Sunday, November 20, 2016

 

Issue #518: How Simple Can It Get?


AR102 and Atlas...
More than a few new amateurs want to take pictures of the night sky. Specifically, they long to take images of deep sky objects, galaxies, nebulae, and star clusters, through the telescope. The time-honored advice given these people is “Start simply. Take star trails photos with your camera on a tripod and move on up to piggyback imaging. Through the telescope? It’s expensive and there is just so much to go wrong. Your backyard isn’t good enough to let you get much of anything anyway. You don’t want to spend all that money and time and have nothing to show for it, do you?”  But is that the correct advice?

I think the first part of the above is valid. Beginning with star trails and then piggybacking the camera, mounting it on the tube of a telescope so it can take advantage of the mount’s tracking while shooting through its own lens, is still the way to go. If nothing else, it gets the novice acquainted with focusing on the sky, operating the camera for long exposure work, and the realities of setting up to do any kind of astrophotography. The rest however? Those cautions about how hard and expensive it is to shoot through the scope and that you can’t do any deep sky work from the average suburban backyard? I set out to prove that wrong just the other night.

Not that I was completely sure I would prove that hoary advice wrong. Especially since my other goal was keeping the cost of gear down just as low as it could possibly be, radically low, and making set up and image acquisition as simple as possible.

First thing any deep sky imager needs is a decent tracking (equatorial) mount. Most of us, however, whether beginning or way advanced, don’t need a 10-thousand-dollar rig. That’s because most of us don’t have the dark and constantly clear skies that justify such an expense. Most of us don’t envisage doing 8-hour LRGB exposures anyway. We just want nice snapshots of the prettier objects to share with our friends and families. If that describes you, you can get a perfectly adequate (used) German equatorial mount (GEM) for 300 – 600 dollars.

What I chose to use for this test was my nine-year-old Synta-made Atlas mount. It is not fancy, but offers decent goto, excellent stability, and (unguided) tracking quality good enough for the relatively short exposures and focal lengths most will want to mess with in the beginning.  A used Atlas (a.k.a. “EQ-6”) can be had for as little as 500 – 800 dollars (for the goto version; avoid the old non-goto variant).

Unfortunately, EQ-6 owners tend to hold onto their mounts, so these GEMs are not as common on the used market as you’d expect given their numbers and the many years they’ve been in production. 500 – 800 might also be a bit much for a novice.

80mm APO on a VX mount...
Another good alternative is a Celestron CG5 goto mount or a Meade LXD-75. These are plentiful used and can be had for 400 dollars or even less. They won’t track quite as well as the Atlas, but they will be good enough for beginners using short, fast (low focal ratio) telescopes, and have the advantage of being much lighter than the Atlas. How about similar non-goto CG5 class mounts? Only resort to one of them if you have no choice. Computerized pointing is a huge help in imaging. Do you really want to spend half your time just getting a target in the frame of your camera?

Now for the telescope. To speak plainly, get a refractor. Yes, I’ve had a long running love affair with Schmidt Cassegrains, but I can think of no more difficult scope with which to begin astrophotography. Even when equipped with focal reducers, their focal lengths are long—meaning tracking is critical and it’s difficult to produce images with round stars—and their moving mirror focusing arrangement is a pain for imaging. That alone can cause trailed stars.

I will admit it is possible to get started using an SCT. I went from fooling around with Newtonians to taking my first successful deep sky photos with an SCT and a film SLR. Course, you really had to want those pictures. You had to focus with a dim SLR viewfinder, guide by hand, and it was never certain whether you got anything until you developed the negatives. It seemed worth the pain to me those long years ago, but even when I was younger and more patient, it wasn't exactly "fun."  

Today, lazy and ornery as I am (my friends have taken to referring to me as The Honey Badger), my least favorite thing in the world is taking long exposure pictures with an SCT. Get a refractor. Specifically, an 80mm to 100mm refractor with a focal ratio of f/5 to f/7 or a bit more. Of late, my 80mm refractor has become my most oft used telescope for imaging.

