Sunday, September 27, 2015
The Nights Everything Goes Wrong Redux…
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| Bertha's backyard M13 |
Sometimes everything goes right under the stars. Not all the time, though. Not hardly. Not if you are an astrophotographer,
or at least dabble in that black art as I’ve been doing for years. Even in the
supposedly simple film days, there were just so many things that could go wrong. Today, when you have to deal with computers and goto mounts and cooled cameras instead of just a
scope with a simple AC powered clock drive and a 35mm camera, there is
tremendously more to go wrong.
The whole mess got started because of my need to check out my
C11, Big Bertha, and her CGEM mount in advance of the fall star party season. I
thought the C11 herself was good to go, since I’d done some fine tuning of her the previous week. Fine tuning because I knew the scope’s focuser needed
attention. My last time out with Bertha months back, it had been noticeably harder to turn the focus knob than it used to be.
That can happen with a Celestron SCT when the screws holding
the “collar” in place around the focuser shaft loosen and the collar becomes
canted with respect to the shaft. The solution is to adjust all the
screws till the collar is parallel to the shaft and focus knob. It only took a
few minutes to sort this out and return the focus action to its normal smooth
feel. I also had a quick look at the corrector’s cleanliness. It was not pristine but not in need of cleaning yet,.
The CGEM? That was an entirely different kettle of fish. I didn’t
know it needed attention, but I also didn’t
know it didn’t. I didn’t know much
about it at all, frankly. In January of this year, I had finally had enough of
the NexStar 11 GPS’s enormous fork, and had purchased a Celestron CGEM to replace it. I was able to test the mount a few times in the
backyard, but only briefly thanks to terrible weather. I had hopes, though; I planned to resurrect Bertha for this year’s DSRSG Spring Scrimmage, to give her a good workout over three April
nights in the wilds of northern Louisiana. Or so I hoped.
Alas, those hopes came to naught. First obstacle was that,
for the second time in the last 30 years, I was summoned for jury duty. While I
was released before I was due to depart for the Feliciana Retreat Center, that coincided
with the arrival of wave after wave of thunderstorms. Since I was paid up for DSRSG, and, as with any star party, there could be no refunds, I drove to Louisiana anyway, but I packed the C8 and VX instead of the CGEM.
Why wrestle with a big SCT when the weatherman said there was absolutely no
chance whatsoever of me seeing anything?
The dire predictions of Weather.com and Scopenights were correct. I had an OK time
at Feliciana, but the Edge 800 never came out of the 4Runner. In fact, the weather
went from bad to worse in the short time I was onsite. I arrived on Thursday afternoon to rain, and left
Friday afternoon to the accompaniment of booming thunder and flashing
lightning.
And that was it for me even thinking about using the C11 for nearly six months. Both because of
this summer’s even more horrible weather and because of non-weather-related factors. What
little observing got done over those months was done exclusively with my simple
Dobbie, Zelda.
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| The new mount |
Yer old Uncle Rod ain’t no dummy—not always—however, and I wasn’t fool
enough to pack up a mostly unused mount and drive hundreds of miles hoping for
the best. That’s how you incur the wrath of the astronomy gods. The CGEM needed
to be tested as thoroughly as I could test it. Not just visually for a few
minutes, but with a camera on the scope over the course of a reasonably long run.
As I wrote last week, I decided it wouldn’t be an overly onerous task to
tear down the Edge 800 and VX, which were out in the backyard, and replace them
with the C11/CGEM. That turned out to be not exactly true. Thanks to the still
and sultry late September weather, by the time I finished getting the big mount on the
tripod and the big OTA on the mount I was dripping with sweat. Stealing an
occasional glance at the partly cloudy sky, I hoped I hadn’t wasted my time.
For once, wunderground.com said I hadn’t. A cold front was
passing and would bring clear skies, if not on this night, then for several nights
following. Since it is not a problem to leave the C11 setup out back for days
on end, I figured I couldn’t lose. Ha!
Once the OTA was on the mount, I still had to rig up the DewBuster and the corrector heater. Mount the Rigel
Quick Finder. Hook up the batteries. Plug in the hand control. Get the
camera on the rear cell (via a make-do prime focus adapter as I mentioned last
time; I couldn’t find my normal one and used an old OAG instead). Finally, since
the camera I would be using, my “backup” body, an old Canon 400D, needed a
Shoestring shutter interface box to do longer exposures, I had to hunt that up
and hook it to the PC.
Standing there ogling Bertha after I finished, I had to admit she looked
mighty fine sitting in the fading late summer light. Of course,
looks mean nothing when it comes to scopes. We’d see how this new incarnation of my
old friend performed.
Once dark, which is finally beginning to arrive a little
earlier, came, I got to work, which was smooth sailing at first. I executed a
2+4 goto alignment, fired up the PC and started TheSky. That’s when my problems
began. TheSky 6 Professional resolutely refused to talk to the CGEM, “There
doesn’t appear to be a Celestron telescope connected.”
I was flabbergasted. TheSky has simply never failed me. Never. I tried
everything. Moved the serial-USB adapter to another USB port on the hub. Restarted TheSky
multiple times. Rebooted the PC multiple times. Checked Device Manager in the
PC’s Control Panel, which claimed I had a perfectly good Com Port 3. Nothing
worked.
Astronomical twilight had now well and truly arrived. I was
wasting time, so I reluctantly decided I’d forget TheSky for now; I’d figure it
out by the light of day. I sent the scope to M13 via the hand control, fired up
Nebulosity 3, and connected to the Canon. Or
tried to. Nebulosity said there was no Canon camera that it could find.
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| M15 |
What the heck was going on? Puzzled, I idly looked over at
the USB hub. Something was different there. Its red LED was not illuminated.
Could that be it? Could the freaking USB hub have gone bad? That didn’t seem
likely. I’ve never heard of one failing…but there are active electronics in
one, and it was years old so… I unplugged
it from the USB port and plugged it into another one. Still no red light.
I ran inside, grabbed another hub, and connected it to a
USB port. Its LED illuminated immediately. Plugged the USB-serial cable into it and
tried TheSky. Still no dice. Went into control panel and reset the driver for
the USB-serial adapter and suddenly everything was fine. Whew! Plugged up the
Canon to the new hub and Nebulosity detected it immediately. My problems were
over. That’s what I thought, anyway.
OK…I put Nebulosity 3 in frame and focus mode and got the
cluster looking as sharp as I could by eye, twitching the C11’s now blessedly
smooth focus knob. I then picked out a non-saturated field star, clicked on it, and switched
to fine focus mode, which zooms in on the star and allows you to dial in focus
precisely. I began focusing and was almost there when the star drifted out of
the small box it was situated in. What the—? That was new. That had never happened before. I went back to frame and focus
mode, clicked on the star again, Nebulosity zoomed in on it again, and away it
drifted once again before I could finish focusing. What now? Was the CGEM down for the
count?
