ATM Are We There Yet?

["Bob Walker" <tac@i1.net>]

All of a sudden, my mirror looks very good--so good I'm afraid to touch it
again until I get some expert opinions!  It's a 16" x 7/8"  f/5.46 plate
glass mirror.  After my last figuring session (the ninth session since I got
it polished out completely)  initial tests looked great, so I left it to
thermally equalize over night.  It sat on the test stand, untouched for 10
hours before I made the measurements to follow.  I'm going to relate my test
methods in some detail just to make sure I haven't made some procedural
error.  I'm using a slitless tester with a blue LED 1/4" in diameter. The
Knife covers 1/2 of the LED (just estimated by eye). The return beam is
never more than 1/2" above the LED.   I'm using a four-zone Couder mask
copied straight from Texereau,  just doubled in size to match my mirror.  As
I move the knife, the various zone openings in the mask do not darken
evenly; the shadow starts from the right (my knife is on the right) and
progress to the left across the zone as I move the knife in to the left.  To
make things a little confusing, the shadow frequently enters and leaves the
pair of zone openings at different times.  It will, for instance, enter the
right Z3 opening before it enters the left opening, but travel completely to
the left edge of the left Z3 opening before it reaches the left edge of the
right opening.  The knife distance at which the shadow enters each opning
simultaneously is quite different from the distance at which the shadow
covers both openings simultaneously.  I therefore made my measurements at
the place where the shadow reached the centers of both openings
simultaneously.  The central zone presented some problems in judging its
darkening for the same reason, and also because it is a bit lumpy.  I
regarded it as if it were two mask openings side by side, as most often the
shadow would start to appear simultaneously in two areas in the center zone.
Following my earlier technique, I judged when the shadows were
simultaneously halfway across the right half of the zone and halfway across
the left half of the zone.  This also seemed to be the point at which the
zone as a whole darkened most evenly.  For this last session, I took a set
of measurements with the mirror's edge mark at 0 degrees, 90 degrees (going
clockwise), 144 degrees, and 216 degrees. I measured the distance from the
knife to the center of the mirror with a tape measure, with the knife set at
the Zone 1 distance, and got 4432.3 mm.  I subtracted the average of the
four zone 1 readings, .5144 mm, and entered the resulting 4431.7856mm in
Sixtests as both the Source Distance and the Knife Edge Distance.  For the
Zone data, I entered the four zone center radii:  35.5 mm, 101 mm, 149.8 mm,
185.3 mm, and the averages of the four readings I took for each zone:  Z1:
.5144 mm,  Z2: 3.1115 mm,  Z3: 5.0165 mm,  Z4: 8.4011 mm.   Turning the
crank revealed a Strehl of .910 and an RMS surface error of 13.4 nm.  This
implies a 1/50 wave mirror!  Did I really do that, or have I overlooked
something??  My next step is going to be to rig some sort of 'scope and do
some looking at stars.  It's supposed to be clear tonight but in St. Louis,
"clear" is a highly relative term.  Since I haven't even begun building the
telescope this mirror will reside in, I'm wondering how crude the test scope
can be and still give some sort of meaningful results.  Could I get away
with using my Foucault test stand with the two pieces of terry cloth from my
grinding post to cushion the mirror, and just clamping a 2x4 on to hold the
diagonal and eyepiece?  I know I won't have time to build a proper mirror
cell today.  I guess I need to Zen-out grinding a few more mirrors.  I still
have a pretty bad case of "I Want It Now".

 *  Best regards, Bob
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