For a 14 inch mirror, what are the advantages/disadvantages for a focal length 4.5 vs 5.O. I know the scope length will get longer with the higher focal length. I have heard some mention the impact of astigmatism with the shorter focal length. Is there an optimum?
14" mirror-- questions concerning focal length
Started by oilmanmojo, 01/30/2004 01:36AM
Posted 01/30/2004 01:36AM
Opening Post
Posted 01/30/2004 02:48PM
#1
Tom, you are probably talking about eyepiece astigmatism. It is nearly in inverse proportion to the f#, so it would be some 11% greater in an f/4.5, compared to f/5. It is much less of a factor than coma, which would make quality field in an f/4.5 nearly 30% smaller linearly, and nearly 20% smaller angularly (counting in eyepiece astigmatism, quality field size gets another 2-3% smaller).
An f/4.5 would be also about 30% more sensitive to miscollimation. The only disadvantage of an f/5 is ~7" extra length/focus hight.
An f/4.5 would be also about 30% more sensitive to miscollimation. The only disadvantage of an f/5 is ~7" extra length/focus hight.
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