By Richard Berry
As any type of observing places a special set of demands on telescopes, so CCD cameras place new demands on the telescopes intended for imaging. This is nothing new. Similar changes have, in fact, happened many times before in the history of amateur telescope making. The move away from the small refractors of the Fifties occurred because the long-focus Newtonian of the Sixties satisfied the aperture and resolution needs of lunar and planetary observers, and the growth in popularity of highly-portable Schmidt-Cassegrain telescopes in the Seventies occurred at least in part because car-portable telescopes became a necessity for many urban amateurs. The remarkable spread of the Dob in the Eighties happened because the Dobsonian answered so perfectly the need for a low-cost, large-aperture telescope for visual deep-sky observing. As new types of observing have become popular, each has placed new demands on telescopes, and the types of telescope that amateur astronomers built and use has changed in response.
Today, we face another great change brought on by the needs of observers who do CCD imaging. The needs of observers who want to use CCDs are not, for example, the same as the needs of visual deep-sky observing. In this series, I will discuss these new requirements are and how we, as amateur telescope builders, can address them. This series will, hopefully, prove useful for amateurs who build telescopes optimized for astrophotography, portability, deep-sky observing, planetary observing, and other semi-specialized types of observing, even if those amateurs do not intend to try CCD imaging for themselves. If nothing else, the series may help everyone to understand what the CCD types are trying to accomplish.
CCD imaging closely resembles astrophotography in its fundamentals. Factors such as field of view, exposure time, and focal ratio as just as important for digital imaging as they are for film imaging. The major differences are that CCDs are generally much smaller than the standard film formats, CCDs require shorter exposure times than film, and the image is read out and stored digitally, which means that the CCD image can be enhanced more easily than film images can be enhanced.
Another major factor that determines what sort of telescope you need for CCD imaging is what type of imaging you wish to do. The characteristics you need in a telescope for taking pretty color pictures, shooting faint galaxies in black and white, tracking comets, or shooting high-resolution planetary images are as much different for CCDs as they are for conventional film photography. It is therefore necessary that you decide what type of imaging you want to do before you start designing a telescope to do it.
Finally, it helps to recognize that telescopes for CCD imaging are not unusual telescopes. We're still talking about refractors, reflectors, and various types of compound and catadioptric systems. The big surprise comes from the fact that CCDs do not call for small telescopes or big telescopes in the same ways that visual observing and astrophotography usually have, and that means that CCDs may offend our habitual sense of "what's right." In a nutshell, for many types of deep-sky observing, the best telescopes are small, fast Newtonians, and for imaging detail on the planets, large Newtonians with good optics outperform everything else. Small, fast refractors with good color correction in the near infrared part of the spectrum also do well for deep-sky imaging.
If there is any one area where home built telescopes have trouble meeting the demands of CCD imaging, it's the mounting. For every type of observing a solid, shake-free equatorial mounting is crucial. Visual observing tends to forgive the vagaries of the mount, and in astrophotography, sheer perseverance and careful guiding can overcome a clock drive with annoying periodic errors. In CCD imaging, where individual exposures seldom run over ten minutes, it's hardly worth the time needed to find and set up on a guide star. That means, however, that the clock drive must be good enough to track without guiding, and to make matters worse, because the pixels on the CCD chip are so small, you can see tracking errors one-third to one-fifth as large as you can in conventional astrophotography.
Largely because of the tough tracking demands CCDs place on the telescope and mounting, small, fast refractors and Newtonians tend to be far easier to use than other types of telescope. For my CCD work, I use a 4-inch f/5 Genesis and a 6-inch f/5 Newtonian with a Byers 812 mount. The Byers mount, designed for an 8 to 12-inch telescope, does a great job with the 4-inch and 6-inch telescopes. On a good night, I count on the drive to perform flawlessly during four-minute exposures, and if the drive hiccoughs during the exposure, I simply repeat it.
However, CCDs do offer an important way to minimize the impact of a
drive system with tracking problems. Using a track-and-accumulate technique,
a CCD camera shoots a series of exposures short enough for almost any clock
drive to handle, for example, making 20 exposures each 15 seconds long.
The camera software determines the positions of one or more tracking stars
in each individual image and then shifts the image to register the stars
and adds it to an accumulating master image. Although track-and-accumulate
with a poor drive takes a long time compared to one-shot imaging on a telescope
with an excellent drive, it makes CCD imaging possible on many more telescopes.
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