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Message: <blockquote data-quote="ronh" data-source="post: 1281375" data-attributes="member: 55514">Ron,Thank you so much for trying to help me. I believe you have argued, and correctly, that a 40 lp/mm target, 1.68 meters from an 8x42 telescope (assuming it could focus this close), would challenge the instrument with its 360 lp/mm in the focal plane. 540 lp/mm in the focal plane is the MTF extinction frequency, or "resolution" of the telescope if it were perfect. I follow your math, and that was a useful exercise.Now, if I can sort of guess what you're thinking but did not quite say, it seems reasonable that this challenge frequency might fall on the MTF curve down around the 25% of maximum contrast level, shown in the paper for "40 lp/mm" in the plot of best prism glass and best coating.Yet I am still puzzled. My simple assumption that by "40 lp/mm" they mean "in the focal plane" cannot be correct, because good binoculars are not down in contrast appreciably at such a low frequency. But, what bothers me about what you did is the rather strange 1.68 meter distance required to reach the desired conclusion. That's not an industry standard, is it? It sounds like you cooked it up to make 40 lp/mm in the object plane do what you wanted it to do. Still it works, I have to admit. If the authors of the paper were photographing the image plane, they would not have been limited by a binocular's focusing ability--they'd have had just the objective and prism out on the table, so could have certainly focused at 1.68 meters if they wanted to. It is, in fact, a convenient distance for a bench test. But without stating it explicitly, they have avoided telling us anything about the absolute performance of the system they measured, so the plots are reduced to unnormalized, relative, touchy-feely type information.So, maybe you have figured out what they did! 1.68 meters, and 40 lp/mm in the object plane! Would you agree that they didn't SAY what they meant, and that it wasn't obvious? Ron</blockquote>

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