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An Introduction to Astrophotography—Part 1: Equipment Choices

by | Nov 24, 2021 | 0 comments

M81, M82
M81 and M82. Photo © Shaphan Shank.

In my early teens, I knew almost nothing about astrophotography, except that I wanted to try it. My early dreams included using a very cheap CCD camera we had, along with our 8” Newtonian reflector. Then, a few years later, we had the privilege of visiting an astrophotographer in Highland County, Virginia. This visit was an eye-opener to us, as we realized that we could take astrophotos with DSLRs and camera lenses that we already owned. In fact, as I learned more, I realized that by using the camera/lens combinations we already had, we could get much better results than we would by using our cheap CCD camera.

You may be wondering why we did not use our DSLRs with our 8” Newtonian. The primary reason that we didn’t use this scope is because, like many Newtonian reflectors, our telescope didn’t have enough focus travel to allow the camera to focus when mounted on the telescope. Moving the mirror forward would have corrected the problem, but this was impractical, especially because we already had several camera lenses that were well-suited for astrophotography.

The only major missing link was a good-quality equatorial mount. A good mount is one of the most important parts of a good astrophotography setup. After doing some research, we purchased a Celestron CGEM DX mount. We were pleased with the performance of this mount, and we still have it today.

There are many different types and styles of astrophotography, which include a wide range of equipment and techniques. One style is wide-angle nightscapes, which are basically photographs of a landscape with the night sky as a major component of the photo. Another form of astrophotography is planetary photography, which is often done by taking short video clips at a long focal length, then using computer software to stack the sharpest frames. Yet another form is that of photographing deep-sky objects such as nebulas, galaxies, and star clusters. This is the type of astrophotography that we have done the most of, although we have also done a little wide-field astrophotography and plan to try planetary photography when we get a chance. The information that follows is most applicable to deep-sky astrophotography.

Good astrophotography (except for some wide-angle photography) requires a good mount. The best camera/lens or camera/telescope combination will be almost useless without a quality mount. Equatorial mounts are the best kind for astrophotography, although fork mounts will work if they are used in conjunction with an equatorial wedge.

To take high-quality astrophotos, it is important that the mount be level and properly aligned with the North Celestial Pole. Celestron mounts such as the one we use have a feature called All-Star Polar Alignment. This feature makes the polar alignment process quite simple, and we use it frequently. If you will be shooting exposures of several minutes, you will also want a mount that lets you program periodic error correction (PEC).

There are many different types of camera lenses and telescopes that can be used for astrophotography. We started our venture into astrophotography by using two Canon lenses we already owned and used for wildlife photography—a 400 mm f/2.8 and a 600 mm f/4. These are high-quality lenses that are actually similar in performance to high-end refracting telescopes. These lenses have an ideal focal length for framing many deep-sky objects, and their fast focal ratio allows for relatively short exposure times. Also, they are excellent for daytime photography of birds and other wildlife. Several minor downsides are that they are rather heavy for their size because of the many glass elements inside, and they can be a little tricky to focus.

More recently, we have started using a Celestron EdgeHD 11” telescope. This telescope can be used for photography at its native focal ratio of f/10, with a focal reducer at f/7, or with a Hyperstar at f/2. The focal ratio (f/numbers) is determined by dividing the focal length of the telescope by its aperture diameter. For example, the EdgeHD 11 has a focal length of 2800 mm and an aperture of 280 mm, so its focal ratio, or f/number, is f/10. With a Hyperstar installed, the focal length is reduced to 560 mm, converting the scope to f/2. The lower the f/number, the brighter the photo will be, allowing shorter exposure times to be used.

We use the Hyperstar for all of our astrophotography with the 11”. The Hyperstar is a special lens that attaches to a camera and replaces the telescope’s secondary mirror. Because of the scope’s large aperture and ultra-fast f/2 focal ratio (with the Hyperstar), we get good results with exposure times of 45 to 90 seconds vs. 3 to 5 minutes with our camera lenses. This has a three-fold benefit.

Astrophotography setup
Here is our first astrophotography setup. Shown is a Canon 600 f/4 lens and guidescope on a Celestron CGEM DX mount. Our camera is inside a homemade cold box. A computer “tells” the camera how many photos to take, and what length of exposure.

