ATM First Light ... uh, kinda...

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

Well, after a month of chasing a dizzying array of troughs, hills, peaks,
valleys, summits and crevasses across the face of my 16" f/5.4, my
frustration level had risen faster than my Strehl ratio by a substantial
factor.  I got the Strehl up to a mighty .636, and then everything I did to
improve it drove it inexorably downward.  Squinting at the Couder mask just
about drove me nuts.  I  finally decided to get out of the basement and look
at something real for a change.  I grabbed some clamps, two-by-fours, the
mirror stand for my tester, borrowed the star diagonal from my 8" SCT (yeah,
storebought) and threw together a truss Dob.  This contraption gave a whole
new definition to the term "kludge".  Baffling? Yeah, I'm baffled, all
right.  Collimation?  We don' need no stinkin' collimation!  So I carted it
out into the front yard, right under the streetlight, so I could see what I
was doing, and pointed it at the moon, which was just about the only
celestial object visible at all.  Well, folks, it looked great.  I don't
mean just better than cheap opera glasses, I mean great.  It looked as good
as the view through my SCT, at least at 75X, the highest I went.  I figured
with my collimation-by-accident and using only a 2" diagonal, I was probably
using only the center 3".  Still, the image looke awfully darn bright.  So,
I made a cardboard mask to knock it down to about 6".  I cut the hole off
center so I could place it in four different positions.  To my surprise, it
knocked the brightness WAY down in all positions.  That must mean I'm not
vignetting much, and really am seeing all or most of the mirror.  So, my
question to the list is:  How is this possible?  Mind you, this mirror is
uncoated, and sports a Strehl ratio of .045.  Yes, forty five thousandths.
How is it that I can see anything but a big blur?
In a closely related issue, Sixtests now shows that I've finally planed down
the center to the same level as the viciously turned down edge.  That's the
good news.  The bad news is that there's a huge hump between them, with its
peak centered at 150mm radius. At least it's a nice smooth curve.  Anyone
care to suggest how I might shear it off without digging out the center or
turning the edge down more?
Well, that's enough catharsis-by-correspondence for now.  I think I'll give
the recalcitrant glass a couple of days to reflect on the harsh words I
applied to it after the last test before I resume massaging it.


 *    Best regards,
*          Bob
         *          *    *

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