focal length for deep-sky observing

opticsplanet.com

This is a very good question I received and I'd like to share it. It is a constant source of confusion for many new astronomers.

opticsplanet.com · Posted: Sat Jul 02, 2005 10:31 am Post subject:

The only reason for going to a lower f ratio is that it gives you lower magnification and hence a wider field of view with any given eyepiece. With your ETX-125 and its focal length of 1900mm, a 40mm eyepiece still yields 48x. The Bushnell with its 1550mm focal length with the same eyepiece gets you 39x. In terms of actual performance in the field, these are identical. Either is plenty low enough for deep-sky work with a 5" scope. Going lower will not gain you much.

The real determinant of performance, especially for deep-sky work, is aperture. The larger the aperture, the fainter the objects that can be seen and in the deep-sky game, locating faint objects is what it is all about. Thus trading your 125 (5") for the same size Bushnell will not move you forward for deep-sky work at all.

Any SCT may need collimation, but it is not a common thing, nor is a thing to be feared. Certainly an SCT is less likely to need collimation than the same size Newtonian and I have used some Newtonians for years without having to collimate. By all means, go with an 8" SCT or other larger scope if you want more performance than your ETX can deliver.
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