From the April 2012 issue

A checklist for astrophotography

June 2012: Keep these tips in mind when searching for, or building, new equipment.
By | Published: April 23, 2012
People have long asked me for advice about astrophotography equipment, so over the years I’ve developed a comprehensive answer. Rather than simply list a multitude of recommended products, I have compiled a checklist of desirable (and, some might argue, necessary) qualities that your equipment should have, regardless of who makes it.

1No matter what kind of camera you’re using, it must sit on a mount to track the movement of the sky. Much has been written about periodic error, a physical problem with the mount, but that’s not what defines good tracking; the lack of “play” between the gears does. You need to guide, which takes care of any periodic error. In fact, you should turn off periodic error correction when guiding to prevent “double correcting” the guide star and creating confusion. With a lot of slop between the gears, the guiding corrections will have a hard time getting the job done. If you have a poorly constructed mount, using something like the Starlight Xpress Adaptive Optics unit will replace your mount’s missing accuracy to effect accurate guiding.

Curvature of field causes corner objects to stretch out and point toward the center. Typically, the center will be sharp while the edges will appear out of focus. Astronomy: Roen Kelly
2You need a flat field. Curvature of field is a common optical defect in imaging. The optics of your telescope must include a field flattener to bring the stars into focus across the imaging device. Failure to do this will result in elongated stars in the corners, all pointing at the center of the image. You can test for this defect by moving the focus in and out to see if the edges become sharp at any point. If they do, and the center has become soft, you need to flatten the field.

3.  Ideally, coma should be absent from the optical design. This lens aberration is responsible for turning stars into the classic “flying seagull” shapes in the corners of your image. To a great extent, coma is dependent on the f/ratio of the telescope: Fast telescopes (f/4, for example) are far more prone to coma than slower ones (f/11). Commercial “coma correctors,” if your telescope maker doesn’t provide one, can deal with this problem.

Astigmatism results in oval stars that rotate 90° when observed on the inside and the outside of focus. Astronomy: Roen Kelly
4The worst affliction to have is astigmatism. This nasty situation primarily results from misalignment of the optics, so perhaps your optical setup only needs collimation to remove the defect. Many telescope owners are mortified to attempt this “tune-up” procedure, but it should be a prerequisite to owning a telescope. In fact, it’s really not your telescope until you can collimate it! How do you know that you have astigmatism? Slightly out-of-focus stars will appear as horizontal ovals on one side of focus, and as vertical ovals on the other.
5Another desirable characteristic is apochromatic optics. If you have a mirror system, your optics are automatically apochromatic, but it’s a more difficult trick to get a refractor to focus all the wavelengths of light into exactly the same focal point. Recall that a prism separates light into a spectrum — that’s what we don’t want! Ideally, light passing through a glass objective will come out the way it went in. Cheap objectives will require refocusing between the colors, something you can do with automated software and a reliable electric focuser. Otherwise, part of your RGB sequence will be out of focus.

6Finally, look for focus stability. Many telescopes change focus as the temperature drops, but this is minimized by constructing the telescope from thermally inert materials (like certain carbon fibers) that don’t change shape with changes in temperature. Unfortunately, these exotic components usually make for more expensive “higher-end” telescopes. Some cleverly made refractors feature designs that compensate for the change of focus due to cooling by changing the length of the telescope tube, thereby keeping the focus constant.

As you can see, building the perfect telescope for astrophotography is no easy trick. If you have one that works, remember to thank the maker!