Quote:
Originally Posted by astro744
The detrimental effects of an oversized secondary are often overstated so don't be too concerned with 42mm as it is on the smaller end for a 6" f5 but it does depend on what focuser and tube you have. SCTs have far more obstruction yet still provide good planetary views. Ideally 25% and under is optimal but at 28% I wouldn't worry about it if the secondary is good.
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I've looked through dozens of different SCT's over several decades (yes I can even go back to the crapola produced in a rush for Haleys Comet in late 1975 and 1976) and I am yet to get a lunar / planetary view in a SCT that comes remotely close to what you get from a high quality equal aperture Newtonian. The laws of physics preclude a SCT from being the optical equal of a Newtonian and that is consistent with my under the stars experience. Planetary imaging is a whole different ball game where the size of the secondary and some of the other factors that affect SCT's visually don't come into play. You might be more easily pleased than me, in which case the size of the CO wouldn't matter. If you're trying to eek out the last bit of performance for a given aperture then it does and ideally it should be under 20% and not under 25%. Under 20% is where the MTF curves of an obstructed scope approach those of an unobstructed scope. At 25% CO they aren't close. My 10"/F5.3 Suchting mirrored SDM has a 1.83" Protostar Quartz secondary and I can guarantee you there isn't a SCT on the planet that gets remotely close to it as a lunar / planetary scope. I don't even let them on the same field
It will all be academic if Stuart sticks with the original Coulter primary as it's quality is unlikely to be high enough for the secondary quality or size to make much of a difference.
Cheers
John B