Sunday, November 27, 2016
Issue #519: How Simple Can It Get II: Kicking it Up a Notch
You’ve accumulated some hours in the backyard imaging with a simple achromat and an Atlas/Sirius/CG5/LXD75/AVX/Bresser Exos-2 class mount. You’re pretty pleased with the results you’ve gotten, and as your
processing skills have advanced, you’ve
been known to mutter, “Hmm, not bad, not bad at all” when looking at your latest batch of astro-snaps. You’re beginning to
think you might be ready to take astrophotography to that vaunted next level, and your
questions now are “Should I?” and “How?”
As for the
first question, “Should I?” the answer, as it so often is in astrophotography
and amateur astronomy in general, is “It depends.” If you are planning on
continuing a program of imaging relatively bright objects from the backyard, a higher quality scope and guided imaging can make some difference.
However, as you can see in the shots below, it is not like night and day.
Look at M27.
The top picture is our original unguided stack of 20 30-second exposures taken
with the AR-102 achromat. On the bottom is a stack of 20 3-minute subs done
with my William Optics 80mm Megrez II APO. The APO shot is better, but it sure ain’t like night and day. The colors are somewhat more saturated, there is less noise, there is a little (but not a lot) more detail
in the nebulosity, and, the main thing, the bright stars are smaller and have
no halos. There is even less difference when comparing the APO/Achro M15 pix. These pictures were taken under marginal conditions, but still represent, in my experience, what you can expect from the backyard when you kick things up that notch.
Could I have
made the difference larger by exposing for longer with the APO? Yeah, if I
could have done that. In my backyard, especially in the presence of the not uncommon haze and
humidity which amplify my light pollution, I can't go any longer than 3-minutes, really.
As you can see in the unprocessed frame, the sky background was already extremely
bright at a modest 120-seconds. I could have used a mild light pollution reduction filter to tone it down, but that
brings its own problems.
Verdict? If you
plan on continuing to do almost all your work from your less than perfect backyard, focusing on the more prominent objects, and the
look of brighter stars in the achromat shots doesn’t annoy you, stick with that achromatic refractor and short, unguided exposures. If nothing else, completing a major project with a simple setup, maybe like imaging the entire Messier catalog from your back forty, will prepare you to take full advantage of more complex rigs if/when you decide to move up to that next level. Your wallet will certainly thank you for sticking with that humble AR102.
Still, there are reasons to think about upgrading to an apochromatic refractor. There’s no denying an ED APO scope is a more versatile scope. One is, for example, more suitable for viewing the Moon and planets—not that the Moon and planets can’t look good in a 4-inch achromat. The main reason for you to switch to an ED scope, however, is if you want to go deeper and intend to do at least some of your imaging from dark sites.
Still, there are reasons to think about upgrading to an apochromatic refractor. There’s no denying an ED APO scope is a more versatile scope. One is, for example, more suitable for viewing the Moon and planets—not that the Moon and planets can’t look good in a 4-inch achromat. The main reason for you to switch to an ED scope, however, is if you want to go deeper and intend to do at least some of your imaging from dark sites.
The longer exposures possible from a better observing location buy a lot. Some time back, I did M33, the
Triangulum Galaxy. First night out I was tired and didn't want to stay on the observing field long and, so, stuck to short (2-minute)
subframes. Looking at the raw images the next a.m., I determined I needed considerably more data, more exposure, to the tune
of 3 – 5 minute subframes to make the galaxy really pop. When you are out in
the dark you can do that, expose long enough to bring out faint details without
the crazy bright background of backyard shots making that a losing battle.
OK, so if
you want to take it to the next step, either because you’re going to start
imaging at a dark site or because you just want a telescope that will do more
things well than an achromat can, step one is getting an ED refractor. My
choices, being cheap as I am, are the Explore Scientific triplets or the SkyWatcher Pro ED doublets. While the three element objectives of the ES scopes should theoretically put them ahead of the game in color correction, it’s really a wash when comparing
them to the SkyWatchers. Unlike the ESes, SkyWatcher's two element objectives
contain one lens made of FPL-53 synthetic fluorite, which makes up for the lack
of a third lens element. Either a SkyWatcher or an ES is a great and economical
choice whether you choose 3, 4, or 5-inches of aperture. I own the SkyWatcher Pro ED 120, which is a great scope, but I could be just as happy with the Explore (Triplet Essential) 127.
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| Achro top, APO bottom... |
Get that new
scope, have fun seeing what an essentially color free refractor can do, and and when you're ready try some longer exposure imaging with it. What you’ll quickly find is that once you
get much the 30-second - 1-minute level, you likely won’t have perfectly round stars with the class of
mounts we are using. If you want to go longer, you will need to guide. You’ll need a second camera that
monitors the position of a “guide” star and issues corrections to the mount to
keep that star centered. To do auto-guiding, you will need three things: a guide telescope, a guide camera, and
software to make it all work.
