ATM 16" Acquired - Lower Standards for Larger Mirrors?

[thomas.d.bennett@lmco.com]

Hi All,

For my second mirror project, I've acquired a 16" F4.5 commercial mirror that the owner 
said was a Canis Major (major dog) and he suspected a turned-edge so it was being sold 
cheap :-). After testing it over the weekend, its Foucault/Tex.exe test results were as 
follows:


        TEXEREAU MIRROR TEST SHEET

           Comments: (null)   Optical diameter: 16
  Readings per zone: 1
Radius of curvature: 144
                f/D:  4.50
   Diffraction disc: 11.8584 * 10^-5

 1 ZONE                1          2          3          4          5          6          
7      
 2 h(x)              3.5500     4.6000     5.4500     6.1900     6.8500     7.4500     
8.0000
 3 h(m)              2.7750     4.0750     5.0250     5.8200     6.5200     7.1500     
7.7250
 4 hm**2/R           0.0535     0.1153     0.1754     0.2352     0.2952     0.3550     
0.4144

 5 hm/4f             0.0096     0.0141     0.0174     0.0202     0.0226     0.0248     
0.0268
 6 D1                0.0890     0.1110     0.1720     0.1990     0.2500     0.3740     
0.4820
 7 D2                0.0890     0.1110     0.1720     0.1990     0.2500     0.3740     
0.4820
 8 D12               0.0890     0.1110     0.1720     0.1990     0.2500     0.3740     
0.4820
 9 D12 -  0.0160     0.0730     0.0950     0.1560     0.1830     0.2340     0.3580     
0.4660

10 LAMDA c           0.0196    -0.0203    -0.0193    -0.0522    -0.0612     0.0030     
0.0516

11 lamda f * 1e5      18.85     -28.69     -33.69    -105.45    -138.48       7.51     
138.48
12 lamda f / rho      1.590     -2.419     -2.841     -8.893    -11.678      0.633     
11.678
13 u * 1E6            -2.62       3.98       4.68      14.65      19.23      -1.04     
-19.23
14 Wavefront           0.00       0.77       1.34       8.75      18.01      13.97       
0.00
        Reference parabola: y = 0.39864 * x**2 + -9.0823
        passing through (  3.55,  0.00) and (  8.00,  0.00)

Maximum wavefront error = 1 / 1.2 wave at zone 5


There appears to be a large hill near the edge but the rest of the figure doesn't look 
bad at all (pretty smooth actually) and the edge is not turned as feared. This seems like 
it should be a relatively easy fix but am not sure what the best way is to proceed with 
this size mirror (my first was a 10"). Since the hill is so near the edge, I don't want 
to do anything that might turn it while reducing the hill. Thought I might try a 4" star 
lap attacking the hill directly followed up by strokes with a full-sized lap to smooth 
out the damage done by the smaller lap. Also still have my old 10" plate glass tool that 
could be used to make a 60% subdiameter lap if necessary. This would have the advantage 
of not having to make a full sized plaster tool / lap if it could be used without 
introducing any roughness. What's the best way to attack this problem?

Also, after many figuring sessions, I was able to get my 10" F4.6 mirror to Foucault test 
to better than 1/20 wave. Star testing indicates it turned out very well and Saturn is 
dim but focus-able, in good seeing at 1000X. I've noticed that the premium commercial 
mirror producers typically quote relaxed specifications for their larger mirrors. Will 
having a goal of getting the 16" to the same level of precision as the 10" a realistic 
one or will trying to do this result in a lifetime ATM project? 

                Thanks - Tom