secondary mirror size

I am building a 12.5" f/4.7 scope that will be used for both ccd and visual. I am trying to decide on whether to get a premium 2.6" secondary or a not so nice 2.76"/2.9". We are talking about a difference of 1/20th pv wave for 2.6" and 1/10th pv wave for the others(maybe 1/15th). How much difference will there really be? Should I sacrifice size for quality?
Need help coming to a decision.
Thanks,
Todd Gray :S

Can yougive us some more specs? How wide is your tube, how tall is your focuser, how much extra in-travel do you need for the CCD, and how big is the chip?

Assuming a 14" tube diameter, a 1.6" tall focuser, and an extra 0.5" of in-focus travel for the CCD to reach, you have a 100% illuminated field of about 0.7" with the 2.6" secondary. That goes up to just over 1" with the 2.9" secondary. If the 0.7" will fully illuminate the CCD chip, then I would take the 2.6" secondary. If it won't, get the bigger one.

If your tube is wider, focuser taller, or you need more in-focus for the CCD, then you need to recalculate the fully-illuminated field based on your measurements. Download Newt 2.5 and enter your specs to see what size secondary you need to illuminate your CCD chip.

Jarad

Todd,
I can only say that after replacing my original 1/10 wave 2.6" in my 15" Dob with a 1/23 wave 2.6" Fuse Quartz diagonal, that all who have looked through the scope say it made the most significant difference of any modification that I have made to the scope.
I also replaced the 2.6" in my 12.5" Dob with a 1/13 wave Pyrex unit.(Picture is of the test on this mirror) Both of the mirrors are from Antares Optical and have full interferometer documentation. This was very dramatic as well and has given me the ability to reach much higher magnification with this scope.
The central area on both mirrors is quite a bit better than the rate p/v by the way and they are 1/66 wave rms for the Pyrex and 1/143 wave rms for the Fused Quartz .
Both mirrors have Enhanced Coatings.
Floyd

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