Re: [ATM] Classical cassegrain secondary

["Bob May" <bobmay@nethere.com>]

Right now, the two surfaces have a hole in the center between
them.  One is the correct ROC (the concave should be) and the
other, convex, surface has a slightly longer ROC.  As you know,
each ring is a half wave of difference between the two distances
so count the number of rings (best to count from one edge to the
other and divide by two if you can't get the spot in the center)
and put that difference into the Sag. formula for generating the
ROC and the difference between the two sag numbers will give out
with the difference in ROC.  If you've already got the concave to
the right shape then all you need to do is get the number of
rings down.
For lines, it is a bit more complex as you have to draw a line
across the glass from the end of one line to the other end.  Each
dark line that crosses that drawn line is a half wave of error.
This doesn't work properly when you are close to the center of
the rings but when you have the center off by a ways, it becomes
effictive.
Bob May
bobmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
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