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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Jim, A year or two ago I helped a guy out in in the South end of Utah Valley with his new 9-inch f/20 Maksutov; he was having trouble using it, for good reason. Magnification was quite high for every eyepiece he put in (even my 35mm Panoptic!), and consequently all the fields were narrow, which made hunting deep sky objects very difficult. This type of scope is very specialized; it's ideal for planetary observing or splitting faint double stars, but not good for general purpose use, because some of what's out there beyond the solar system is rather large -- and is only best appreciated if you have a wider field of view to put it in (the great galaxy in Andromeda for example actually is so big you might find the "best" views of it in binoculars). So a simple answer to your question begins with a question. What do you want to do with your scope? If the answer is "as much as possible", then you need to find a more moderate focal length, and that's why scopes in the f/4.5 to f/8 region are the most popular. If your passion is detailed planet observing, a longer focal length is probably what you are after. Every choice is a compromise, and that's why there is no "one ideal telescope", and why many of us end up with more than one! :-) -Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:
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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