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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1103
Loc: Fort Worth TX
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John (aka DrDyno) and I have been PM-ing on his diagonal size for his 10" f/4.5 Coulter mirror, so I thought I'd share what I came up with ZEMAX to compare with the other diagonal prediction programs. He told me he's making his own tube, which is 12.5" OD and 1/4" wall thickness, with the 10" mirror face 4" from the back end. His focuser is 2.25" high fully racked up, so I put his image plane 1/2" higher, or 9" from the optical axis. He doesn't know his EFL exactly so I assumed 45.0".
This shows that the 2.500" minor axis diagonal with an in-plane aperture offset of -0.185" (0.131" away from the focuser and toward the primary) can give an unvignetted circular field 0.57" in diameter. What do the other diagonal codes predict? I will do the 2.25" and 2.6" diagonals next.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1103
Loc: Fort Worth TX
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I ran the 2.25" and 2.600" minor axis diagonals. No need to repeat the graphics.
The 2.25" minor axis diagonal, with an in-plane aperture offset of -0.172" (0.122" away from the focuser and toward the mirror) gives an unvignetted field diameter of 0.266".
The 2.600" minor axis diagonal, with an in-plane aperture offset of -0.190" (0.134" away from the focuser and toward the mirror) gives an unvignetted field diameter of 0.70".
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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Chris Z
super member
Reged: 03/24/04
Posts: 112
Loc: Chicago, IL
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Mike,
My spreadsheet duplicates the calculation on Mel Bartel's diagonal calculator, which I think is based on the S&T caculation from the 1970's. I also took Mike Lockwood's numbers (linear interpolation). I get the following fully illuminated diameters for this geometry:
100% illuminated diamter
minor axis_Zeemax__MelB's__MikeL
2.25"______0.27"___0.30"___0.31"
2.50"______0.57"___0.62"___0.625"
2.60"______0.70"___0.74"___0.75"
Looks like Mike L's calculations are close to Mel Bartle's, both a little larger than Zeemax. Presumably the Zeemax results are correct.
Chris
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Orion SVP 120mm f8.3
Obsession 12.5" f5
Edited by Chris Z (12/17/07 09:15 PM)
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DrDyno
member
Reged: 09/08/07
Posts: 19
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Quote:
I ran the 2.25" and 2.600" minor axis diagonals. No need to repeat the graphics.
The 2.25" minor axis diagonal, with an in-plane aperture offset of -0.172" (0.122" away from the focuser and toward the mirror) gives an unvignetted field diameter of 0.266".
The 2.600" minor axis diagonal, with an in-plane aperture offset of -0.190" (0.134" away from the focuser and toward the mirror) gives an unvignetted field diameter of 0.70".
Mike
Mike,
Many thanks for your incredible presentation! The last question I have concerns the only other appropriate diagonal commercially available... a 2.1". Would appreciate your results on offset and diameter of 100% illumination... and if 2.1" would vignette the mirror.
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Regards,
John
St. Petersburg, FL
Cold Astronomy (less than 68 degrees)...sucks!
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1103
Loc: Fort Worth TX
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OK - here's the same analysis for the 2.1" diagonal.
The 2.1" minor axis diagonal, with an in-plane aperture offset of -0.164" (0.116" away from the focuser and toward the mirror) gives an unvignetted field diameter of 0.08". It basically provides 10" worth of light to an almost zero FOV.
It's WAAY too small for a 10", especially at that short f/#. That size diagonal is more in the 8" primary range. Go with the 2.5" diagonal. At 100(2.5/10)^2=6.25%, the CO is nothing, and you get a decent-sized field with full illumination.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1103
Loc: Fort Worth TX
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Thanks, Chris, that was interesting. I think you're right, the ZEMAX method gives the most accurate answer, as no approximations are taken. That's easy to see from the footprint plot on the diagonal aperture. And it's easy to do the same analysis for non-circular FOV's like CCD or DSLR chip formats.
Do the other diagonal calculators do the decentration? I was interested in what values they were giving.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 1964
Loc: California
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I ran your numbers against NEWT. Here is what I came up with
2.25" / 0.125" offset / 0.313" 100% illum diameter
2.50" / 0.139" offset / 0.625" 100% illum diameter
2.60" / 0.144" offset / 0.750" 100% illum diameter
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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DrDyno
member
Reged: 09/08/07
Posts: 19
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You guys are amazing! When I started building telescopes we used yardsticks and masking tape!! I doubt I'll ever get to the depth of optical knowledge you fellas' possess... it's a new age. Many thanks for your collective input!!!
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Regards,
John
St. Petersburg, FL
Cold Astronomy (less than 68 degrees)...sucks!
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1103
Loc: Fort Worth TX
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You are most welcome, John, and I started there with you in the Stone Age as well. I built my first scope in 1971 down in good old Corpus Christi, TX at age 19, with a 6" f/6 (my first mirror), and did all the calculations, both for the Foucault data reduction and for the tube layout, with a slide rule. Yep, I trusted a slide rule to guide me where to drill the big hole for the focuser. Went together perfectly.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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Chris Z
super member
Reged: 03/24/04
Posts: 112
Loc: Chicago, IL
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Mike,
If decentration means the impact of moving of axis, then some of the calculations provide this information (Ex: Mel Bartle's). The calculation I did is a first order approximation (described a bit below). The result is the percentage illumination as a function of radius at the image plane. Mel Bartle's web page below does this calculation and provides results (link below).
Mel Bartles Diagonal Caculator
Description of calculation
The secondary is a circle with diameter equal to the minor axis of the diagonal located in the center of the tube at the optical axis of the focuser. The circular size of the light cone from the primary when it reaches the secondary can be calculated (the distance from the primary to secondary must be specified). The overlap of these circles determines the illumination percentage and can be calculated as you move off axis. The same idea could be extended to calculate vigneting by the OTA and focuser. The impact of the secondaries shadow can also be incorproated. I've been to lazy to do all this, but intend to get around to it at some point.
Chris
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Orion SVP 120mm f8.3
Obsession 12.5" f5
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