I checked everything. RA and declination locks were firmly locked. Mount was
definitely in normal sidereal tracking mode. Everything seemed OK. So why wasn’t
it tracking? I didn’t know what else to do, so I reset the hand controller to
factory values, reentered my site/date/time data and did a new goto alignment.
Got M13 back in the field using TheSky—at least that was working now. Started
Nebulosity, went back to fine focus mode, and watched that darned star slowly
drift off into never-never land...
I was well and truly stumped, but luckily, maybe, this wasn’t exactly the first time I've been at a loss under the stars. At least I had the sense to STOP, not do anything else, and
just THINK. What causes stars to drift other than problems with the mount?
Well, poor polar alignment. But I had done an Allstar polar alignment with the hand
control, just like I always do and… Wait
one freaking minute. What AllStar polar
alignment? I hadn’t done a polar alignment
following the goto alignment and that was why the star kept drifting no matter
what I did. All I’d done was sight Polaris through the CGEM’s hollow polar
bore. Hardly good enough when you are imaging at nearly 1800mm.
In my defense, I’d had the VX out in the backyard for nearly
a week. After the first night, when I performed the goto and polar alignments,
all I’d had to do was wake the mount up from its hibernate mode each evening and go to work.
I’d gotten good and used to that and that was why I’d forgotten to do the AllStar, I
supposed. Abashed, I did an AllStar alignment on Antares, went back to the fine focus mode on
Nebulosity and finally got going nearly two hours after I began.
Again, so many things can go wrong when you're doing astrophotography—computers,
computer programs, cables, hand controls, batteries, cameras—that it’s a wonder
something doesn’t go wrong every blasted time. You just have to accept the fact that you are going to have occasional disastrous nights no matter how experienced you are. Yeah, I had egg on my face, but
I hoped that meant that I was due some good nights over the coming months.
When I finally sent Murphy and his pal Finagle packing, I
began grabbing images of M13. This was special, since this was—get this—the first time I’d taken prime focus (non-video)
deep sky images with the C11 I had owned for over a dozen years. In her
former incarnation, it just wasn’t practical. The alt-az fork was fine for
video exposures but not for DSLRing or CCDing. Yes, I had a good wedge for the scope,
but mounting Bertha on it was a two-person job, easy, so I rarely did that.
So, what turned out to be the deal with the C11 and CGEM once the images started hitting my monitor? Nearly
1800mm (with the f/6.3 reducer corrector on Bertha’s rear port) is a lot of
focal length to deal with in my book, and the CGEM is hardly a top-of-the-line
mount. I like it just fine, but there’s no use pretending it’s a Bisque or an A-P.
Nevertheless, I was pleased with what I was seeing on the laptop, especially
considering the fact that I had to go unguided.
I don’t have a guide scope mount for Bertha, and the guide
camera I have, an old Orion StarShoot, is not sensitive enough to use with an
off-axis guider. That didn’t mean I couldn’t take pictures, though. I found I
could go 30-seconds without guiding and keep a respectable number of frames. Backing off to 15 –
20 seconds meant almost all were perfect, and, frankly, all could have been
used in a stack if I weren’t being picky. 15 – 20 seconds is perfectly fine for
me at the moment given my interest in short-sub imaging, but I might think
about a better guide cam or at least a guide scope mount for Bertha eventually.
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| M56 |
Frankly, I was rather impressed by the images the C11
delivered as compared to a C8. More aperture will not give you brighter nebulae or galaxies. Extended objects just
get bigger in larger aperture scopes (assuming the focal ratio stays the same)
not brighter. Stars are another matter. The C11 pulls in more, and combining
that with the larger image scale of its longer focal length made M13 look nice
indeed. Star colors were good and the core looked real sweet.
The next morning, I found it easier, I
thought, to process the cluster without burning out the core than it is with
a C8's images. Since star clusters, open and globular, are prime targets for my
backyard imaging, I think the C11 will be preferable for that. Might make it
easier to tease some detail out of planetary nebulae, too. Of course, there is
the fly in that ointment: I have to
convince myself to set the big thing up.
I didn’t stop with M13. I’d sweated too much
blood on this night to leave it at that despite the now rather late (for me)
hour. From M13, I went to M92, M15, M56, NGC 6888, and a few others. All were
easy enough to capture, and all were easy enough to process. The old 400D
acquitted itself well. While I would have been tempted to go to an ISO higher
than the camera’s maximum of 1600 if that had been available, in truth it would have been a mistake in the light pollution and with a Moon in the sky. As it
was, processing wasn’t a pain, but I still had to deal with some gradients.
How about the mount? I have nothing whatsoever to complain
about. While it is an “inexpensive” GEM, it did bring home the bacon. I will
be interested to see how easy it is to guide. I don’t expect perfection, of
course. The C11 is a heavy enough and even reduced has enough focal length that it
really cries out for a mount two or three steps above a CGEM. A Losmandy Titan
or a Bisque MyT or an A-P Mach 1. The chance of me acquiring any of those fine
machines seems slim, however, and why should I worry about it if the CGEM can
do what I want it to do? For me that is casual imaging of the sort I did on this evening and deep
cruising with the Mallincams, and it’s already obvious the CGEM is more than
capable of both things.
There is one area where the CGEM bows to no mount: goto accuracy. Do a good 2+4 alignment, and
the CGEM will put anything from one side of the sky to the other in an eyepiece
at 150x – 200x. You might get your high-priced-spread of a GEM mount to do
better, but not without doing a 100-point T-point run, and you can do T-point
with the CGEM too. Frankly, I am satisfied; as I hoped, the CGEM is an improvement in
both portability and performance over the fork (except for goto accuracy; the
fork was outstanding there, too).
What’s next? When the Moon is New again, I will be at a star party, the Peach State Star Gaze, with Bertha. I may actually try to get
out sooner than that for some more short-subbing once Luna begins to wane, though.
That is assuming the weather gods and my moods allow, which is always a tossup, natch. Stay tuned.
Sunday, September 20, 2015
More Short Sub Imaging
Since I had pretty good results with the little ZWO ASI120MC
on the deep sky the other night, I decided
to give the minimalist camera a second outing, applying what I’d learned. If
you weren’t here last time, what I’ve been doing is experimenting with a
current craze in astro-imaging, taking many short sub(frames) with a CCD camera
or a DSLR and stacking them into a final image. Keeping your exposures to
30-seconds or considerably less means you will likely not have to guide and
will still obtain a respectable image when you are done.
