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.

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.

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.