There are lots of cool things out there in deep space for you and your friendly CAT to look at and admire: star clusters, nebulas, and galaxies. The SCT is not only capable of showing these deep sky objects (DSOs), it is able to deliver remarkably detailed visual images of them under good sky conditions. It can do that because of its generous aperture (the diameter of the main mirror). To see an inherently faint object like a galaxy well, what is needed is plenty of light. Not all telescope designs are created equal in this regard. A very large refracting telescope, for example, will have an objective lens 6-inches in diameter. An average SCT has a main mirror 8-inches in diameter, which will collect nearly twice as much light as the 6-inch lens (objective area. not diameter, is what counts). Also, a fine 6-inch refractor is a fairly heavy and very expensive instrument. An 8-inch SCT, in contrast, is light, easily transported, and inexpensive enough to be within the financial reach of just about anybody.
It is not just optics that have allowed the SCT to pull ahead in the contest for the hearts and minds of amateur astronomers who are interested in deep sky observing. Almost all SCTs currently available have easy-to-use go-to computers. What's
"go-to"? Select the object of interest on a little TV remote control-like "hand controller," push a button, and a pair of motors automatically points the scope at the target and tracks it as it moves across the sky. This is a boon for people more interested in looking at objects rather than looking for objects. Big Dobsonian telescopes, which are often recommended for deep sky observing, usually do not have go-to, and finding objects to observe often involves squinting at star charts and peering though dim finder scopes. Some Dobsonians can be adapted for go-to and can use other computerized pointing aids, but in general they are still not as accurate or easy to use as a go-to SCT.
Another Schmidt Cassegrain advantage for visual workers is the comfort inherent in CATs. An SCT allows its user to observe anything in the sky while comfortably seated. A big Dobsonian telescope can deliver a lot of that prized light, sure, but to see anything, the observer will often be swaying at the top of the tall ladder required to reach the eyepiece observing position of a large scope. A DSO may be brighter in the Dobsonian, but if it can be viewed in comfort while seated, almost as much— or more—may be seen in an SCT with a considerably smaller aperture. Nearly all Schmidt Cassegrains can track stars and other objects across the sky via built-in motor drives, allowing an observer to sit and stare at a galaxy for as long as desired, until the object sets or the Sun rises, anyway. Most Dobsonians lack any kind of motor system to make up for Earth's rotation. "Dob" users must continually nudge the scope along to follow objects, which can be distracting. Push a button to find an object. Sit comfortably to view it. Stare at it for as long as desired as it sits centered in the eyepiece. What could be better for visual deep sky observing?
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