The VX and the Edge 800, Ms. Emma Peel were still set up in
the backyard, so all I had to do with remove the Desert Storm Cover, wake the mount from its hibernation, and soldier on. If you didn’t read last week’s
entry, the camera, the ASO120MC, is a
one-shot color job with a small 1/3rd inch CMOS imaging chip. I
had high hopes for the second round since I’d had such surprisingly good luck the
first time.
After checking focus on M13, I decided I might as well do
another sequence of everybody’s favorite (everybody north of the equator, anyhow) globular star cluster. As before, I used FireCapture as my image
acquisition/camera control software, and also as before I saved my results (or,
more properly had FireCapture automatically save the frames) as .bmps that were
converted to color on the fly before they hit my screen.
When I was ready to begin, I set my exposure. Initially, I tried 5-seconds, which seems to be what a lot of the short-subbers are
using. I Had FireCapture take an image and subtract a dark frame, and
had a careful look when it was done. Hmmm...not so hot...
The globular was there,
and stacking one-hundred or more 5-second frames would probably have yielded a recognizable final product, but I thought the image was just too thin and too close to the
background’s brightness level in my light polluted backyard. Yes, stacking many
frames reduces noise and allows you to more easily process the resulting image,
boosting its levels, etc., than you could with just a single frame, but
stacking images does not increase detail.
The detail must be there in a single exposure. If it’s not, stacking will not
create it.
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| M13 with XTi... |
So, I compromised. 15-seconds seemed a good exposure length
to me. Short enough to keep the stars
round without auto-guiding, but long enough to deliver detail aplenty in brighter objects. I did go
up to 25-seconds for my dimmest targets, Stephan’s Quintet and the Crescent
Nebula, NGC 6888, however, since in a 15-second exposure both were just barely,
barely visible.
I do believe 5-seconds could work from a dark site. But my
backyard? Just no. My Zenith Limiting Magnitude is usually around 5.0, and it’s
a struggle to pull some objects out of the background glow. Also, since, most
equatorially aligned scopes can do 15 – 20-seconds without guiding, I don’t see
much point to 5-seconds unless you are trying to image with an alt-az mount. The
ground truth is that longer subs are just better all things being equal.
How many of those 15-second frames did I accumulate for each
object? 50 was my average number. Didn’t take long to do a sequence of that
length, and it also gave me enough of a stack to keep the background of my finished
images looking reasonably smooth. Remember, this is an un-cooled camera, and my nighttime
temperatures are still in the mid-70s.
I kept going a good long time, doing more objects than I
thought I would. It was quite pleasant outside. Yes, it was still in the
mid-70s before midnight, but it was also relatively dry and I was quite
comfortable on the deck at the laptop. I did pop inside a couple of times to
look at the Arrow episode the CW was showing, but unlike on my previous run,
the bugs didn’t chase me indoors.
When I was done, I’d re-shot M13 and M92 and also done M2,
M27, NGC 6888, the Blinking Planetary, the Cat’s Eye Planetary, Delphinus’
little NGC 6934, and Stephan’s Quintet. A good variety of objects to help me
assess what the little camera could really do. When I was finished, I shut down
the scope and covered her with the Desert Storm again, since I fancied I might, amazingly, get a third night of imaging or even a fourth judging by the weather forecasts
I was hearing. That kind of streak has been awfully rare over the last year.
Next morning, I fired up Nebulosity 3 and stacked and
processed my frames. Not only does Nebulosity make processing images easy, it
includes what I believe to be the best and easiest to use deep sky stacking
routine in the business. After my subframes were stacked, I gave each image the
Digital Development treatment in Neb, tweaked levels a little, sometimes did a
little sharpening, and, finally, exported the fits images to Photoshop and did
a small amount of tweaking. It was all relatively easy.
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| M71 with the ZWO... |
With the exception of the Crescent Nebula, which was just
too thin, I had plenty of signal in all the images, and was able to deal with
background noise easily enough in most of the shots. I was quite pleased. No,
NGC 6888 and Stephan’s Quintet were most assuredly nothing to write home about,
but they were there, and, as I reminded myself, this was from a light polluted
site, my backyard, which is only a couple of miles from Airport Boulevard with its endless strip
malls and car lots. It was thrilling to get anything
that dim with a cheap camera on an unguided telescope.
I’ve been praising the ZWO, and that praise is well
deserved. I don’t see how you could do better for the miniscule price. What
should also receive praise, however, is the VX mount. I didn’t throw out a
single frame—well, one or two satellite trail plagued ones. Even at 30-seconds
the mount’s behavior was exemplary. Yes, I had the C8 reduced to f/3.3, but
even then, the “magnification factor” inherent in such a small chip—M13 is
almost too big for the frame—means that tracking has to be darned good. It was.
So, the ZWO had acquitted itself well as a deep sky camera
fully capable of fulfilling the wants and needs of those of us who don’t aspire
to being the next Bob Gendler. Those who, like your correspondent, are happy
with simple deep sky postcards, mementoes of our cosmic journeys to show off to
our wives and girlfriends and mates. If that is all you want, this camera and a
600mm or shorter focal length telescope is all you need.
Which got me to thinking…how could Joe or Jane
Cashstrapped-amateur kick it up a notch and maybe spend even less? One way might be with a DSLR, a really cheap used DSLR. I still have my old (unmodded) Rebel XTi,
which I use as a backup (terrestrial) camera body. It’s been far outpaced even by
the Canon Rebels in the last five years, but I thought that it might be an improvement over
the ZWO in some ways.
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| ZWO Stephan's Quintet: at least they are there... |
One big way is its much larger imaging chip, a 10-megapixel
APS-C senor that produces large 3888x2592 images as compared to the ZWO’s 1280x960
pictures. More importantly perhaps, the Canon’s individual pixels are larger, 5.7µm
in size compared to the ASI120’s 3.75µm. Usually, larger pixels result in more
sensitivity and less noise.
Before I could try the old Rebel on Emma, I had to round up
a couple of items. First was the long exposure shutter module, the DSUSB from Shoestring Astronomy. Unlike
more recent Canons, you cannot enable long exposure with the USB connection to
the camera. You have to have a DSUSB or a remote shutter release connected to
the camera.
I also needed my good, old SCT prime focus adapter. Which
wasn’t in the big tackle box I use to store my small astro-gadgets. In fact it
wasn’t anywhere. I probably had not used it since last January when I was
shooting the comet, since we haven’t had much prime-focus-DSLR-worthy weather since. I am pretty maniacal about putting stuff back in its place, so
it not being in the tackle box was a bad sign. Indeed, after much ill-tempered searching I had to admit it was just not anywhere. Well, maybe it was somewhere, buried in the tall grass at the club dark site.