But exactly what sort of refractor? In order to keep the price of the telescope down, naturally you’ll be buying a Chinese scope. Possibly a used one. What would be ideal? An 80mm ED model. The “ED” business means the false color, the purple fringing around bright objects, that is a characteristic of non-ED (achromatic) refracting telescopes will be low.

An 80 ED can be an incredibly powerful tool for astrophotography, allowing you to take wide-field shots of even very dim objects. What matters for extended objects like nebulae and galaxies when imaging is not aperture, but f-ratio. The lower it is, the less exposure time you will need, the deeper you can go, and the wider your field of view will be. How much money are we talking? Explore Scientific will sell you a nice 80mm triplet ED/APO telescope for just a little over 500 dollars.

You’ve just bought a mount, though, and 500 new or used sounds like a lot. Can you go cheaper? You can:  with an achromat. Yeah, I hear the veteran astrophotographers howling: “Rod, how can you recommend an achro? Especially a medium-fast achromat? You can’t take pictures with one. There’ll be horrible purple halos around even dimmer stars.”

Yes, I know there will be the dread color purple. But I also know 100mm achros are dirt cheap right now. The above mentioned Explore Scientific offers the very fine 4-inch f/6.5 AR102 for as little as 300 dollars on sale. Almost everything you need is in the box, including a decent finder and an excellent star diagonal. Despite the conventional wisdom, I decided to see whether one of these scopes—which is superior to an 80mm ED for visual use—could deliver pictures that would please a newbie, at least.

Of course, you’ll need a camera. If you’ve got a DSLR of any brand, use that. If you don’t, there is but one choice for the dollar-conscious newbie:  a Canon Digital Rebel. They aren’t expensive new (see the website of my fave dealer, B&H), and are dirt cheap used. Only caveat? Don’t go too old. Try to at least get the Rebel Xti. One of these classic Rebels still has more than enough features and capabilities for any beginner. My Xti is nearly a decade old, and I still use it for astro-imaging—frequently. While its top ISO (sensitivity) is 1600, its relatively large pixels mean it is quite sensitive.

To mount the camera in the scope’s focuser you’ll need a (2-inch if possible) prime focus adapter, available for a few bucks from most astro-dealers or from B&H. The DSLR is attached to that prime focus adapter using a T-ring, available for your camera brand from the same sources.

Do you need a computer? You will for image processing, and one can make focusing and image acquisition easier in the field (I use the wonderful program Nebulosity to control my DSLRs during picture taking), but you don’t need one. A simple and inexpensive remote shutter release for your camera, an “intervalometer” will do.

So, into the backyard. While the Atlas’ GEM head is heavy, it’s actually somewhat less awkward for me to lift onto the tripod than my CGEM for some reason. Once I had it on the tripod with the counterweight on the counterweight shaft, it was pretty simple to finish the setup:  mount the scope, in this case the AR102, and balance it so it was slightly east-heavy (to keep the RA gears meshed). That only required one 11-pound Synta pancake weight halfway up the declination shaft. Plug in power (an AC adapter I got from Orion) and the hand control and I was done with the preliminaries. And, naturally, right after that, the clouds came.

Before the evening was over, I was able to get a few cloud free minutes, however. Enough to allow me to polar align the mount and check it out (I hadn’t used the Atlas since the 2015 Peach State Star Gaze).  To polar align, I follow a two-part procedure. The first part uses the mount’s built in polar alignment borescope.

M15 before processing for chromatic aberration...
First, I rotate the mount in RA until the little circle on the polar borescope reticle where Polaris goes is on the bottom, and set the RA setting circle to “0”. I then turn on the mount and after I enter time/date/location it gives me Polaris’ current hour angle. I rotate the mount in RA until that “time” is under the RA circle’s pointer. With the little circle where it should be, I move Polaris into it using the mount’s altitude and azimuth adjusters (only).

The above will generally give a good enough polar alignment to allow reasonable length—two or three minute—sub exposures on the camera. It’s easy enough to tighten the alignment up a bit if desired, though, with part two of the process, using the built in polar alignment in the hand control. To do that, I complete a three-star goto alignment with the mount and then select Polar Alignment from the setup menu. From there, the process is nearly the same as the AllStar polar alignment used in Synta’s Celestron branded mounts.

