[ATM] star testing uncoated mirrors, and trepannind secondary.

[Douglas S Angle <Douglas.S.Angle@can.dupont.com>]

At 11:41 24-04-04 -0500, Jerry B. Hillman wrote:
>I got no responses to my question about testing this telescope with both
>mirrors uncoated. Either the answer is obvious, or no one knows so I will
... or I'm not on-line every day.

Other's have pointed to the 4% of 4% problem, but I think that answer is
incomplete.  If each mirror reflects only 4% of the incident light, then
the combined effect is 0.16%, or a reduction by a factor of 625.  However,
your primary is also collecting and concentrating light, probably more than
that amount.  I don't remember your mirror parameters, but I tried this
with our club's 24".  It has a light collecting area of about 10,000x, so
the combined effects of bare glass and 24" aperture are a brightening by
10,000/625 = 16x or about 3 magnitudes.  My setup was a little rickety, so
I didn't actually conduct a star test.   I can attest that the moon was
painfully bright.  It also had a double image - one reflection from the
front of the secondary, and one from the back.  Make sure you know which
image you are testing.

Also, on trepanning the secondary, there was a comment that sandwiching
between layers of plate glass insures no chipping.  We sandwiched it this
way, but still had edge chipping.  I think this was due to a couple of
factors:
- we used parafin wax, which didn't cement the glass together.  As the
grinding progresses, the plate separated from the mirror.  The suggestion
in an earlier posting was to use beeswax, which may have worked better in
our case.
- the care in setup.  We used a piece of stove pipe (big secondary), which
had a rough brazed joint, and wasn't perfectly round.  As the pipe wobbled
around, it was clearly adding stress in directions we didn't want.  This
probably contributed to the wax separating from the glass too.
- pressure.  I don't have any particular knowledge or advice, but I suspect
if there is too much pressure, there will be chipping out as the cutter
pushes through the last bit of glass, even overcoming the support of the
wax and plate glass.

Photos of the diagonal construction are at
http://astrosurf.com/jwisn/diagonal.htm.

Doug



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