What to do? I’ve got a couple of old Newtonian/refractor
prime focus adapters, but they would put the camera way too far back from the
Edge f/7 reducer. What would I do? What would
I do? Still looking for the adapter, I opened a cabinet in the shop and spied
my ancient Meade off-axis-guider. Heck, that would work. I’d plug up the
eyepiece port and just use it as a prime focus adapter. Threaded it onto Emma’s
reducer, mounted the camera, and I was ready to roll.
Since I’d be shooting with the Canon, the image acquisition
program would as usual be Craig Stark’s fantastic Nebulosity 3. All I had to do
was select “Canon,” enable the DSUSB option, set an exposure and have at it. I
thought that to be fair in comparing Canon and ZWO I probably should have given
the Canon shots a little more exposure than 15-seconds, since the Xti’s ISO tops out at
1600—which I thought might make it less sensitive than the ZWO. A look at the
first M13 to appear on my screen said otherwise.
Not that the ZWO isn’t a sensitive little camera; it’s
just that the Canon is more than sensitive enough. More importantly, perhaps,
the backgrounds in the images were obviously smoother and more noise free than those of the
ZWO. No, the Canon isn’t cooled either, but its makers had gone to great
lengths to keep noise down even in long exposures and it shows. It also
helped that the camera subtracted a new dark frame from each image
automatically. If M27 looked a hair dimmer on screen than with the ZWO, and I
wasn’t sure it did, I could tell it would be easier to pull out of the
background glow during processing.
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| Canon M27 and ZWO M27 (inset)... |
One other good thing inherent in the Canon? A bigger chip
meant more field. While every object I’d requested had been in the frame of the
ZWO thanks to the VX mount’s outstanding goto, some were on the edge.
Everything was well within the bounds of the frame of the Canon and most of the
objects didn’t even need centering.
This is the time to give a shout-out to my mount control
program, my planetarium program, the ever popular TheSky. In my case, that's TheSky6 Professional. Yeah, I know TheSkyX is even better, but I am
cheap and I am used to 6 and it does everything I need. One odd thing? I have
it hooked to the VX using an ASCOM driver. That being the case, there is no way
it should be any more accurate in gotos than any other ASCOM capable
astro-soft. BUT… For some reason, with my setup gotos using TheSky are better.
Go figure. Anyhow, I love the program, and the combination of it and Knightware’s
Deep Sky Planner is a powerful one.
Since for comparison purposes I would be shooting the same
objects on this night that I’d shot on the previous one, I didn’t spend a lot of
time outside. Once it was clear the scope, mount, camera, and computer intended
to behave, I’d start a sequence of 50 15-second frames, and immediately retire
to the den for the 25-minutes (including time for the darks) it would take to
complete that. There were no surprises. I did M13, M92, M27, M2, and a couple
of others before calling it a night.
What were my findings the next morning when I processed the
images using the same software and procedures I’d used on those I'd got with the
ZWO? The ZWO images were easy to process, but those from the Canon were easier. I could play around with levels
and sharpening without having to be as careful not to pull out more background
noise.
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| NGC 6934 with the ZWO... |
Otherwise? On the globulars it was a tossup. The Canon shots
looked good, but in the ZWO images, the globs nearly filled the field of the
camera and just naturally looked very impressive. Yes, I could shoot at a
higher focal ratio, f/10, with the C8 and make the globs bigger, but even in
15-seconds you can run into problems with trailed stars at f/10 if you don’t
want to do a drift alignment, and I do not.
The Dumbbell Nebula was another story. While I didn’t
have quite enough subs to make it look really smooth given my suburban skies,
there was no doubt the nebula showed considerably more detail than in the ZWO
shots. It’s a large object for that small chip, and this time the object being
bigger hurt more than it helped.
The big takeaway for me, however? This observing run showed
that a DSLR in the C8 at f/7 can do a good job on brighter objects, even in
suburbia, and that guiding is not required. Neither is a drift alignment. My
stars were round at 15-seconds, and probably would have been fine at 30, too.
At 15-seconds I had to throw out exactly zero frames.
My success with the Canon got me to wondering how far I
could push this. What would the C11 do? Its longer focal length would give
larger pictures. But how about guiding? Would that heavy OTA require it at
15-seconds with my CGEM? I thought it would be fun to find out. In the months
I’d had the CGEM I had yet to take on single prime focus shot, guided or
unguided. You hear a lot about the mount’s capabilities or lack of them in that
regard, so maybe it was time I found out.
There was more than just idle curiosity to discovering how the
C11/CGEM would do for deep sky imaging. What is the biggest problem facing the
backyard imager? Those darned light pollution gradients. Use an SCT with a
focal reducer and it gets worse. The sky background isn’t just bright or uneven;
you have a “porthole” effect. There’s light falloff toward the edge of the
frame due to the reducing lens, the telecompressor. You expend most of your
effort in processing trying to suppress that while bringing out as much detail
in the object as possible.
What can help? Keeping exposures short helps with the
gradients, but that makes background noise worse—TANSTAFFL as Bob Heinlein
used to say. “There ain’t no such thing as a free lunch.” What else? Flats can also
work if you’re not too lazy to take them. If you are, an excellent Photoshop plugin, Gradient Xterminator, can make
your shots a lot more even. So can a little more focal length, which tends to
spread out background skyglow. Course, there is the TANSTAFFL factor there,
too, since your image scale increases with increasing focal length, which, as above, isn’t
always a good thing. Still, I thought some more millimeters via the C11 might
help.
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| The Crescent: not bad considering the light pollution... |
I had planned to disassemble the C8 after this night’s run
anyway, and convinced myself it wouldn’t be much more work to not just pack up
the Edge and the VX, but to replace them with the C11, Big Bertha, and her
CGEM. Ha! A C11 ain’t a C14, but don’t kid yourself; it is a handful at
close to 35-pounds. And so, of course, is the CGEM head (the normal tripod is
almost identical to the VX tripod and is no heavier). By the time I was
finished, I was soaked to the skin with sweat and wondering if the effort had
been worth it.
I planned to reduce Bertha’s focal length via the good, old
Jim Riffle designed f/6.3 reducer/corrector, which would take her from about
2800mm to 1764mm which is a darned sight better than nearly 3000, but still… My
experience over the years has been that as soon as you pass 1500mm, getting
good tracking becomes harder. And as you approach 2000mm it becomes absolutely
tough. Even at short exposures in the 15 – 20 second range. Would the CGEM,
which is most assuredly in the “bargain mount” category, be up to the task?
