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ETX Etc Blah Blah Blah

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Posted by Daniel Johnson on April 22, 2003 11:37:59 UTC

Your neighbor has a point. Atmospheric fuzz limits resolution. Larger scopes can give brighter images at a given magnification, but on any given night no scope can exceed the resolution that the atmosphere allows that night. On a typical night of good viewing, that limit is around 1/2 arcsecond. I usually think of a 10-inch scope as meeting the atmospheric-imposed limits, but the exact limit varies. An 8-inch scope will be good enough many nights. A 12 or even 14 inch scope, in brief moments of stability on good nights, will give you half-second peeks at better resolution, but mostly they just give you brighter images at any given magnification, or equal brightness at higher magnification for tiny, dim objects such as distant galaxies.
It's possible to photograph through a Barlow. For deep sky, you'll never want to. For a digital camera and planets, you probably won't need to with the SCT but you'll need to with the Schmidt-Newt. It adds one more thing that can wobble or throw your scope out of balance.
There are 2-inch Barlows. I've never tried one and don't have much insight here. Still, Barlows are for high magnification, such as planets. You don't need a 2-inch eyepiece for that. However, a two-inch focuser is nice. It allows you to use two-inch camera adapters for film photography, which results in less vignetting. The SCT can be fitted with two-inch accessories if you want. However, it does have a 1.5" limit on the opening in the rear, so some of that 2" is wasted.
In sum:
Schmidt-Newt is probably a bit better for deep sky. Be aware that it is such a low-mag scope that you won't be able to use the 32mm or 40mm eyepieces well, since the exit pupil (cylinder of light leaving the eyepiece from a star) will be larger than your eye's pupil, so some of the light will just be wasted. Even the 26mm will be useful only in dark skies when your pupils can dilate fully.
Schmidt-Cassegrain is probably better for planets, and a good can-do-it-all choice. I'm not sure how its weight compares with the 8" or 10-inch Schmidt-Newt, since weight is a limit for that mount if you want photography. But really, photography is hard stuff with a long learning curve. You may end up using the mount for a camera alone rather than the scope, when you photograph--see below.
ETX mount is good for visual work but not for photography, period. ETX is good for planets but not so good for deep sky--aperture is too small, focal ratio is a bit too long.
LXD55 mount can be used for photography, magninally (a last-second thought here: difficulty of photography goes up with focal length. So the f/4 is a little easier to guide than an SCT with an f/6.3 focal reducer).
What's your biggest love, planets or deep sky?

Some day, if you find the LXD55 mount is a constant struggle for through-the-scope photography (and I only have the S&T review to go on there), try using it with a 35mm film camera piggybacked. You'll get spectacular views of the Milky Way through an ordinary 50mm lens, and any lens of 20mm to 135mm will be easy to guide with that mount. You'll be amazed what you can do with ordinary camera lenses, and your friends will think you're a genius.

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