A software package for designing newtonian scopes. You can download it free from
http://home.att.net/%7Edale.keller/a...t/newtsoft.htm
Its a great tool and way to learn about the way scopes work, if nothing else.
I would hazard a guess that GS use the same size secondary in most of their scopes. A 2" would be about 17% in a 12" (about perfect), 20% in a 10" (OK) and 25% in an 8" (too large). Hopefully they don't use it in the 6"!! It would be a way for them to keep costs down when making the scopes by only having one size for all scopes.
The 1.5" one I now have is an 18.75% obstruction by diameter which is about perfect for the rule of thumb of between 16-20%.