Stop back by next week to find out.
Sunday, September 13, 2015
My Yearly M13
One of the reasons I’ve remained interested in deep sky
imaging is that just when I am getting bored with it something new always comes
along. In the film days, shooting with Tri-X in a cold camera gave way to
hypering Kodak 2415 and exposing at room temperature. Then film itself went as
those CCD thingies came in. Good as they were, the CCDs didn’t have an
exclusive lock on astro-imaging for long. DSLRs soon got better and less noisy
and became a simpler, less expensive way to get pretty pictures of the deep sky.
Then some of us gave up stills altogether and began imaging the faint and dim
with video. And so it goes…
The latest deep sky imaging method that has to come to my
attention? The “short subs” movement. What that is about is taking short
exposures, 30-seconds or less—often much less—and stacking many of them, fifty,
one-hundred, or more, into a finished picture. Why is that good? It puts far
less stress on the telescope mount. A plain vanilla un-tuned and un-guided Meade
or Celestron GEM can usually produce many good frames at 30-seconds and can probably
produce almost 100% good frames at 15-seconds. No sweat, actually, as long as the mount
has a halfway decent polar alignment (no drift alignment required) and the
telescope’s focal ratio is less than about 1000mm.
More than 1000mm or so of focal length? You might want to
guide at 30-seconds of exposure, but you’ll still find guiding much easier to get going right than
you would at 300-seconds. With short subs and a focal length less than 1500mm, an inexpensive guide camera like the Orion StarShoot on a 50mm finder-type
guide scope like the ones Orion and KW sell is good enough. One thing is
sure: minimizing guiding or eliminating
it altogether goes a long, long way to making deep sky imaging fun and easy. Assuming you can get decent looking
pictures out of 15-second subs, and you know what they say about that word.
I’ve always been told longer CCD (or CMOS) integrations are
necessary to keep detail up and noise down. Yes, stacking many frames will reduce
noise, but one-hundred stacked 10-second frames will never have as much signal or look as good as the
result of stacking 10 one-hundred second
frames. Still, I thought it might be fun to see if there was anything to this
new idea. In addition to that, it would make taking my Yearly M13 as simple as humanly
possible.
I'd set up the mount, do a goto alignment and a good enough
AllStar polar align with the NexStar hand controller. Slap a camera on the
telescope. Start an exposure sequence and run back inside to get away from the
bugs and humidity and I’d have M13 in the bag. No guide scope or PHD Guiding to
worry about. No constant checking to make sure the stars were still round.
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| C8 and CG5... |
What’s this “yearly M13” business about anyhow? If you’ve
been reading here long, you know I try to get out and shoot Messier 13, the
Great Globular (star cluster) in Hercules, each summer. It’s been a tradition
with me over the last decade and ensures I keep my hand in even during lousy
summer weather. I have to get out at
least once and shoot that glob, even if it’s through sucker holes.
This had been a particularly bad summer for us on the Gulf
Coast, but as the calendar flipped over to September, the skies finally began to
clear, though they remained hazy. At least the thunderstorms were now more an
afternoon thing than something to plague me all night long. I’d finally get out
into the backyard and get that glob. But
with what? “Which mount, scope, and camera?”
The mount, I decided, would be my decade old Celestron CG5,
which I haven’t used much since I got a VX going
on three years ago. Why? I am at the stage where I don’t want astro-gear I
don’t use hanging around. For one thing, it seems a shame to let a scope or
mount sit idle when it could be making some young person or a cash-strapped
fellow amateur happy. I decided I’d sell the CG5 for a reasonable price this
autumn. Before I did that, I’d need to make sure it still worked of course.
Which telescope? That was easy: a Celestron C8. The C8 is a
good match for the CG5, and if the mount behaved with one that would give the
CG5 a clean bill of health. Since I thought I might sell one of my excess standard
C8 OTAs with the mount, I’d use one of them rather than the Edge 800, Mrs. Peel.
Which camera, then? Canon 60d or SBIG ST2000? The answer was
“none of the above.” I’d been surprised lately to learn people are using the ZWO ASI120MC one-shot color “planetary” cameras
for deep sky imaging. Not just “using,”
but getting fairly impressive results. I thought it would be fun to see how my
uncooled small-chip (1/3-inch) ZWO would do on M13 from my hot, light polluted
backyard.
If this less-than-three-hundred dollar camera could deliver
on the deep sky, it would be a boon for a lot of folks. There are just not many
inexpensive astro-cams out there anymore following the disappearance of the
Meade DSIs, the SACs, and the scaling back of Orion’s budget offerings. That
may be changing with the coming of new lower-tier cams from StarLight Xpress
and others, but right now there just isn’t much.
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| ASI120MC |
It wouldn’t get much simpler for me than shooting with the
ZWO. No power supply, very light camera, easy to use software. I expected to
take dark frames, sure, but biases and flats and all that stuff? Uh-uh. Which
seemed in keeping with the spirit of the Yearly M13 outing.
But what should I expect? How would the little cam do? Even for
bright M13, I wasn’t sure. However, I knew not to count the ZWO out. I’ve used it
for everything from Moon pictures to stellar spectroscopy and it has not failed
me yet. While this camera has small
pixels (1.2 megapixels of 3.75 microns in size), which is what makes it such a
planetary imaging powerhouse, I figured I could bin it 2x2 if sensitivity turned
out to be a problem.
The final piece of the puzzle would be the software. Some people
have a hard time understanding how you can use a planetary camera like the ZWO
for still deep sky images. Normally, after-all, it produces .avi video. You use a program like
Registax to stack those individual frames into finished stills. The answer is in
the software many of us use to control the camera, the amazing freeware program
FireCapture.
The ZWO is a camera like any other astro-camera. It’s not a
video camera or a still camera; that is determined by the software running on
the PC that controls it. The camera
hardware is capable of exposing for up to 16-minutes; you just need the proper software
to access that.
With FireCapture, that is simple. Set the exposure to a
value you want, say 15-seconds, select a still frame file type appropriate to
your needs—you can choose tiffs, bmps, fits, and other formats—and fire away.
In this mode, the camera works just like a normal still CCD, exposing and
saving each frame as an individual picture, not as part of a video stream.
When we finally got a night when I could at least make out
some of Hercules’ stars, I moved the CG5 into the backyard and got down to
business. I’d updated the mount’s firmware some time ago, so it really wasn’t
much different from using the VX. In fact, I still like the CG5’s non-Plus hand
control better. I essayed a 2+4 goto alignment, did the AllStar polar
alignment, and redid the goto alignment for good measure. Naturally, by the
time I finished all that, the clouds had returned with a vengeance. My time wasn’t
wasted, though; the alignments were complete and I was able to test the mount for
proper operation before the sky closed down completely.
