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Re: ATM 16" Acquired - Lower Standards for Larger Mirrors?
Tom,
If you can press the 10 inch polishing tool into contact, working with
tool on top,
you want to imagine that you are pushing that hill over the edge of the
mirror but
beware of too much overhang.
I'd try starting with the tool centered and push glass over the edge until
I got a
max of 1 inch overhang, do 3 or 4 of these then rotate the mirror and walk
a bit
before doing it again. Try 3 trips around the barrel and look at the
Focault test
to see your progress. By concentrating on the raised portion and avoiding
getting the center of the tool too far out, you'll end up just fine. Then
you can go
for the accuracy you want to invest your time in.
Good luck,
Ken Hunter
At 04:10 PM 12/11/00 -0700, you wrote:
>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
Happy Grinding,
Ken Hunter
http://www.geocities.com/atm_ken_hunter/
ICQ 1189536