First, we do not need to use an autoguider to get good results. An autoguider is a small camera that tracks a star and sends commands to the mount when minute adjustments need to be made. This is not what makes the mount track with the stars, but, rather, it makes tiny corrections to compensate for tracking errors. With our camera lenses, we needed to use a separate guidescope with a guide camera, adding weight, complexity, and flexure to the system. With the shorter exposures we use with the Hyperstar, the autoguider becomes unnecessary.

Second, by using shorter exposure times, the chance is reduced of something going wrong and ruining an exposure. Hence, the percentage of useful photos is increased.

Third, the total amount of time spent photographing an object can be reduced. This is because the final quality of an astrophotograph is not dependent on the total time of all exposures, but rather the number of exposures stacked to make the final photo. Therefore, in one night, you can get photographs of several different objects that are the equivalent quality that you would get by spending all night photographing one object with a slower optical system.

Photographing with the EdgeHD 11 at f/10 or f/7 is also an option, but we haven’t tried it at this point. One reason is that autoguiding would be necessary because of the longer exposure times and the much higher focal length. (Higher focal length equals more magnification, and more magnification magnifies tracking errors.) However, if one has a good autoguiding system, shooting at f/10 or f/7 would definitely have merit for smaller objects such as planetary nebulas and most galaxies. The focal length at these higher focal ratios provides too much magnification to be ideal for large galaxies and many emission nebulas.

These are the two optical designs that we have used for most of our astrophotography, but there are other options. Many astrophotographers use high-end refracting telescopes. These refractors are the epitome of optical quality, and, yes, you pay for that. For the same money, you can get a much larger scope with very acceptable quality. I won’t say, though, that high-end refractors don’t have their place.

Another very viable option for astrophotography is a Newtonian reflector designed for astroimaging. These imaging reflectors are cheaper per inch of aperture than most, if not all, other designs, and their optical quality is often quite good.

Two main types of cameras are used for deep-sky astrophotography. CCD imagers are designed specifically for astrophotography and are used solely for that purpose. An alternative to a CCD camera is a DSLR (digital single-lens reflex) camera. DSLRs can be used for daytime photography as well as astrophotography.

There are two different types of CCD imagers. The best type is the monochrome CCD imager, which shoots in black-and-white instead of color. With these cameras, photos are shot in four different stages. First, luminance frames are taken without a filter. These photos are used to get fine detail in the final photograph. Then photos are taken through red, green, and blue filters. Finally, the photos are combined using computer software to create a color photo.

The other kind of CCD camera is the OSC (one-shot color) camera. These cameras are often cheaper than monochrome CCD cameras, and, although they can take very nice photos, they are not as good as monochrome imagers. The general consensus among many astrophotographers is that if you want a CCD camera, you should get a monochrome one; otherwise, you are better off buying a DSLR that can also be used for daytime photography.

We have always used modified DSLRs for our astrophotography, and I am pleased with how they perform. The results that can be obtained with modern DSLRs are amazing, and, to me, it’s not worth the money and hassle to buy a monochrome CCD imager and learn how to run it. (They are operated quite differently than DSLRs.)

A modified camera has had an internal filter removed so the camera records hydrogen-alpha light. This light is the red color seen in many emission nebula photographs. Most people prefer to have this work done by a professional. Be aware that this will void the camera’s warranty.

Good CCD cameras do have some nice features not found in DSLRs. One such feature is a built-in cooling system. This cools the camera sensor, greatly reducing noise in each picture. We used to use a homemade cooling box to cool our camera when we were photographing in the summer. This helped some, but it was built for use with our camera lenses, and it can’t be used with the 11” and Hyperstar.

One other tool is very useful for astrophotography—a computer. Although it is possible to take astrophotos without a computer, you won’t get quality results with pinprick stars, a dark sky, and a bright subject, unless you have one.

These four components—mount, optical tube, camera, and computer—are the basic tools you need to get started taking beautiful astrophotographs. Next month we plan to cover the process of actually taking the photos.

North American Nebula
North American Nebula. Photo © Shaphan Shank.
This photo was taken with a modified Canon 60D on a 400 mm f/2.8 lens.
The photo is made up of thirty-four, three-minute subframes.

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