Secondary Size for 6" f8

Started by Bruce N., 05/05/2002 02:37PM
Posted 05/05/2002 02:37PM Opening Post
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
Posted 05/05/2002 05:20PM #1
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