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Erfle Question

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Posted by Ian McKernan on December 21, 2001 02:06:33 UTC

Hi All! Just got a 38mm Erfle yesterday. Tried it out last night and am very pleased as it gives a 50% wider true field of view than ive had before. Am a bit confused about the numbers though. The manufacturer quotes a 60 degree field (i assume this is apparent field of view) in their advert. I measured the inside of the barrel (which is the best i know how to determine the field stop diameter) and using AFOV = 57.3 * d/f i come up with 41.32 degrees. On my 8" f5 that gives me 1.57 degrees true field of view by calculation. I did the "how many moon diameters" thing, and also compared distance (in degrees) between stars in Orion and everything agrees that i am getting just over a degree and a half TFOV. According to the manufacturers specs (60 degree field) i should get just over 2 degrees. Couple of questions. On my 8" f5 would a field like that be wasted due to vignetting / distortion? Have i made a wrong assumption about measuring the field stop diameter?

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