Astrophotography: Choose Stars, Planets, Nebula, or DSOs!
Noted author Douglas Noel Adams (1952-2001) wrote a series of books about another book. It was an entirely fictitious book that acted as a cheapskate’s reference for touring all the civilized places in the Milky Way for just Five Altairian Dollars per day. It was called The Hitchhiker’s Guide to the Galaxy.
For us, here and now, there can be no promise of locating those "civilized" portions because we haven't yet met any other people that might be out there. Like the protagonist of Adams' book, we are human. We haven't yet developed the technology to go over and visit Betelgeuse, Sirius, or even something relatively close to our neighborhood, such as Proxima Centauri.
We haven't even been to our next-door neighbor, the Moon, since the 1970s. Though there are plans to return before the end of this decade, which is nice. In fact, for the moment, we can’t even scoot up to Low Earth Orbit to spend three days at the Orbiting Hilton, Marriott-in-the-Sky, or the Holiday Out (of-This-World). So what’s a person to do? Well… we can still look…
What you will Need
- A good solid tripod, for camera or telescope;
- The good sense not to put your equipment in an air-conditioned car and then take them out in a humid environment, so all the lenses or mirrors fog up! Closed cases, in the trunk, so they are pre-adapted for when you arrive. If you’re at home, put them outside somewhere safe for a couple of hours;
- Fully charged batteries, and spares because long exposures are draining;
- A hat and thermos of hot drink because even the desert gets cold at night;
- Remote shutter for the camera (Bluetooth, infrared, or lockable cable);
- An interval timer (optional);
- A Star Chart to find your target;
- MP3 player to fill the time during long exposures.
Recording what you see is terrific. It gives you a chance to share your experiences with other enthusiasts, or show something brand new to a person who has never troubled themselves to “look up” before.
The first question you have to ask yourself is: What do I want to take pictures of? There are two basic answers, and they will determine what equipment you need.
Answer 1: Pictures of Stars (or the Moon)
A perfectly ordinary film camera is suitable for this type of photography if you have an assortment of lenses. These are generally wide-field shots that include portions of the horizon or surrounding objects to frame a part of the sky. A telephoto lens can give good shots of the Moon.
However, nowadays, most people have switched to digital cameras because actual film is expensive, and you have to wait for it to be processed. Besides, a typical roll of film maxes out at 36 images, and you can't preview it. Digital Single Lens Reflex (DSLR) cameras with mirror locks, or non-reflex, mirrorless cameras, are preferred. This eliminates the shaking from operating the shutter, or from multiple shots.
One of the most popular image-types is to create star trails with long exposure shots. With the camera centered on Polaris (aka, the North Star), the stars will appear to orbit around it, like the upper portion of this image.
If the camera is directed at the eastern or western horizon, star trails can turn into long artful streaks like the bottom part of this image. You can kid-around with amateurs and tell them it was the most massive meteor shower in history—and you got a picture of it!
Timing is everything
Exposures for acquiring star trails will require a minimum of 30 minutes with a 50 mm lens, 45 minutes with those 35-28 mm, and at least an hour with 24 mm and smaller. The bigger your field-of-view is, the smaller the apparent motion over time. You can go much longer, of course, to get dramatic arcs and sweeping trails…
Even in complete darkness, long, uninterrupted exposures are bad. The image starts to accumulate video-noise (visual artifacts) that decreases the picture quality. To solve that, your images can be automatically taken with an Interval Timer (~$35) for, say, two minutes, and then the next image is taken right away, and the next… Once they are chained together in the software, you get a contiguous image showing the full period you shot, but there are far fewer artifacts for you to contend with.
Just remember, there is a period called Astronomical Twilight (AT) that occurs well after sunset. That is when “true darkness” arrives. Until AT, there is still too much light for long exposure shots. Lots of Apps can tell you when it occurs locally.
As mentioned, people generally use picture editing programs because getting the perfect, once-in-a-lifetime shot only happens once-in-a-lifetime. Instead, they take a series of shots and, once the images are loaded into the computer, they can be processed into a composite image. Cheating? Maybe…but it works. There are plenty of programs (with tutorials) to help you accomplish astronomical tasks, many of them free, such as Autostakkert, DeepSkyStacker, or StarStaX. Images can certainly benefit from some final assembly and tuning in LightRoom or Photoshop.
Answer 2: Pictures of Planets or Deep Sky Objects (DSOs)
You’re going to need a telescope. Telescopes are like the world’s best telephoto lenses, and that is how you’ll be using them. The big difference is that usually, you would attach a telephoto lens to a camera to get a close up of something far away. With astrophotography, it just the opposite; you connect the camera to the telescope to record something distant.
Planetary photography is best accomplished with a scope that has a powered equatorial mount. See our article Tripods & Mounts [Author’s Note: insert a link to Mounts & Tripods article here] for information about setting it up to give you genuinely stellar pictures.
Set your camera to record in RAW format instead of JPG, to avoid in-camera editing by the camera software, too. It’s better to edit on your computer with ALL of the data than to suffer from the camera's "choice" of what is essential.
At high magnification, you can take a string of 100 ms exposures of, say, Mars. The images themselves may be individually horrible. Still, when you get enough of them, they can be dumped into a photo-analysis program that looks through hundreds of shots and collects all the consistent elements.
Every time a pixel is confirmed by being repeated in several images, the program moves it to a Master photo file. Even with just minimal processing, it could look many times better than the luckiest single shot.
Of course, with long exposures, the planet will rotate, because that’s what planets do. Most imaging programs have an automatic “de-rotate” function, so it builds the image out of the parts that will remain in the picture. It ignores bits that rotate away and won't be in the final image, as well as the “late arrivals” that won’t be included, either.
If you want to take a picture of a small cluster of distant stars, and don't have an equatorial powered-mount, there is a formula to calculate your maximum exposure time. Use the Rule of 500 to avoid unwanted star trails:
Divide 500 by your focal length (FL). If it happens to be 50 mm then:
500 ÷ 50 = up to 10 seconds of exposure time; you can double that with a 24 mm: 500 ÷ 24 = 20.8, so stay under 21 seconds to avoid trails.
Of course, with a telescope, the focal lengths are much longer than cameras. If your FL happens to be 500 mm, then you would get a trail in about one second. If it’s 1500 mm, you’ll only have about 1/3 of a second to grab your image before it blurs… This is why powered equatorial mounts are so desirable to capture an image of a DSO or Deep Sky Object. Pictures of galaxies require such an arrangement to be successfully imaged, as do images of even local nebulae.
Of course, it is impossible to cover all the details you’ll need for stellar photography, but this should have certainly set you on the right track. Our galaxy, the Milky Way, has up to 400 billion stars in it, with at least 100 billion planets, but probably much more. So even if you don’t look at the other 2,000,000,000,000 galaxies that surround us, each with its 300,000,000 stars, you should still be able to find something that you want to take a picture of…