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| FireCapture in operation on the deep sky... |
After a few gotos, I could tell the CG5 was her same old
self: a little noisy but accurate pointing.
That would be vital for using the ZWO. Without accurate goto, it would be a
pain to get objects on that tiny chip, even using an f/3.3 reducer.
Not that I had a prayer of seeing what the camera could do on this night. By the time I did those three gotos, thunder was beginning to rumble. I hibernated the CG5, covered the scope and mount with a Desert Storm cover, and ran inside.
Not that I had a prayer of seeing what the camera could do on this night. By the time I did those three gotos, thunder was beginning to rumble. I hibernated the CG5, covered the scope and mount with a Desert Storm cover, and ran inside.
The following (semi clear) evening I woke up the CG5 and got
started, mounting my old f/3.3 Meade focal reducer and the ZWO on the rear
cell. I didn’t bother testing the pointing accuracy with an eyepiece before hooking
up the camera; the Celestron hibernate function has always worked perfectly for
me. I punched “Arcturus” into the HC so I’d have a bright star to focus on, the
CG5 made her famous weasels-with-tuberculosis sounds, and I headed for the PC
where I started FireCapture.
To begin, I selected “16-bit” image depth, since I thought
that might deliver easier to process images.
That allowed me to select “fits” as my image file type, which is the
astronomical standard. Next, I set the exposure to a short but not crazy-short 15-seconds with
FireCapture’s exposure slider. Finally, I told FC to automatically take and
subtract a dark frame. With an uncooled camera, that is vital if you don’t want
to see a snowstorm of false stars, hot pixels. I started out with 70% gain
(think “ISO”) on the ZWO to give decent sensitivity without introducing more
noise than necessary.
As usual, the CG5’s pointing was good. Arcturus, a big bloated
donut, was easily in the frame. I mashed the appropriate button on the JMI Motofocus
hand control till the star looked more like a star than a pastry, centered it
up a bit, and placed the Bahtinov mask over the C8’s aperture. A Bahtinov masks
makes getting sharp images fairly easy. Just adjust focus till the spikes
around the star created by the mask are symmetrical (for want of a better
description) and you are there--or close, anyway. I removed the mask, walked over to the scope,
and entered “M013” into the HC. Rubber meets road time.
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| M57 with the ZWO with minimal processing. |
Well, my yearly 13 was in the can, but I’d only been outside
for a little while, so I thought I might play around a bit. Why not make things
even easier? I’d switch off 16-bit, which meant I could have FireCapture
debayer my images on the fly—convert them to color as they came in. The Fits
images would have to be converted from their raw form to color later. I told FC
to debayer and to save the images as .bmps. Doesn’t get much simpler than that.
Surprising thing? I really didn’t notice much image quality difference.
I left the settings as they were and did a few more bright
targets after I’d captured another sequence of M13. Next was the nearby smaller
globular M92. After that, M57 was a natural. I was much impressed by how good the
Ring looked onscreen, showing striking color and its tiny central star. The
“magnification” factor inherent in the ZWO’s small chip meant that even at
f/3.3 with the C8, the Ring was large enough to show plenty of detail.
Before pulling the big switch, I thought I’d do one slightly
dimmer target. NGC 7331, the Deer Lick Galaxy was high in the east, if in a
particularly hazy area, so there we went.
There was no trouble seeing the galaxy onscreen, though there was no
doubt the haze and light pollution were taking their toll. I could even see
hints of detail, but only hints. I could have binned the camera to increase
sensitivity, but forgot to try that. Probably wouldn’t have made a huge
difference anyway. The real problem was sky conditions.
While the galaxy didn’t look great, I was amazed to see I
was able to make out one of the Deer, one of the dim little NGC galaxies near
NGC 7331, in raw frames. In my processed images, several are apparent, showing
the ZWO doesn’t lack sensitivity. Anyhow, after getting a sufficient number of
frames, 30 (should have done more, but I was ready to cover the scope), I shut
down and headed inside.
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| Crop of ZWO NGC 7331 picture... |
The next morning it was time to see what I’d got. I’d
restrained myself from looking at my shots when I went inside—no matter how
good your pictures are, they will always look terrible the night you shoot them.
Verdict? Not bad. Surprisingly good as a matter of fact. I stacked with
Nebulosity, processed with that excellent program as well, and tweaked with
Photoshop.
Perfect? No, the backgrounds were not that even, but, no, not bad. I’ve had DSLR images that were
more difficult to process. M57 in particular was good looking when it came out
of Nebulosity 3. One real strength
of the ZWO in comparison to other budget cameras is the size of the images it
produces in unbinned mode, 1280x960, which means your finished pictures are
nice and large.
Speaking of Nebulosity, I could have operated the ZWO with
Neb instead of FireCapture. While the program does not have a built in driver
for the camera, the ASCOM driver furnished by ZWO allows the 120MC to work just
fine with Nebulosity. While Neb won’t automatically subtract dark frames, that
might be a good thing. Taking multiple darks under varying camera temperatures
might help. While the temp of the ZWO as shown by the temperature indicator in
FireCapture seemed fairly stable, lots of good darks might make a difference
when you are trying to wring every last ounce of performance out of the camera
on cold, clear winter nights.
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| ZWO vs. DSI (inset)... |
While I was quite pleased with my shots, I did think M92 was
a little thin and slightly out of focus. Since the scope was still set up in
the backyard and we were slated to have another good (relatively speaking)
evening, I thought I’d give Hercules other globular another visit. I was also
curious to see how my old Meade DSI I stacked up against the more modern ZWO.
Everything went smoothly with the ZWO, and I got an M92 that
I was pleased with. The DSI? Not so much. It worked as well as it ever did, but
what that meant was that shots under my light polluted skies on a hot night were
not so good. The DSI can do a heck of a job on a winter evening, but despite
the designers’ efforts to reduce thermal noise, there is plenty there on Gulf
Coast summer nights. There were also some odd artifacts visible. Probably due
to me not setting up the cryptic Meade Envisage software’s stack-on-the-fly
feature correctly. However you sliced it, there was no denying the 300 buck ZWO
ASI120MC blew the ten year old DSI out of the water.
Final thoughts? Other than being impressed with the ZWO and
being anxious to see what comes out of the company next (they are readying a
more advanced and cooled camera), I concluded that there is something to this
short subs business. Until I can get a camera out under better conditions, I
can’t say how much there is to it,
but I plan to do further experimentation at the club dark site and at star
parties this fall. Stay tuned.
