Sunday, March 19, 2017
Issue #535: The Final Piece of the Puzzle
In our pre-spring observing season drive to get novices (and maybe even a few not-so-novices) set up with a rig for deep sky imaging, we’ve addressed mounts, telescopes, and, last week, auto-guiding setups. This Sunday we’ll finish with suggestions for a low-cost camera. I’ve talked about imaging cameras with y’all fairly recently, but the difference is that this time I’ll try as hard as I can to keep the cost as low as possible.
So, you need a camera and a few accessories. Where do you start? The first question to answer is, “Do I want color?” While a monochrome CCD/CMOS astronomical camera can take color images by exposing successive frames through three or more colored filters, it’s not something you want to face when you are just getting off the ground in imaging. Unless you enter the ranks of the hard-core someday, you may never want to face it. In the beginning you will find just processing a “one-shot” color image enough of a challenge. Properly calibrating and combining three + separate frames into a color frame and then stacking and processing a bunch of those? Uh-uh.
So, it’s a color camera, a one-shot color camera, you want. How does one work? A color camera is different from a monochrome camera in that red, green, and blue color filters are built into the sensor chip. Software, either in the camera or in an image processing program, automatically combines the R, G, and B to produce a full color image. That is usually transparent to the user—with a digital single lens reflex (DSLR), anyway. You take a picture, you see a color image, end of story.
Some astrophotographers say a monochrome camera can produce visibly higher resolution images because it doesn’t waste pixels on the production of a color image. In truth, in the beginning at least, and especially on deep sky objects, you won’t notice any difference.
The next question is “CCD or CMOS?” That is not much of a question today. Unless you are interested in some special applications, mostly having to do with obtaining scientific data, there is no reason to choose a CCD chip over a CMOS chip. Today, the formerly preferred CCD has lost ground to CMOS sensors even for use in “astronomical” cameras. CMOS chips are now very sensitive and very low in noise. At any rate, almost all cameras in our price range, which I am topping out at 450 dollars, have CMOS chips, so the choice has already been made for you.
|What a ZWO ASI120MC can shoot...|
Next up, cooling. “Does a camera for taking long-exposure images need to have its sensor chilled to reduce thermal noise?” Today, probably not. With some camera/chip combos, an internal fan, at least, can be helpful to reduce the false stars of thermal noise, but the low-noise characteristics of today’s sensors usually means subtracting a dark frame is enough to deal with thermal noise.
And the Final Jeopardy Question… “Astro cam or DSLR?” There are some interesting low cost astronomical cameras coming on line, like those from China’s ZWO, and I’ve actually taken credible deep sky image with one of their 1/3-inch cameras that cost a measly 200 dollars. However, I think for most of us a DSLR is just a much more sensible choice. A much more sensible choice.
Why is a DSLR better? There are several reasons, but there is one real big one: when you’re not taking pictures of the night sky, you can be wowing everybody at your mother-in-law Margie’s birthday party with your snapshotting skills. There’s also that big elephant in the living room. Like many wannabe astrophotographers, a few nights wrestling with camera and scope may convince you you are actually more of a visual observer. If that be the case, you can still get years of use and enjoyment out of the DSLR, even if you never take another astrophoto with it.
Another big plus (for astro imaging) of the DSLR? Their relatively big chips. A less than 500 dollar camera will have an APS-C size chip. Lower cost astro-cams tend to have small chips that restrict your field of view, focal length for focal length, and also tend to make guiding more critical.
Finally, while I control my DSLRs with a program running on a laptop (“tether them,” as we say in the photography business), which makes focusing and framing much easier, you don’t have to do that. You don’t have to have a computer out in the field when you are taking pictures. You can do just as we did in the SLR days: telescope, mount, camera. You will, as in those SLR days, need a remote camera release (an intervalometer, preferably), but that is it.
OK, so which DSLR? The safe thing to say is still “Canon.” In some ways they still lead the pack in astrophotography. The Canons are remarkably low in noise over long exposures, and are easy to use in the field with a laptop if you choose to do that. Things are changing now, but until recently camera control software (like Nebulosity) was unheard of for other brands.
|SCT Prime Focus Adapter|
There’s also Canon’s longstanding involvement in our game. While Nikon and, now, Pentax are coming on strong for astrophotography, until the last couple of years only Canon acknowledged people were actually using their cameras for astronomical imaging and produced cameras with astronomy in mind.
Canon is a safe choice, in my opinion, but which one of their many DSLRs? If you are buying new and must keep the price tag low, the Rebel T6, which is available for about 450 dollars, is a remarkable value. Not only do you get a DSLR that will perform well for astro-imaging or anything else, you get a pretty good (zoom) kit lens for use in wide-field astrophotography or at Margie’s above mentioned b-day party.
Just don’t want a Canon for whatever reason? The equivalent Nikon is the D3300, which is even less expensive than the Rebel. And it can perform very well for astronomical imaging. BUT… Computer control options for this camera are (very) limited—it is not supported by the major Nikon astrophotography program, BackyardNikon—so if you want to tether camera to computer, a Canon is a far better choice.
How about buying a used camera? Is that a good idea? That depends. A fairly recent camera or seldom used older camera can push prices even lower. A perfectly serviceable older Rebel, like a 450D, for example, goes for 150 or fewer dollars with a kit lens and a few accessories. Be careful here, though. While the Rebels, Canon’s introductory DSLRs, and Nikon’s comparable models are well-made, they are not professional grade cameras and won’t stand up to real abuse. So, when considering an inexpensive camera it’s best to limit yourself to one that’s for sale locally so you can examine it in person and make sure it’s fully functional.
