ATM Re: Secondary Size and Diameter of fully illuminated

[BRYAN LEENHEER <BLEENHEE@millersqa.com>]

Hi All!

Just was contemplating the trade-offs here with small aperture fast
telescopes.  Namely a 4" f/4 RFT.  I've written a wee spreadsheet (using
the formulae in Appendix G of Texereau), and the results aren't pretty. 
With a .46" focal plane; I get a secondary size of 1.25"!  This is a 31.32%
obstruction!

Speaking of obstruction;  What exactly is the cutoff point for toleration of
obstruction?  I'm halfway thinking of going with an off-axis design!(well,
not really).

Actually, the obstruction on a four incher is always a rough point, isn't it?
For an f/10, you can go down to a .875" secondary with .5735" focal
plane, (21.88% obstruction); and an f/12 with .875" secondary has a
.628" focal plane. Still on the high end with the obstruction, though, isn't
it?

What exactly would be the focal plane you would expect from a scope
this size?  Texereau suggests a focal plane of .43", but on Mel's page,
he suggests a 1.91" focal plane! (the calculation gives really, REALLY
scary numbers then...!)  Which should I believe, or should I just reverse
engineer to a standard size secondary the focal plane size?

Later!

Bryan