10 TIPS FOR DEEP SKY
My recommended techniques for shooting images of deep-sky wonders
Deep-sky imaging always uses some form of motorized mount to follow the sky as it turns from east to west. It might be a simple tracker on a tripod, or it could be a hefty equatorial mount carrying a massive telescope. Whatever the level of complexity in your deep-sky system, I find these same tips apply. And as with Nightscapes, the edict is to always shoot Raw.
#1 Go for a Modified DSLR
I always advise anyone who is serious about deep-sky imaging with a DSLR to consider purchasing a modified camera. A number of suppliers (I recommend AstroHutech) sell DSLRs which they have modified by replacing the infrared cutoff filter that sits in front of the sensor with another filter that still cuts off IR but lets through more of the deep red visible spectrum. This is important for picking up the photogenic red nebulas that dot the Milky Way. With a stock camera (the image at far left here) most nebulas will appear pale and dim. But a modified camera really snaps them up (as at right in the example here). Modded DSLRs are compromised for daytime use but can still be used by creating a Custom White Balance to counteract the pink caste these cameras lend to all images. Canon has made two factory-modified models, the 20Da and 60Da, but both are discontinued.
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#2 Go for Good Lenses
Stunning deep-sky images are possible by shooting with nothing more than a DSLR equipped with a wide-angle to moderate telephoto lens, and riding along on a tracking system. As with nightscape photography the best choices are fast, prime lenses. I use everything from 8mm fish-eyes to 200mm telephotos, with my 35mm, 50mm, and 135mm lenses personal favourites for framing Milky Way segments, constellations, and rich starfields. Low-cost manual lenses will work fine. However, the best quality lenses from Nikon, Canon and Sigma (their new “Art” series lenses are superb) will provide tack-sharp images to the corners at all but the widest apertures.
#3 Start with Trackers
Before investing heavily in telescopes and elaborate mounts, buy one of the affordable “star trackers” that allow long exposures of wide areas of the sky with lenses up to 135mm telephotos. They are simple to set up and run, and allow you to capture far more detail in Milky Way shots than you're can with a fixed camera on a tripod, and at lower, smoother ISO speeds. I’ve used the iOptron SkyTracker and Sky-Watcher Star Adventurer units (about $450 each) and can recommend them. Be sure to get a solid ball-head for attaching and aiming the camera. Even if you go crazy with high-end telescope gear, you’ll always have a use for a portable camera tracker. They also work great for capturing deep images of the Milky Way over landscapes.
#4 Focus on Focus
Stars are the most merciless subjects for revealing poor focus. As with nightscape shooting, it’s worth the taking the time to precisely focus any lens or telescope. Live View makes it easy. Set the camera’s Live View options to “Exposure Simulation,” with the ISO set to high, the aperture set to wide open, and the shutter to set 30 seconds or Bulb. Aim at a bright star and manually focus until the star is as pinpoint and colour-free as possible. With lenses even the slightest turn of the focus ring will bloat stars or add coloured haloes. With telephotos and telescope refocus through the night. As the air cools and tubes contract, the focus can shift. Clever image processing can cure a lot of imaging ills, but it cannot make up for poor focus.
#5 Seek Out Dark Skies
Nightscape photography benefits from moonlight. Not deep-sky photography. We also need skies as free from artificial light pollution as possible. While some specialized filters can reduce the effect of man-made skyglow to some extent, there is no substitute for seeking out the darkest skies you can for your deep-sky expeditions. Plan on getting away on New Moon weekends to a lakeside cottage, a favourite campsite, or the local astronomy club’s “dark site.” If you already live in a rural area free of interfering lights, you’re all set to shoot from your backyard. The best accessory you can give your cameras and telescopes is a dark, moonless sky.
#6 Take Dark Frames (Always!)
For deep-sky images I always turn on Long Exposure Noise Reduction, so the camera takes a dark frame to reduce noise. Some photographers prefer to take dark frames separately at the end of a session (by capping the lens or telescope), then subtracting the dark frames in “post-production.” In A-B tests I’ve never found that procedure as effective as having the camera take a dark frame at the same time as the exposure. The reason is that only then is the sensor at the same temperature for both the dark and “light” frames, essential if noise levels are to match and subtract accurately. Yes, this increases the time it takes to acquire images, but the results are better. But here's a tip: If you are using Canon full-frame cameras (5D and 6D models) and you control them through the shutter port, not the USB port, you can take 4 or 5 images in quick succession before the dark frame locks up the camera. One dark frame then gets applied to each light frame, shortening total exposure time.
#7 Shoot Slow and Long (Camera)
It is possible to shoot at very high ISO speeds (3200 or higher), with the benefit of reducing tracking errors or eliminating the need to guide the telescope. To counter the noise of the high ISO speeds, you take lots of frames (16 or more) and stack them to average out the noise. However, I find that the best results come from shooting at slower ISO speeds (400 to 1600), then stacking several (4 to 8) frames. Yes, the exposures are longer (and will require guiding – see point #9) but images shot at slower ISOs have a wider dynamic range and certainly lower noise, made even smoother by stacking them in Photoshop, or other popular stacking programs. Stacking four frames improves noise by a factor of two. As with nightscapes, expose each frame well, erring on the side of overexposure. Use the camera’s histogram to ensure you “expose to the right.” ETTR is a key tip to ensure the lowest noise in DSLR images. Do not underexpose.
#8 Shoot Fast and Short (Telescope)
Keeping exposures short certainly has its benefits. For one, you get more images shot in a night. When shooting close-ups of deep-sky objects through telescopes I prefer to shoot with fast f/4 to f/6 “apochromatic” refractors. Their focal ratios afford a good balance of fast speed (allowing use of slower ISOs) vs. freedom from image aberrations and flaws such as darkening of the outer areas of a frame. An 80mm to 100mm apo refractor doesn’t require a massive mount, making for a solid yet portable system that is easy to set up. For a light but capable mount ideal for a small refractor, look at the Sky-Watcher EQ3 or HEQ5, the Orion Sirius, or the Celestron VX. Avoid big 10- to 11-inch reflector telescopes – you will find every excuse not to heft them out to the country and set them up. The fanciest equipment does you no good sitting in the basement or garage. Buy what you can handle, literally.
When shooting with focal lengths over 200mm and exposures over 1 to 2 minutes, most mounts will require guiding. Yes, they have motors that follow the sky, but minor errors in the gears, among other sources, will trail stars slightly during long exposures. To eliminate tracking errors we employ an “autoguider,” a little CCD camera looking through a small guide scope. It watches a guide star and sends a pulse to the motor to counteract every time the guide star wanders off. Autoguiders vary, but most require a laptop to run guiding software (the free PHDGuider program for Windows and Mac is popular). The Orion Starshoot guider works very well – it is sensitive enough to find and reliably guide on stars looking through no more a compact 50mm guide scope. Once set, an autoguider allows exposures of any length, all perfectly guided, while you nap.
#10 Keep It Simple at the Scope
As you can see, deep-sky imaging can get complex. I recommend people stay with DSLR cameras for their simplicity of operation. I even tend to avoid using a laptop to control the camera, to avoid the fuss of cables and fiddling with keyboards and cursors in the field. I stick with hardware intervalometers to run a camera, and use a “stand-alone” autoguider (the Santa Barbara SG-4) that doesn’t need a laptop to run. It’s costly, but oh so convenient to use! In portable systems resist the temptation for accessories and cameras that all demand power and cable connections to laptops. You can spend all evening just getting your gear connected and running when you really should be shooting. Keep it simple for productive results under the stars. Add complexity only if you have the luxury of a permanent observatory at a dark site.