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[ATM] Rebuilding the mirrorcell
Hello all,
I am rebuilding the lightweight Dobson I built in 1993 (12,5 inch, f 4,8
Thickness of primary = 2,00 inch, made by Galaxy Optics).
I have problems with the collimation, probably because of the mirrorcell.
At this moment the mirrorcell is 9 point and is a crude version of the
Kriege design.
I have not used PLOP yet, but after following the ATM-forum for some months,
I expect that 9 points is far more than necessary. (I understand that a 16
inch of 1,5 thickness can be placed on a 6-point, though properly built).
Question 1: What do I need for my new mirrorcell? 6-point? 3-point?
Question 2: are there any good examples described in words and images on the
net?
Question 3: I once read that someone advised to place a mirror on plastic
Bubble-foil (not only for Figuring, but also for mirrorcell. I remember this
was a person form Scotland). What do you think of that?
Very many thanks
Greetings from a half-cloudy Holland
Harro Treur
-----Oorspronkelijk bericht-----
Van: Jeff Anderson-Lee [mailto:jonah@eecs.berkeley.edu]
Verzonden: donderdag 11 november 2004 8:28
Aan: Jerry B. Hillman
CC: ATM
Onderwerp: Re: [ATM] plop
Hmm. What's the f-ratio on that mirror? Six points is likely enough.
Nine is harder to construct well. And yes, it should be easy to support.
6-pt f/4 P-V=1.22e-5 RMS=2.15e-6 r=0.581 refocus=-0.028mm
6-pt f/4 P-V=1.83e-5 RMS=2.87e-6 r=0.625 refocus=-0.016mm
6-pt f/4 P-V=2.07e-5 RMS=4.29e-6 r=0.687 refocus=0
6-pt f/5 P-V=1.23e-5 RMS=2.10e-6 r=0.578 refocus=-0.041mm
6-pt f/5 P-V=1.82e-5 RMS=2.86e-6 r=0.625 refocus=-0.024mm
6-pt f/5 P-V=2.01e-5 RMS=4.16e-6 r=0.684 refocus=0
At the no-refocus setting, the RMS is fine and the P-V is slightly high
but you would probably never notice.
At the refocus setting, the P-V and RMS are both very good but the
refocus distance is slightly more than the focus tolerance (not that you
are likely to notice).
Part way in between still gives very good RMS, with a fine P-V and
refocus tolerance.
Any of the three (or in between) is likely to be fine.
To give some perspective, 1.70e-5 is about 1/30th wave surface or 1/15th
wavefront on the P-V, while 4.25e-6 is about 1/120th wave surface or
1/60th wave wavefront on RMS. Unless you paid beaucoup bucks, this is
likely very significantly better than you mirror, even if it claims to
be 1/10th wave.
Jeff Anderson-Lee
Sacramento, CA, USA
Jerry B. Hillman wrote:
>Hi All,
>Okay, everyone knows I am mathmatically challenged. I am redesigning
>my OTA from the 16.5" mirror to accept an 15" x 1.5" pyrex mirror. I
>ran Plop for a 9 point cell and got the following. Visible PV error
>1.62362e-05 Visible RMS error 3.37887e-06
>
>I think this is good but I am not sure. I had the refocus turned off.
>This should be an easy mirror to support, shouldn't it? I thought about
>using the 27 point support I built for the 16.5" mirror then decided
>that it might not be a good idea. Clear skies, Jerry
>
>_______________________________________________
>ATM mailing list http://www.atmlist.net/
>
>
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