Guide Camera
Any camera,
still or video, capable of sending images to a computer over a USB connection
is capable of working as a guide camera. However, for best results you’ll want
one that is sensitive and delivers monochrome images. The reason you need
sensitivity is clear: you want to always
be able to find a guide star in the field of your target. A star that is good
enough in the signal to noise ratio department to allow your guide software to
stay locked onto it. The reason to pick a monochrome rather than color camera
is that monochrome cams tend to be more sensitive and also less noisy.
So, which
one? One of the best guide cameras in the business is Starlight Xpress’
Lodestar. Unfortunately, it’s not just a great guide cam; it’s a fairly
expensive one at 650 dollars. At the other end of the price scale is Orion’s
StarShoot Autoguider at about 250 bucks. The Orion works—I used one for
years—but there is no denying it could be more sensitive. Also, while
most guide cams can be used as imaging cameras as well as guiding cameras—many
can do a good job on the planets or even the deep sky within reason—Orion’s
StarShoot is really for guiding only. It can be made to deliver images with special software, but
they are not very good.
So which one?
After agonizing over the guide cam question for a long time after I decided to
replace my Orion, I settled on a QHY 5L-IIM. It is the same price as the
StarShoot (actually, the StarShoot is a rebadged, earlier model QHY camera),
but is far more sensitive and is an impressive planetary imager, too. It’s
small, it’s cute, and it is oh-so-sensitive. In my years of using the
StarShoot, I never landed on a field where there wasn’t a single star I could use for
guiding. Frequently, however, there were only two or three even marginally usable stars in the frame, and seeing them took 3-second or even longer guide camera exposures.
The typical QHY 5L-II field is filled with dozens of good guide stars in short exposures.
How long
your guide cam needs to expose to deliver a suitable star is important because
of the inexpensive mounts we are using. If you are forced to use three second or longer exposures, the mount’s periodic error over those three or more seconds may cause your
stars to trail slightly. With the QHY I can always get by with one to
1.5-second guide exposures.
 |
| Achro top, APO bottom... |
Guide Scope
The guide
camera needs a telescope to look through. That can be the imaging telescope if
you use a device called an “off-axis-guider,” which diverts a small amount of
the light from the main scope to the guide camera. An “OAG” is difficult to
use, however, and unless you are attempting to image at focal lengths above
about 1300mm, especially with an SCT, it is a tool you’ll want to leave for
later. Instead, use a guide scope, a small telescope piggybacked on the main
instrument.
That guide scope
can be any sort of telescope (excluding a CAT with moving mirror focusing).
80mm achromatic refractors like the Synta Short Tube 80s are often used. The 80 f/5 can indeed
work well if it is securely mounted. If it is not securely mounted, if its mounting flexes as the telescope changes attitude, etc., stars will trail
no matter how good the auto-guiding. Mount that sucker as sturdily as possible
using high quality solutions from Losmandy or ADM. Or, if your imaging scope is
less than 1000mm in focal length or so, think about a 50mm finder-guider.
A
finder-guider is basically a 50mm finder scope that has been modified to accept
a guide camera with a 1.5-inch nose-piece instead of an eyepiece. The advantage
to the finder-guiders is that they are light and are securely mounted in the
average 50mm finder mount and not likely to flex. With a QHY or similarly
sensitive camera, one will pick up plenty of guide stars across a wide field. I
have even used one semi-successfully with my 8-inch SCT reduced to f/7.
Software
There are
numerous guiding packages available, but what is most everybody using? PHD Guiding. Talking the ins and outs of setting it up
and using it is the subject for an entire article, which I did a couple of years ago. I will say, though, that the latest iteration
of the program, PHD2, is almost plug
and play. You will likely get good, if not necessarily perfect, results just using
the defaults. Anyhow, start with PHD2 if for no other reason than that so many
people are using it that there are oodles of tutorials on how best to adjust
its somewhat bewildering array of settings.
Hooking Up
Like working
with PHD, setting all the gear up for auto-guiding is a subject for an entire article (here). Basically, though, what you will
do is mount guide scope and guide camera on the main scope and plug in two
cables. Assuming your mount has an auto-guide input, you’ll run the (included
with the QHY) RJ type ST-4 guide cable from the RJ plug on the camera to the RJ
plug (the auto-guide port) on your mount. Then, connect a suitable USB cable
from the camera to the computer. What if your mount (like the LXD75) does not
have a guide input? You can still guide using the mount’s serial port. See the
above article for details.
Finally,
start PHD and begin taking frames with the guide camera. Follow the instructions
that came with the guide scope to achieve initial focus. Getting the guide
scope in decent focus is critical for good guiding performance. Some gurus will
tell you that being just ever so slightly out of focus yields better guiding,
but you still need to be close to focus for good results. One tip? Clicking on
a star on the PHD video display will give the current signal to noise ratio.
Adjust focus on the guide scope until that number is as high as you can get it.
