, problems
like flexure and mirror flop instantly disappear.
Unfortunately, there’s a price to be paid. The OAG will only
pick up stars around the periphery of the telescope’s field. There may be few
of the them, and their shapes may be distorted if you are using an older “standard”
SCT whose field edge is not perfect. In this day of sensitive guide cameras, the problem of finding a suitable
guide star is not as bad as it used to be, but it can still be difficult. I used an OAG all through the film days, but never
found it to be a pleasant experience.
Get a Good Polar Alignment
Declination drift due to poor polar alignment just makes the
task of guiding more difficult. Strive to get within a couple of minutes of the
celestial pole if possible. That used to be tough, but innovations like the Polemaster
polar alignment camera, and the polar alignment routine in Sharpcap (which uses
the guide scope and camera to do the alignment) have made it positively easy.
Tips for Getting it
all to Work
Balance
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| C8 Edge plus Atlas EQ-6: not quite perfect but mine... |
With a sub-Losmandy mount, a Chinese GEM up to
and including the iOptrons, be scrupulous about balance. That means
balancing the mount so it is slightly east-heavy. Of course, you will
likely have to rebalance if you move far from your initial target. That is not
a big problem for most of us, since we’ll usually only image one or two objects
a night and it’s easy enough to pick two subjects in roughly the same part of
the sky. “East heavy” can make a big
difference in how an imported mount performs, since it ensures the R.A. gears
are always properly engaged.
Keep Subs Short, but…
With a C8 riding on an AVX or similar mount, you may
find it to your advantage to keep individual exposures short. To pehaps a
minute or two. If you have a bad spot on your gears, just throw out
that sub-exposure and be on to the next one. Over an exposure of 5 – 10-minutes,
there’s a lot that can go wrong with a light mount’s tracking ruining that
whole, long shot.
Do remember, though, that sub-exposures have to be long enough to
capture desired detail. While stacking subframes will make a shot less noisy and
smoother, no detail not visible in a
single sub-frame will show up in the final, stacked, photograph.
Keep Working with PHD
Settings
I didn’t for the longest time and am now sorry I didn’t. The
settings I had were good enough for the APO refractors I usually use for
imaging these days, yielding RMS guide errors of 2” or a bit more on my AVX and
CGEM. Couple that with my laissez faire approach to polar alignment, and most
of my shots with a C8 (reduced) didn’t have perfectly round stars if I
zoomed in enough in Photoshop or whatever.
Eventually, I decided I needed to do something about my
guiding, since I wanted to begin imaging with the Edge C8 again once in a while.
I read up on the PHD2 Brain settings and devoted one entire evening
to tinkering with them. In just that one night my RMS guide errors went from 2”
to 3” to a bit more than 1” at best, and under 2” at worst. That,
coupled with Sharpcap polar alignment, has meant that for me imaging with the
C8 is easier than it ever has been.
Shoot Appropriate Targets
If a target, a medium-small galaxy or globular cluster,
perhaps, cries out of an 8-inch SCT, by all means use one as the imaging
scope. If it doesn’t? Use a nice 3 – 4 – 5-inch ED refractor instead. Why make things hard
on yourself for no good reason? In addition to less focal length, a refractor
in this range will be lighter than the SCT, and an inexpensive GEM mount will
always track better with a lighter load.
And that is that. Don’t be afraid to try long exposure deep
sky astrophotography with an 8-inch SCT, no matter what you may have read on the darned Cloudy
Nights BBS. A little experience and you may find it’s not as difficult as you'd been led to assume,
and that the focal length and aperture of your friendly, neighborhood C8 or M8
brings a new dimension to your astrophotography.
