To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 


Scott,
  You were completely right.  I did about 3 two-minute sessions 1/4 to 1/3
diameter, wide W, and it walked right toward a sphere. You can see the
latest Foucaultgram at:
  http://host67.everstrive.com/~mlbrown/ASTRONOMY/Foucaultgram22Mar8pm.jpg
The image shows more TDE than my eye says, and I can see a slight
diffraction ring on the KE side that isn't in the photo. Ronchi shows the
straightest lines I've ever seen, with a little apparent TUE followed by
just the slightest hint of TDE.

I did some measurements, using the 3 zone mask I'd already made. Sixtests
says Strehl 0.862 RMS 16.8.

Tex says 1/4.6 wave:
           Comments:    Optical diameter: 4.15
   Readings per zone: 4
Radius of curvature: 76.625
                 f/D:  9.23
    Diffraction disc: 0.00024328

  1 ZONE                1          2          3
  2 h(x)              1.1980     1.6942     2.0750
  3 h(m)              0.5990     1.4461     1.8846
  4 hm**2/R           0.0047     0.0273     0.0464
  5 hm/4f             0.0039     0.0094     0.0123
  6 D1                0.1445     0.1507     0.1205
  7 D2                0.1445     0.1507     0.1205
  8 D12               0.1445     0.1507     0.1205
  9 D12 -  0.0956     0.0489     0.0552     0.0249
10 Lamda c           0.0443     0.0279    -0.0214
11 Lamda f * 1e5      17.30      26.33     -26.33
12 Lamda f / rho      0.711      1.082     -1.082
13 u * 1E6            -4.52      -6.87       6.87
14 Wavefront          -3.34      -4.68       0.00
         Reference parabola: y = -1.44071 * x**2 + 0
         passing through (  0.00,  0.00) and (  2.08,  0.00)

Maximum wavefront error = 1 / 4.6 wave at zone 2

Results for individual sets of readings:
         Reading set 1: 1 /  4.6 lamda, Max Trans Abberation=1.16
         Reading set 2: 1 /  4.8 lamda, Max Trans Abberation=0.99
         Reading set 3: 1 /  4.6 lamda, Max Trans Abberation=1.05
         Reading set 4: 1 /  4.5 lamda, Max Trans Abberation=1.12

So I plan to do a little more of the same wide stroke, to move it a little
more toward a parabola.  I'm not planning on perfection on this one -- I
really need to get it aluminized and in the mail so the scope can be made
while Jupiter and Saturn are still easily visible.  The 1/4 wave
measurement says I'm done in that regard, but I don't really trust those
numbers, not when I can still see that TUE.  I think I'll take it to
something like 1/8 lambda by measurement to know for sure.

I'll be a perfectionist on my next mirror (a 4.25" f-6 waiting for polish now) .

=Matt


>Delivered-To: mlbrown{at}0
>From: "scottythefiddler" 
>To: "Matthew L. Brown" 
>Cc: "ATM" 
>Subject: Re: ATM Getting less edgy
>Date: Fri, 21 Mar 2003 12:02:59 -0500
>X-Mailer: Microsoft Outlook Express 6.00.2800.1106
>
>Matt,
>
>the hole in the center is a sign that you are reducing the hill.   Further,
>the edge is better than it was in the first focogram you showed.
>
>Right now, the mirror is oblate...i.e. it has 'correction', but in the wrong
>direction.
>Try a parabolizing stroke, MOT, wide side overhang, 1/4 diameter, and the
>same overhang top and bottom.  Do 3 minute spells.  Expect the edge to start
>looking worse, before it starts looking better.  With MOT, the figuring
>action may take place on the outside of the TUE, which will make the TDE
>appear to getting worse.  The TDE will will start to improve when the TUE is
>almost gone.  It might take 10 or more 3 minute spells to get to a sphere,
>or it might take 2!  Take it in small steps, and you won't overshoot your
>target.  Bear in mind that it might take 15 minutes to get rid of half the
>TUE/TDE, and then 2 minutes to finish it off.....
>While figuring a 10" mirror, it took 7 steps to reduce the abberation in the
>middle zones by about .040", and then 1 step to reduce it the last
.015".
>The key is to take measurements after each step, so that you can see your
>progress.   As long as there is progress, continue with the same stroke.
>
>Best of luck,
>Scott Donaldson

--- BBBS/NT v4.00 MP