To do a polar alignment with the hand control, I choose a bright star (one due south is best), slew to it, and begin the polar alignment routine. The hand control instructs me to center the star in the eyepiece, and then slews away from it. I use the altitude adjuster on the mount to get the star as close to the center of the field as I can get it. After I press Enter, the mount slews again, and I re-center the star using the azimuth adjusters on the mount. When that’s done the process is complete. The manual warns you may want to redo the goto alignment after a polar alignment, but I usually find that unnecessary.

All done, I did a few gotos to see how the mount was performing. It’s no secret the pointing accuracy of the Atlas is not nearly as good as that of the CGEM, with its famous 2 + 4 star alignment, but the Atlas’ three-star alignment is usually quite good enough with a widefield refractor onboard.

Anyhow, anything I requested from one side of the sky to the other was always somewhere in the field of my 8mm Ethos ocular (83x). The Atlas would not be my choice for video astronomy, where I might want to go to 20 or 30 targets over the course of an evening, and where I’d need the mount to put those targets on the small chip of a video camera, but the Atlas with its SynScan goto system is more than sufficient for visual use or for going to a couple of astrophoto targets a night.

Just after the mount centered the Dumbbell Nebula dead center in the field of my 25mm Bresser eyepiece (we’ll address the current crop of wide field bargain basement eyepieces like the Bresser some Sunday soon), the clouds poured in again and M27 faded out. I threw the big switch, covered the scope, and repaired to the den for some Agents of Shield action.

After...
While we had plenty of clouds for several days, we didn’t have a drop of rain—this has been one of the driest falls I can remember—so I was able to leave the Atlas and the AR102 set up in our secure backyard covered by one of the excellent Telegizmos scope covers. Finally, last Friday evening, the clouds departed and I was able to get started.

While I purposefully kept things as simple as possible, not even controlling the mount with the laptop, I did use Nebulosity for image acquisition. With the way my eyes are in these latter days, I simply find it too difficult to focus on a DSLR’s small display, even with zoom enabled.

Using Nebulosity, I can focus with the 17.3-inch screen of my Toshiba, and, using the program’s fine-focus mode, get images as sharp as possible. I thought that would be critical when using the AR102, as any misfocus would make chromatic aberration all the worse. I focused on Vega and the dimmer stars in its field, and when done sent the scope to my first target, M27, using the SynScan HC, which put the nebula almost in the center of the frame.

Let me pay Explore Scientific a big complement right here. The Crayford style focuser on the AR102 proved to be just about perfect. Not only did it have plenty of range, more than enough to focus the the Canon and the (excellent) Hotech field-flattener I used in lieu of a prime focus adapter (couldn’t find my plain prime focus adapter anywhere), its fine focus control made dialing in exact focus a joy. The draw-tube never slipped or threatened to with the Xti onboard, even when I pointed at M15, which was riding high.

In order to eliminate the necessity of guiding, I set the camera’s sensitivity to the maximum, ASA 1600, and limited my exposures to 30-seconds. That resulted in perfectly round stars in almost all my frames and had the benefit of keeping the background reasonably dark given my somewhat bright skies. Despite the typically bright suburban skies, it was apparent sky darkness was good enough to allow even a novice without a lot of image processing experience to get plenty of good stuff.

Alrighty, then. I told Nebulosity to give me 30 30-second sub-frames, and it began clicking them off. How was the chromatic aberration? Oh, the brighter stars definitely had purple haloes. I didn’t worry about that, and didn’t add any kind of filter to the imaging train. I’d decided that in the interests of simplicity I’d do any "filtering" after the fact, during post processing. I am also of the opinion that deep sky results are usually better if you don’t use filters of any kind during exposures. I wandered back inside to watch TV. The mount was tracking well, and Nebulosity was doing its thing without a hitch, so there was no need for me to stay outside kibitzing.

When the M27 sequence was done, I used the HC to go to M15, the great globular cluster in Pegasus. There is a magnitude 6 star in the field with M15, and I figured that would provide a good test of my ability to suppress chromatic aberration during image processing. Indeed, I could see the star had a pretty extensive bright purple halo even in the short subs. Again, I didn’t worry, just let the mount, scope, and camera do their thing.