Sunday, September 06, 2015
Are my Eyepieces Good?
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| Measuring eye relief... |
Well, are they?
Sure they are if you enjoy using them, but are they as good as they could be? Are you getting your money’s
worth? What got me started thinking about testing eyepiece quality was that I
recently completed a magazine review of a new series of oculars and had spent
several weeks putting them through their paces. Frankly, despite what the
eyepiece fanboys on the Internet forums would have you believe, it’s easy to test eyepieces, and I thought
you might like to learn how to do so if you don’t already know.
Eyepieces or “oculars” are fairly simple things in the grand
scheme. They are essentially sophisticated magnifying
glasses. Your telescope produces an image without an eyepiece, but
it is really too small to show much detail to the eye. You need a magnifying
glass to bring out details, and that is the role of the eyepiece.
Let’s get ocular vocabulary definitions out of the way
before we begin. Apparent field of view
(AFOV) is the size of the eyepiece field circle measured in angular degrees. It
is not how much area of the sky the
eyepiece takes in. That is true field of
view (TFOV), which is also expressed in degrees. Apparent field is
analogous to the size of your TV screen, while true field is the size of the
scene shown on the TV. A 12-inch TV can display a shot of the whole globe of
the Earth, but the view is much more immersive on the 60-inch. Apparent field also
determines how much true field you get; more AFOV equals more TFOV in eyepieces
of the same focal length.
The lens you look into is the Eye lens, the out-facing lens of the eyepiece. The field lens is the lens element closest to the telescope. In modern wide-field eyepiece designs there may be as many as six (or more) internal lenses between eye lens and field lens.
Eye relief is an important ocular characteristic. That is the measure, always in millimeters these days,
of how far you can hold your eye from the eye lens of an eyepiece and still see
the whole field of view circle. If you must wear glasses while observing (because
you suffer from astigmatism), you need as much eye relief as you can get.
Focal length of
an eyepiece is the number that determines its magnification. Magnification is
equal to the focal length of the telescope divided by the focal length of the eyepiece.
So, a shorter focal length ocular delivers more power. A 10mm focal length eyepiece in a
2000mm focal telescope yields a power of 200x.
One last term you will hear occasionally is “Field stop,” which is the metal ring inside (some) eyepieces
that limits the size of the field. It is analogous to the iris diaphragm in a
camera lens, but unlike a camera iris it cannot be
changed.
Now, let's have some ground rules as to what to expect. First, simpler
eyepieces, eyepieces with fewer lens elements, generally produce brighter
images. That is still true despite modern lens coatings, but not as true as it
used to be. Simpler eyepieces perform less well in fast focal ratio telescopes,
though, with “fast” beginning at about f/5.
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| Coma... |
Wide-field eyepieces tend to be harder to make well. It’s difficult
to make an eyepiece that has a wide field of view and few optical aberrations,
especially at longer focal lengths. Wide field eyepieces also tend to have shorter
eye relief. So, the best reasonably priced eyepieces are often of medium focal
length (15mm – 20mm) and possessed of moderate apparent fields (less than 70-degrees or so). If you want “long” and “wide,” expect to pay for it.
How about color rendition? Some eyepieces tend to produce warmer
tones, and some cooler. Saturn might look more yellowish in one design, and
have a slightly blue cast in another. This color cast is usually subtle and not
disturbing.
OK, let’s start testing. The first things you’ll probably
want to know are the eyepiece’s eye relief and its apparent field. The
manufacturer probably lists these things on their website, but if you’re like
me, you want to know for sure.
Eye relief is easy to determine. Hold a white card up to the
eye lens of the eyepiece (out of the telescope, natch) under scrutiny, and
point it at a distant light source. Move the card back and forth until the
image projected on the card is sharp. Now, measure the distance between card
and eye lens as precisely as you can with a ruler.
Apparent field is also easy to determine, if a bit more
work. The simplest way to figure it is to work backwards from true field. How
do you calculate the true field of your scope/eyepiece? You can calculate it
from the eyepiece field stop diameter. Divide the field stop size (in millimeters) by the
telescope's focal length and multiply the result by 57.3. Unfortunately, you
probably won’t know the field stop
diameter; most manufacturers don’t publish that figure and it can be hard to
measure. Luckily, there is another way to arrive at true field, by timing the
drift of a star near the celestial equator across the center of the eyepiece’s
field.
Choose a bright star as close to the equator as possible,
get your stopwatch (your smart-phone should have one) ready, and shut off the
scope’s drive if it has one. The only problem is positioning the star so that
it drifts precisely across the center
of the field. There’s a trick to get around that. The human eye/brain combo is
very adept at centering a bright point, like a star, in a circle, like an
eyepiece field. Center the star, let it drift, and time how long it takes for it
to leave the eyepiece. Multiply that value by two and you are done with the
field work. To reduce error, repeat your drift timing two more times and average the three resulting values.
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| TeleVue coma corrector... |
Back inside, your first task is to convert your
minutes/seconds figures to minutes and decimal minutes. 1-minute 30-seconds won’t
work in the formula we’ll be using to determine TFOV. That time has to
be expressed as 1.5-minutes. Covert to decimal minutes by dividing the seconds
figure by 60. When you have done the conversion, determine true field of view
by dividing your result by 4. For example, if the time was 7.5 minutes, the TFOV
of the eyepiece in question is 7.5/4 = 1.8-degrees.
While it’s nice to know TFOV, what we are really after is
the size of the spaceship porthole, the apparent field, the size of the field
of view circle displayed before you in angular degrees. To find that, determine
the magnification of the eyepiece under test by dividing the focal length of the
telescope by the eyepiece’s focal length. Multiply that by the TFOV and you
have AFOV. If the eyepiece has a measured TFOV of 1.7-degrees, and its
magnification in the telescope is 59x, its AFOV is 100-degrees (59 x 1.7 =
100).
It’s sometimes useful to know the size of an eyepiece/telescope
combo’s exit pupil. That is the
diameter of the light cone emerging from the eyepiece. Why would you want to know
that? If your pupils, like those of most middle-aged folks, don’t dilate more
than 5 or 6mm, an eyepiece that delivers a light beam 7mm in diameter is
wasting light (which can be the case with small short focal length scopes and
long focal length oculars). To get the exit pupil diameter, divide telescope aperture
in millimeters by magnification. For example, a 100mm aperture scope with a
10mm eyepiece gives a power of 100x. That means the exit pupil is 1mm in
diameter (100/100 = 1).
What else can you/should you check? How heavy is the thing?
