Re: ATM BK 7 Ideas

[Anthony Stillman <atmer@flash.net>]

Snipped from Bob May's post:
>...from the same melt which is one of the requirements
>for the Mangin mirror in the Schupmann design.


It is the common wisdom that the glass for the Schupmann Medial primary and
mangin must come from the same melt.  I too, use to believe this.  Then, a
few years ago, my sophism was corrected by Gerry Logan while I stood in
front of his 7 inch Schupmann.  He hadn't used glass from the same melt and
his telescope worked (works) fine (actually, great.)

How closely matched do the glass need to be?  After Gerry challenged my
certitude with pesky facts, I went to the ray tracer.  Using James A.
Daley's prescription for a 6 inch Schupmann (ATM Journal Issue #1 p30), I
explored using different glasses altogether for the two elements.  The
index and dispersion of the primary and mangin do need to be close, but
Schott's ordinary melt to melt tolerances are quite within the needed
bounds.  Consequentially, two different BK7-alike glasses from two
different companies may also work.  For instance, using cataloged values, a
BK7 primary with a BSC2 mangin, or a BSC7 mangin, or a BSL7 mangin, all
perform well.  Only slight changes in the location of the focal plane, and
no other changes to the prescription, are needed.


Anthony