Telescope Choices

Any CAT can be used for planetary imaging, but some CATs are more equal than others. The most critical requirement? Quality optics, but most CATs fulfill that these days. Almost as important is focal length. A detailed planetary image requires a lot. It's possible to extend the focal length of any telescope by the use of a Barlow lens, but it's usually best to start out with a "slow" scope to begin with. A telescope that brings a lot of millimeters to the table is the Maksutov Cassegrain, and it can blow other designs out of the water on the Solar System. Its only failing? Aperture. In the interest of sharp images, it's desirable to keep webcam frame exposures as short as possible. That's no problem for a 6-inch Maksutov on the Moon or Jupiter, but it can be a problem on Saturn, or when very large image scales on any planet are desired.

All in all, the good old SCT is, once again, the workhorse. Its images, although maybe not as sharp as those of other designs, are still very good, it offers plenty of focal length, plenty of aperture, and its moving mirror focusing means almost any camera can be brought to focus. The SCT's main problem for Solar System work is focus shift. At high power, focusing can drive the image of a planet right off a webcam chip. This problem can be cured by the simple addition of a rear cell Crayford focuser. Browse through the astronomy magazines and have a look at the high-resolution planetary images done by the expert imagers. What do they have in common besides their amazing detail and beauty? Most were shot with Schmidt Cassegrains.

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