If you’ve got a postal scale or similar you can find out. How are the lens
coatings? When you hold the eyepiece up to an oblique light do the coatings
(usually green these days) look even? You should see minimal internal reflections.
How about the barrel on the scope end of the ocular? Is it painted a flat, dark
shade of black on its interior to suppress reflected light? Filter threads
should be there, but be aware that since nobody has ever standardized eyepiece
filter thread pitch, not all filters will screw smoothly onto all eyepieces.
Finally, how about the physical properties of the ocular?
Does it look professionally made and feel solid? Does the eyepiece's shape get in the
way of putting your eye to the eye lens? Unfortunately, a few ultra-modern
barrel designs make that hard. Some newer Celestron eyepieces, for example,
seem designed to give you a hard time
when it comes to eye placement.
After you have determined where your eyepiece stands
vis-à-vis its specs and its physical characteristics, it’s time to check it for
aberrations.
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| "The Stig," Astigmatism... |
Coma
Nothing makes Joe and Jane Newbie
more depressed and disappointed in their eyepieces than coma, the aberration
that makes stars out toward the field edge look like fraking comets. That’s why
it’s called “coma,” natch. The irony is that eyepieces don’t typically introduce coma.
It’s an effect caused by fast telescope optical systems, mainly in Newtonians.
Coma is almost unnoticeable at f/8, visible at f/6, prominent at f/5, and disturbing
below that.
So what do you do about coma?
With one exception, an eyepiece, no matter its cost or pedigree, won’t help.
There was one eyepiece design, the
Pretoria, that corrected for coma, but it wasn’t a very good eyepiece design
and has long since been made obsolete by coma correctors like the TeleVue Paracorr.
To reduce coma in a Newtonian, you use a coma corrector which, to put it simply, is like a very special
sort of Barlow. Or you just concentrate on the field center and ignore the "comets" toward the edge. If you can’t do that and don’t want to buy a coma
corrector, use higher magnification. Higher power eyepieces and narrower field
eyepieces won’t show the hairy edge of the light cone where coma is worst and
your field will look better.
Astigmatism
Not all of the nastiness at the
field edge is caused by coma. Some may be from astigmatism, an aberration found
in many eyepieces. You’ll know you have it when you plug in the coma corrector
and the stars at the edge of the field of your Acme 100-degree Wonder still look like hell. Now, astigmatism
can be in your eye, in the telescope, or in the eyepiece, so how do you know if
it’s the eyepiece or not? A tip-off is that in oculars it’s sorta like coma;
stars at the center are good, but those at the edge are distorted things that
look like lines, seagulls, or even crosses.
What do you do? What do you do? There’s not much you can do, really, except buy a better
eyepiece. TeleVue makes a corrector lens, the Dioptrix, for astigmatism, but
that is to correct astigmatism in your eyes, not the eyepiece. If the stars at
the center are round, even when thrown slightly out of focus, the problem is
likely the eyepiece. If the elongated axis of the star changes its position
angle, its orientation as you move your head around the eyepiece, when you change the
angle of your head in relation to the eyepiece, the problem is (or is in part)
in your eyes. Keep in mind that there can be a combination of the three causes,
astigmatism in eye, eyepiece, and telescope. Luckily, most people find
astigmatism in eyepieces less disturbing than telescope coma.
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| Curved field... |
Kidney Beaning
You are happily enjoying your
view of NGC Umptysquat when suddenly the field blacks out. It’s like what your
old Admiral portable TV from the 1970s did when it went on the fritz. Maybe the
field doesn’t completely black out. Perhaps what you see are dark patches in
the eyepiece, oval, bean-shaped
patches.
The cause of kidney-beaning is
improper eye placement. Get your eye off the optical axis and you will see
these effects with many eyepieces, even rather expensive ones. Cheer up. After
some practice with a new eyepiece, you’ll probably find it easy to keep your eye
centered. The TeleVue 35mm Panoptic is a wonderful ocular, but when I first got
mine I had kidney-beaning out the ying-yang. Wasn’t long before I got used to
centering my eye on the new eyepiece, though and all was well. Be aware
kidney-beaning is common with longer focal length oculars. In fact it is almost
inevitable with them.
Curved Field
If the stars in the middle of the
field are in sharp focus, but those at the edge are slightly fuzzy (but not distorted)
you are suffering from field curvature. Many if not most scopes, and
particularly faster refractors and non-corrected (non Edge design) SCTs, have
curved focal planes. But eyepieces can contribute to the problem. How can you
tell if the problem is in your eyepiece? The easiest way is to check it in a
longer focal length telescope. If the problem is mostly the scope (which it
usually is) field flatteners are available for both refractors and SCTs, the scopes
that generally need them most.
Lateral Color
What’s troubling you, Bunky? Not
only do you have comet shaped stars at the field edge, they have blue and/or
red fringes? Most modern eyepiece are well color corrected for objects near the
center of the field, but some—especially longer focal length and wider field
oculars—are not so good at the field edge.
What can you do? Mainly, position
your eye properly. Having your eye off the eyepiece’s optical differential refraction. If you are using an achromatic refractor, check
the eyepiece in a known color free telescope like a Newtonian. What can you do
about lateral color? Stop obsessing about the field edge or open your wallet is
about it.
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| Lateral color... |
Pincushion and Barrel Distortion
In Pincushion distortion, parallel lines are bowed in as they cross the center of the eyepiece. Barrel distortion is the opposite; they are bowed outward. You can check for these problems by viewing distant boards on a fence or on the side of a neighbor's house during the day.
What can you do about these things? Get another eyepiece. But keep in mind that you probably won't even notice barrel or pincushion distortion during normal observing. It tends to be obvious only when you are looking through the eyepiece while slewing the scope fairly rapidly for a good distance.
Sharpness and Contrast
Now we come to the subjective
stuff. How good is this hunk o’ glass really? How good does stuff look in my eyepiece? After a few years in the
game, you’ll come to know what a good image is. If you are a novice, you can
learn by looking through your friends’ expensive eyepieces. Yes, that Ethos
will probably ruin your Orion MegaDoodle ocular for you, but you need to know
what perfection is. Even if you decide you don’t need (or want to pay for)
it.
I know what a perfect image is,
but if you are a long-time reader here you know I absolutely love my humble
Happy Hand Grenade, a 100-degree Zhumell eyepiece I bought when it was one of
the few 100s that didn’t cost close to a grand. Not perfect, but I enjoy using
it, I can overlook its imperfections (it probably suffers from every one of the
above problem to one degree or another), and it makes me happy in spite of them. That’s the real task when it comes to
choosing eyepieces, deciding what can make you happy. Do that and you are
golden, Ethoses or no.
