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Message: <blockquote data-quote="ronh" data-source="post: 1347614" data-attributes="member: 55514">I think Ron's argument about the booster flaws not being magnified is good.  I can think of a little more circumstantial evidence, which you will have to judge, since it's hardly proof, that the technique is valuable:1) The booster is at a great optical advantage, since only the central part of it's optics, corresponding to the size of the binocular exit pupil, is being used. A 50mm f/4 finder scope, for example, would be working at f/40 when testing a 5mm exit pupil. It's is likely to be sensibly perfect in that case, if reasonably well made.2) Different binos, and different barrels in the same bino giving different results argues very strongly that the aberration is dominated by the bino, not the booster.In the (it seems incredibly unlikely, but could happen) event the binocular and booster had significant but cancelling errors, I think you'd just be burnt and not know it. Rotating the booster about its axis would help reveal this if the errors were asymmetrical, but if the errors were circular, like spherical aberration, gee, what could you do?I tried my 7x50 Fujinon tonight, on Vega, using a good 8x50 finder scope. The bino was mounted, but I had no way to mount the booster, and just held it up, lens lightly against the rubber eyecups. It was shaky but not too bad. With 16mm stops on the bino, corresponding to daylight conditions with the eye shrunk to 2.3mm, Vega looked very neat. I couldn't make out a diffraction pattern, but the spots in both barrels were neat and circular, as far as I could see with my poor setup. With the stops removed, both sides showed considerably larger and comatic spots, with bands of color inside the comatic tail. That's about the flavor of result I was expecting, but it seemed a little funny that these spots looked quite similar in the two barrels, with the comet tails pointing up and to the left in each. I wish I'd had the presence of mind to rotate the finder scope at that point but didn't think of that until now. I'll try again soon, and do that to see if the booster is contributing, or if it's really the case that the two barrels are flawed, but very similar. I need to figure some kind of shelf to support the finder scope behind the binocular.Ron Harper</blockquote>

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