Quote:
Originally Posted by Rainmaker
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 isnt as critical.
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I did the same with a 10.1 f6.4 (Suchting) with 2.1 and 1.83 secondary. Installed the 2.1 for the same reasons but also I prefer less magnitude drop at the edges in my wider field low power eyepieces. I can interchange them fairly easily.
I got the best view ever of Saturn in a 16 f5 (Suchting) but with a 2.6 secondary. Almost like having a 16 refractor. However I felt the low power wide field views were noticeably dimmer at the edges. If it were mine Id probably go for a 3.1 secondary at least maybe 3.5. Note other factors such as tube diameter, tube thickness and focuser height come into play, (also use of Paracorr).