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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1985
Loc: Central Texas
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I'm considering an 18 point mirror cell design for my 18" x 1.6" f/4.2 mirror that uses a triangle of welded square tubing where the mirror support triangles are supported by bars balanced along side the main triangle.
Somewhat along the lines of this:
http://pw2.netcom.com/~ahighe/17_5ultra.html
My question is 2 parts:
1. What is the optical consequence of the the primary mirror being off-center laterally (side to side) with respect to
the mirror box? Does the tilt of the secondary compensate?
Say I was off .25 - .5" ?
2. My ability to place the mirror support triangles exactly where PLOP wants them is dependent on my ability to cut and weld the main frame steel tube triangle exactly.
If my mirror support points were off be .25" , how much will the figure of my mirror start to deform?
In a nutshell, how important is the exact placement of mirror support points in an 18 point cell for an 18" x 1.6" mirror?
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David
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Olivier Biot
Amused
Reged: 04/25/05
Posts: 17277
Loc: 51°N (Belgium)
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Answer to 1st question:
When collimating, you will try to make the optical axis of the primary and the focuser coincide. If the primary is off-center from the mirror box, then you will tilt the primary so the secondary mirror will probably not be at a 45° angle. The net effect of an offset primary is not a major concern. Offsetting the secondary will only affect the illumination of the field of view.
Regarding your 2nd question, a first run with PLOP (assuming Pyrex) yields a visible RMS error of .89nm and a visible P/V error of 7nm for an 18 points cell. The support circles are at 38.67% and 79.59%.
If I allow both support radii to vary +/- 8% (~16mm or ~2/3") and both support angles to vary +/- 5° to allow for some construction tolerances, then I get the following extremes:
| | Radius of support circle | Angle of 1st point of support circle | RMS error |
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| | inner circle | outer circle | inner circle | outer circle | RMS error |
|---|
| Minimum | 31% | 72% | -5° | +10° | 0.96 nm |
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| Maximum | 47% | 88% | +5° | +20° | 4.74 nm |
|---|
I believe this 18 points cell is quite sturdy to construction errors 
Cheers,
Olivier
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Computer entomology: study of computer bugs
Tal-200K (#199) with JMI NGF-Mini2M focuser on GEM3 • Astro-Tech AT80ED • Orion Sirius EQ-G with wireless EQDIRECT • Astro-Tech Voyager • Celestron Regal LX 10x42 • Helios 15x70
ATM 14" f/5 (redesigning) • ATM 10" f/6 Portable Truss (polishing) • ATM 10" f/25 Dall-Kirkham (optics)
AstroForecast
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Olivier Biot
Amused
Reged: 04/25/05
Posts: 17277
Loc: 51°N (Belgium)
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If you would like to run these tests for yourself, go to the Cell Design tab and select "Scan range" for each of the 4 parameters.
Or load the following PLOP input file:
Quote:
;18 point cell with fixed angles
diameter 456
thickness 40
focal-length 1920
n-mesh-rings 20
rel-support-radii 0.386654 0.795916
num-support 6 12
support-angle 0 15
basis-ring-size 6
basis-ring-min 0
obstruction-diam 0
scan-var rel-support-radii0 0.31 0.47 5
scan-var rel-support-radii1 0.72 0.88 5
scan-var support-angle0 -5 5 5
scan-var support-angle1 10 20 5
part triangle 6 point 0 0 point 1 0 point 1 11
part bar 3 part 0 0 part 0 1
Don't forget to select a file for saving the output:
--------------------
Computer entomology: study of computer bugs
Tal-200K (#199) with JMI NGF-Mini2M focuser on GEM3 • Astro-Tech AT80ED • Orion Sirius EQ-G with wireless EQDIRECT • Astro-Tech Voyager • Celestron Regal LX 10x42 • Helios 15x70
ATM 14" f/5 (redesigning) • ATM 10" f/6 Portable Truss (polishing) • ATM 10" f/25 Dall-Kirkham (optics)
AstroForecast
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1985
Loc: Central Texas
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Oliver.
I never expected to receive such a specific detailed answer.
I think I can speak for quite a few people who frequent Cloudy Nights when I say you provide some invaluable help and information .
Thank you!
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David
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Olivier Biot
Amused
Reged: 04/25/05
Posts: 17277
Loc: 51°N (Belgium)
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Thanks David
--------------------
Computer entomology: study of computer bugs
Tal-200K (#199) with JMI NGF-Mini2M focuser on GEM3 • Astro-Tech AT80ED • Orion Sirius EQ-G with wireless EQDIRECT • Astro-Tech Voyager • Celestron Regal LX 10x42 • Helios 15x70
ATM 14" f/5 (redesigning) • ATM 10" f/6 Portable Truss (polishing) • ATM 10" f/25 Dall-Kirkham (optics)
AstroForecast
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sixela
Postmaster
Reged: 12/23/04
Posts: 11497
Loc: Boechout, Belgium
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Yeah. If he were smaller, we could all start calling him "Kabouter Plop"  .
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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