Quote:
Originally Posted by Stevo69
Thanks Mark, I remember you said in passing comment that a 3.1" should be okay. I was going to get back to you on that. Good to see both my poor maths skills and poor knowledge of optics have not let me down this time around!!
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Another consideration in choosing a secondary size is if you want to use a paracorr you will need to bring the focal plane out another 15-20mm, which will demand a larger secondary.
In the designing of my 15" f4.8 SDM, the calcs suggested that a 2.6" sec was just big enough, but once I discovered that I like using a paracorr, it would not be. I'm glad that I went the next size up and at barely over 20% secondary obstruction, Im not losing anything.