ATM Advice on Secondary Sizing

[Brad Davy <bdavy@fred.net>]

I need advice from the list.  I'm ready to order my secondary, but I
can't decide which size.  Here are the specifics:

Primary Diameter 7 inches
Focal Length  29.62 inches
Distance to focal plane 6 inches

Primary use will be for CCD Imaging (cookbook 245).  The 6 inches to the
focal plane is based on focuser height .75 inches, .9 inches extra for
Cookbook, and .25 extra travel, for a tube diameter of 8 3/16 inches.

Secondary Size Options 1.52 inches or 1.83 inches.  The competing
concerns are obviously
1)  The 1.52 inch secondary drops off to 90 percent illumination at the
edge of the CCD field.

2)  Vignetting with the 1.83 inch secondary, plus 26% obstruction. 

I'm currently leaning toward the 1.83, but I'm not sure how much of a
problem vignetting would be.

Thanks for any advice.

Brad
bdavy@fred.net                  http://www.fred.net/bdavy