Secondary Size for 6" f8

I am a bit confused on how to figure the best size eliptical secondary for my Edmund White 6" f8 scope. I have heard 15% to 20% obstruction but not sure how you figure that with an eliptical vs. a round primary.

What quality should I be looking for?? Will a 1/10th wave be OK or are substantially better results had with higher quality.

Thanks

Bruce

With the elliptical secondary the measurement of interest is the minor axis. That corresponds to the obstruction diameter when it is mounted at a 45 degree angle.

I figure the actual size by looking at the illumination levels off-axis and considering the type of observing that I would like to optimize for. Mel Bartels has a calculator at

http://www.efn.org/~mbartels/tm/diagonal.htm

But SEC makes the comparison easier (see middle of page at)

http://skyandtelescope.com/resources/software/article_328_1.asp

Probably both 1" and 1.3" would be reasonable depending on your preferred type of viewing. 1" is a little aggressive, 1.3" a little conservative. Improvements to better than 1/10 wave would be very subtle or nonexistent, depending on your primary. The better the primary, the more I would be willing to spend extra on the secondary.
--
Mike