M27
I was smart enough not to examine the M27 and M15 sub-frames after the last target, M15, was done. Pictures always look much better in the morning. I just shut down, covered the scope, and hauled the laptop inside. Despite not examining the subs, I was pretty sure  what I had gotten, and gotten so easily and simply, would have more than thrilled me when I was a novice.

Next morning, I set about to process my pictures, beginning by stacking the sub-frames into single images of M15 and M27 using Nebulosity’s built in image stacking routine (best in the business in my opinion). When I was done, I was not surprised to see that the brighter stars were really purple, but, again, I did not panic.

There are various ways to remove the purple halos of chromatic aberration in post processing. In the interests of simplicity, I decided to do as little as possible. There are ways to reduce not only the color purple, but the sizes of the haloes around the stars using Photoshop. I’ve experimented with that in the past with a friend’s achromatic images. Photoshop is expensive, however, and the procedure not overly simple for a novice. Instead, I used the built-in routine in another Adobe program, Lightroom.

The advantages of using Lightroom is that it is relatively inexpensive and does a lot, even including a built-in routine to remove that nasty purple. All that is required is to move a couple of sliders and you are done. True, the haloes remain, but they are no longer purple and are much less intrusive. Again, there are ways to reduce the size of the halos and the star disks themselves, and if you out there in blog-land have a good (and simple) method of doing that, I’d love to hear about it.

And that was that. Well, except for a little level-adjusting and some minor sharpening on M15. My resulting images are not masterpieces, but they certainly blow away many of the astrophotos I took in the film era. As above, I know, know, I’d have been thrilled to get these results when I was wet-behind-the-ears. I’d have been thrilled to get deep sky pictures so easily.

What’s next? While these pictures look pretty good, they could have used a little more exposure, so why don’t we talk about the art of autoguiding, autoguiding simply some Sunday soon?  For now? Why don’t y’all get outside and see what you can get of the deep sky without a lot of effort?

Comments:
Great article Uncle Rod. After a couple year hiatus, I'm finally getting the itch to capture more photons.
One thing that I keep messing up is with balancing. I do it after scope set up, but I always forget to consider the weight of the cameras... (I'm going to claim age on this one...lol )
Clear dark skies to you my friend!
 
Perfect timing for this great entry! A 450D finally showed up on BH used, so I grabbed it and then ordered an AVX GEM and that Explore Scientific 102ED triplet. Was going to get the 80, but found out that its weird shoe doesn't like the AVX saddle. An ADM 14 universal can get it to mate...but why bother, when that 102ED is only 750 bucks right now. And....I start progre$$ing down the slippery slope of AP!
 
Great write up on "simple" astrophotography for beginners Uncle Rod! Your images of M27 & M15 are fantastic. Maybe in the near future you can do a write up on solar/eclipse photography. With the big solar eclipse coming nest August, Im sure there is a lot of interest in that area of endeavor.
 
To make things even simpiler check out Astro Toaster, freeware that stacks images live using DSS. You can observe the object and, if you wish, save a stacked image for later processing. It works amazingly well for casual imaging.....Dwight
 
You know, Dwight, I tried that, but didn't find it overly simple in the field, nor was I impressed by the quality of the final images as compared to Nebulosity.
 
Rod, do you downsample your images in order to conceal imperfections at the pixel level? I do. The pixels on modern cameras are so small that they reveal very tiny optical and guiding problems. I find that if I downsample so that the resulting scale is about 3" to 5" per pixel, things look a lot better.
 
HI Michael:

No, I never do that. I always find results better if I deal with small imperfections with techniques like applying a mild Gaussian blur, noise reduction, etc.
 
Can you give us some thoughts on non-Photoshop (pricey), Nikon-compatible processing? Would Photoshop Elements work? Is Lightroom adequate, despite its (I think) lack of layers? This is somewhat confusing prior to having experience. Thx.

 
The main reason to have Photoshop (or Elements), for me, anyway, is to be able to use plug-ins like Gradient Xterminator. I rarely mess with layers, and find Lightroom more than adequate for my simple efforts. :)
 
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