Hi Jim: To the best of my knowledge, there is no optimum focal length. Focal length determines magnification, so it should be chosen to suit the application. Magnification should never exceed the diffraction limit for a given aperture. Telescopes are like airplanes in that if you design one that does "everything", it will do nothing well. Some of the large observatories use auxilliary foci with some whopping final f/ratios, sometimes at or over the f/100 you speak of. Of course, they have apertures larger than our houses, so there is plenty of light to support these image scales. Even so, long exposures (or integration times, or many short exposures, stacked) are sometimes needed to record the data. C. --- Jim Gibson <xajax99@yahoo.com> wrote:
I was wondering since we explored the bottom end of the spectrum, and discounting the physical length for a moment, ( I hope this isnt too dumb of a question) is there an optimum focal length? For example is an f15 all that much better than an f10. Is an f100 really 10 times better optically than an f10? Is there and upper end for focal length? I was thinking that if there is a diminishing return and an f10 is only slightly less optically than an f15 then when physical restraints are considered I could deal with it. But if the optimum focal length is really up there, then I will be back to where everyone else is the maximum minima or what will fit in my car. Or, like David what is the best way to fold the optics.
It seems to me that folding the light is like running a long cable. You are going to have some line loss. Of course the light loss will probably only occur at each juncture or device its run through.
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