Prime Focus Adapter
|Prime focus adapter (1.25-inch)...|
Once you’ve got a camera, of course you’ll need accessories. You always need accessories in astronomy, you know that! First off, you will need a prime focus adapter in order to connect camera to telescope. “Which” depends on your scope style. SCT prime focus adapters screw onto the SCT’s rear port. Those for other telescope designs, like refractors, typically have 1.25-inch or 2-inch nosepieces and slide into the scope’s focuser. I like the 2-inch models, not because you have to worry about vignetting or something like that with an APS-C size sensor, but because they allow me to dispense with a 1.25 – 2-inch eyepiece adapter and seem to provide a more secure mounting arrangement.
You’ll also need a t-adapter for your camera, aka a “t-ring.” This is a, yes, ring shaped adapter with T-threads on one end to screw onto the prime focus adapter, and a lens mount for your particular camera on the other end. These two things in hand, you can remove the camera’s lens, mount the combo of T-ring/prime focus adapter in its place, and then mount the camera on your scope by inserting everything into the focuser or screwing the prime focus adapter onto the rear port of an SCT.
As you may know, DSLRs, most of them anyway, and certainly all the Canons, can’t expose for more than 30-seconds without the addition of a remote shutter release. Even if your camera could expose for longer without a remote, you’d still want one as it allows you to trip the shutter without bumping the scope and causing trailed stars.
An intervalometer is a remote shutter release, but it’s also much more. Not only will one of these (usually) wired controls allow you to trip the shutter from a distance and expose for as long as you like, it will allow you to shoot sequences of images. Say 30 3-minute exposures, which is exactly what we want to do. An intervalometer allows you to do many of the things a tethered computer would allow you to do, but without the computer. How much? A Vello is about 50 bucks and a genuine Canon is about three times that. Guess which one I’d choose?
If you’re not using a tethered PC, you’ll have to have a memory card, digital "film" on which to store your images. An SD card (used by almost all DSLRs, now) with at least 64gb capacity is my recommendation—you’d be surprised how much space an evening’s images can take up. Get a good, decently fast card. I like the Sandisk ones. About 40-bucks.
If you’re going to use a battery, make sure you keep an extra, or, better, two extras in your gadget bag. During long exposures, the camera is drawing current from the battery continuously, and you’re unlikely to get a full evening out of one cell, especially on cold nights. There are lots of third party batteries available, but I have had noticeably better performance out of genuine Canon, so that’s what I recommend here, the real deal, for a change.
Yes, batteries are a problem during astrophotography, so don’t use one, or use a real big one. Hop on over to Amazon and buy yourself either a 12vdc or 120vac power brick for your Canon (or whatever). I do most of my shooting at locations with mains power, so I prefer the AC option. The DC supplies have cigarette lighter plugs that will plug right into your jumpstart battery pack.
What do you plug one of these things into on the camera end? These power supplies have little plastic (wired) widgets that take the place of the normal battery in the battery compartment and supply power to the camera that way. I’ve found one of the inexpensive—less than 15-dollars—units on Amazon to work just fine, but Canon will sell you one for considerably more if you like.
Anything else? Well, a few things, maybe. If you are new to DSLR photography, you probably want a camera bag, a gadget bag, to keep camera and lenses and, well, gadgets, together. A nice piggyback bracket so you can mount DSLR and lens on your telescope tube is a nice addition and you may find you like doing wide-field shots from dark locations. A lenspen is good to keep your lens’ surface pristine. A broadband light pollution filter can be helpful if, like me, you do some of your imaging from an at least somewhat light-polluted backyard. And that is really more than enough to get you started.
You’ve now got all the pieces to the complicated astrophotography puzzle, but how the heck do you put them together? We’ll talk about that, about getting started with all this stuff, next week.
Addendum: How good can a VX be?
Auto-guiding wise, that is. Some of you considering a Celestron Advanced VX mount (or the similar mounts on the market today) have expressed grave concern about my statement last week that 2” (arc seconds) of RMS guiding error is about what you should expect of this group without some fine-tuning (of PHD’s Brain Icon settings, I mean).
Anyhow, while 2” is perfectly suitable for some image scale/camera pixel combos, naturally it would be nice to do a bit better with this inexpensive and highly portable GEM. So, I set about the other night to see how much and how easily I could tweak the VX.
Surprise! I really didn’t have to do much tweaking at all to get this modest mount’s RMS guiding error down. I did do a decent polar alignment, and I did spend some time carefully balancing the scope (east heavy with a little declination bias as well). As for the settings, I backed off on a couple of them. Cutting aggressiveness in half and reducing hysteresis as well. Oh, and, conversely, I increased Max Duration both for RA and declination.
The result? Despite OK but hardly great seeing, my errors were immediately halved with me getting just under 1” of RMS error most of the time. Even when my target got low in the sky, and seeing began to deteriorate, the error was just over 1”, easily good enough to yield round stars with an 80mm f/6.9 despite the fairly small (1/2-inch) sensor of the camera I was testing.
While I warned you not to start chasing lower and lower numbers with these GP/CG5 clone mounts merely for the sake of lower numbers, given the small amount of effort involved in this substantial improvement, the few minutes I spent was well worth it.
The other take-aways? People naturally worry about their guide-software settings, but what makes one of the very largest differences? Seeing. Without good seeing you will not see great guiding, so don’t start messing with your settings on an unsteady night. Oh, and good polar alignment is important for good guiding as well. Having to continually chase alignment-caused drift just muddies the water and makes guiding more difficult to get right. Finally, with this class of mounts, correct balance is just as important as polar alignment and seeing. If you want 1” or less guiding errors, you’ll likely need to rebalance if you move to a radically different part of the sky—cross the Meridian, etc.