When you are done, click on a bright (but not saturated) star, and click the
bullseye reticle icon. PHD will then “calibrate,” move your mount in the
cardinal directions to get a feel of how it responds, and will begin guiding.
 |
| Before processing... |
From there? Take pictures just like you did in the 30-second days, only with longer durations,
maybe beginning with 1 – 2 minutes. Another tip? As I hinted at last time, use
Nebulosity to control your Canon camera. It makes everything so much easier. Be
aware that to use longer exposures with Nebulosity and the early Canons like
the Rebel Xti, you’ll need to connect a shutter interface box between the
camera and the laptop. Those are readily available from Shoestring Astronomy.
Processing
It’s
morning. The birds are chirping, the sun is shining, and you are ready to see
what those hard won long exposure images look like. Process them the same basic
way you did your 30-second shots. Stack them using Nebulosity’s built in
stacker or the freeware Deep Sky Stacker. Is there anything you will have to do differently
when processing longer exposures? In the backyard, the sky background will be considerably brighter, so you’ll have to deal with that using your processing program’s histogram
adjustments. You may also have some light pollution gradients. These are the effects of the bright backyard sky and will cause some areas of the image background to be brighter than others. One typical effect of this is vignetting.
Vignetting is what I call “the porthole effect.” The center of the image is brighter than the edges. It’s like you are looking through a round
porthole at your object, and will limit how much you can brighten the target.
There are two ways to deal with that, the hard way and the easy way. If you
want to go hard, take flat-field frames: illuminated, evenly illuminated, shots
of the twilight sky or a white card or through a translucent mask. Apply those flats to your images
(with Nebulosity or your astrophoto processing program of choice). Or, if you
are lazy like me, you can take the easy way out and use a software tool called Gradient Xterminator.
Gradient
Xterminator is a simple plug-in for Adobe Photoshop that will virtually eliminate any light pollution gradients. It is extremely simple to use, and the only real
“problem” is that you’ll need Adobe Photoshop to use it (it will also work with
some versions of Photoshop Elements). Adobe Photoshop is something you probably want anyway as you grow as an
imager, and there are options today for getting it that aren’t quite as painful
on the pocketbook as in the past.
In addition
to light pollution gradients, the sky background in light polluted areas,
especially if there was haze present during the exposure, may be badly discolored, brownish or even red as
in the example here. It will be even worse if you, like me on the night I shot these pix, allow a little dew to
accumulate on the objective without noticing it (I was inside
watching TV while Nebulosity took my pictures). The easiest way to fix this yuckiness is with the background color offset tool in Nebulosity. That
turns a pain into a pleasure when it comes to getting the sky the correct hue.
And, well, you know what? That is about it. Going
from unguided imaging to auto-guiding is quite a leap, but it is the biggest
leap you will encounter in astrophotography. Everything else is incremental
improvements: better mounts, cooled CCD
cameras, imaging through filters with a monochrome camera, etc., etc. Once you
have mastered setting up for and doing guided photography with your simple rig,
you have conquered 90% of the astrophotography learning curve, and can now, I
hope, actually start having fun.
Comments:
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I came really close to getting a new mount to replace the LXD75, but stacking 30-60 second stuff works for me in my yard - I only do visual when I am at a dark site a few times a year. I know realize that for the 5 to 7 times a year I image from my yard the LXD75 and my FR C8 is really all I need. I picked up a Panasonic Lecia 100-400 zoom this year and am amazed at how well it works on one of my M43 cameras which can build a live view image. It is perfect for the M stuff and I found that with the C8 + FR I got 7331 and the rest of the group. Before I started this type of shooting I was even thinking of getting ride of my scopes. I don't want the hassle of setting up - I use the built-in intervelometer and can take enough shots (99 on one camera and whatever the battery will do on the other). I often think about gettting a guider (I got the 909 clone for the LXD75), but I know "keep it simple stupid" is what works for me. A properly cooled and with proper maintenance (bob's Knobs)works well for 30 second exposures. Like every things its what works for you...in this for me its keeping it very simple.
Great article...and exactly the path I'm on. Going to start short exposures unguided to learn my equipment and the software.
Question: for my ES102ed, should I get a Flattener/Reducer right away? Pretty much just doing DSOs, no planets.
Question: for my ES102ed, should I get a Flattener/Reducer right away? Pretty much just doing DSOs, no planets.
Hi Jack:
It depends on how large a chip your camera has. Try it without the flattener and see what you thing? Me? I use the Hotech flattener even with my 5-inch f/7.5 APO. ;)
It depends on how large a chip your camera has. Try it without the flattener and see what you thing? Me? I use the Hotech flattener even with my 5-inch f/7.5 APO. ;)
NB: macOS Sierra is (temporarily) incompatible with the Canon ESDK use by Nebulosity to control Canon cameras.
Thanks for the article, very useful for anyone interested in astrophotography. Just one question, How do you do the polar alignment, ASPA, drift alignment, Polemster?
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