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Re: Focal Ratio

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Posted by Tom on March 23, 1999 10:40:35 UTC

: What is the significance of the focal ratio for the objective mirror,except of the length of the tube? : f/6 or f/7 meens also smaller or bigger enlargement of the observed object? : Thanks!

Steve: Like all aspects of optics, you are dealing with trade-offs. Shorted focus mirror have become popular in the last 20 years because it allows a larger diameter telescope to remain somewhat portable. The trade off is, with shorter focal ratios, abberations like coma and astgimatism increase, and inexpensive eyepieces have problems dealing with the steep light cone. Because of these, short focal length mirrors need more expensive eyepieces to help keep stars sharp accross the viewing field especially at low power, because the "abberation free" area of view is smaller as the mirror gets "faster". "Fast mirrors are VERY sensitive to alignment and most fast scopes I look through are off enough in alignment to make noticable errors visible in the eyepiece. Many fast mirror owners blame the mirrors on the bad images, when in reality a few minute of "tweeking" sharpens the image greatly. My 16" F/4.5, when properly aligned, will give high power views of Jupiter that would knock your socks off - yet my 6" F/8 refractor has shows stars that are "pinpoint sharp" with just about any eyepiece. The other end of the "trade-off" is portability. You will not gain much optically, with a mirror longer than F/10 except a scope that is unwieldly and difficult to mount ridgidly - a 12" f/10 scope would be 12 feet long!!! On the lower end I wouldn't go shorter than F/4.5, cause your eyepieces would end up costing as much as your mirror. My advice is for scopes 6" and smaller, f/8 is great, 8" - 12.5" f/6 will give great all around performance, yet fit in most cars if built carefuly.

Hope this answers your questions.

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