oversize secondary mirror

[rajiv 1972]

Does a secondary mirror of 50 mm makes a big difference in diffraction and coma VS a 30 mm for a primary mirror 188 mm newt f/6.53,and distance from the secondary to the focal plane is 185 mm? My main interest fully illuminate an APS-c size (15.6 x 23.4mm) DSLR chip ? secondary and primary mirror , both are 1/4 to 1/8 wavelengh.

i mean 50/188 =0.26 big difference in diffraction and coma ?

[michael.h.f.wilkinson]

No difference in coma, because that is caused by the primary. Your current secondary mirror only gives about a 2mm fully illuminated circle, so really has been optimized for planetary viewing. A 50mm mirror (which should illuminate the APS-C format sensor properly) will reduce contrast a bit, but my C8 has 0.31 central obstruction and still gives very good planetary views.

[Peter Drew]

Slightly undersize secondaries are more problem optically than slightly oversize ones.

[michael.h.f.wilkinson]

Slightly undersize secondaries are more problem optically than slightly oversize ones.

Absolutely! I once bought a Konus 4.5" F=500 scope, which came with a 27mm secondary, which give the 160mm from secondary to focus meant I effectively had an 86mm scope. I first replaced it with a 40mm, which meant at least the centre was illuminated by the whole mirror. Later I rebuilt it into a mini dob for the kids, and got a bigger secondary (46mm). No problems despite a 0.40 central obstruction. It is more of a rich-field than planetary scope, but the views are good (when properly collimated).