Contrast Problems

SCTs are obstructed telescopes. What that means is that there is an obstruction—a "secondary" mirror—placed in front of the main (primary) mirror. Optical experts say obstructing the primary mirror of a telescope in this fashion will inevitably degrade the contrast of its images because light is scattered by the secondary into places where it should not go. Any reduction in contrast is potentially harmful for planetary observers. When straining to make out an almost-invisible cloud band on Jupiter, the last thing that is wanted is reduced contrast. Any telescope that uses a secondary mirror to divert light to an eyepiece will be affected by this problem, but the SCT is particularly troubled by this effect due to the size of its secondary mirror. To keep a Schmidt Cas-segrain's tube short and easy to mount, the secondary mirror's diameter must be relatively large, often as much as 30% the size of the primary mirror.

That is pretty big, true, but the simple fact of the matter is that an obstruction of any size in a telescope's light path, no matter how small, will damage contrast. Even a Newtonian reflector with an obstruction of less than 20% will have lost out when compared to an unobstructed design like that of a refracting telescope. The question is, does the larger secondary of the SCT make things much worse? Based on my 43 years of observing experience with telescopes of all types, the answer is, "No"—or at least, "Not much."

Listening to scope "experts" down at the local astronomy club or on the Internet go on and on about this issue, the novice will get the idea that a C8 must produce planetary images not much better than those of a 60-mm junk-o-scope from a discount store. This beginner will then be amazed at his or her first look at, say, Jupiter, through a Schmidt CAT. The job an SCT can do on Jupiter or any other planet is simply astounding. There are plenty of belts to see, and subtle colors are easily discernible on Jove's huge globe. The Great Red Spot will not just be visible; there may be detail within it. Maybe this image will not be quite as high in contrast as one in a refractor, but as mentioned, the SCT at least delivers more light than all but the most horrendously expensive lens scopes (priced an 8-inch refractor lately?), and in my opinion, this extra light does a lot to make up for the Schmidt Cassegrain's contrast faux pas.

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