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I tested my 18” F3.5 scope extensively with my 2 secondaries, first with a 4” 1/10th wave and then a 4.5” 1/12th wave.
After several nights with each secondary on different targets I decided to keep the 4.5” (25% obstruction) ahead of the 4” (22.2%) as the difference visually was not really noticeable as far as contrast and illumination goes and using the larger mirror means the flatness of very edges of the secondary and the actual placement isn’t as critical.
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