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I am also looking at making a DIY ONAG using a 50mm square cold mirror from Edmund.
6 lambda wavelength for the surface accuracy does not sound good, however the on-axis light cone at that point is only about 10mm ( if the CCD chip was 50mm away at F5). Being so close to the CCD may mean the 6 lambda is irrelevant in practical terms. (wishful thinking here)
Biggest issue with these is the increase in back focus required which probably means for a Newt that the only coma corrector suitable is the RCC1 with 90mm back focus.
As Peter said another mirror to collimate and adjust.
For $90 odd dollars I think its worth a gamble.
Regards
Bill
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