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siriusandthepup
super member
Reged: 02/14/06
Loc: Central Texas, USA
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Re: Tolerance on DK Cass secondary radius
[Re: Mike I. Jones]
#5661126 - 02/04/13 04:20 AM
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OK - secondary radius tolerancing done, with results you will like. First, the prescription and tolerance table.
I just love it!
Mike, stunningly beautiful work! That tells the story way better than my puny words can express. This insensitivity of primary to secondary matching is a key benefit of the wsDK. The targets are wide, and figuring primary and secondary together is not required for success; as it usually is in construction of a traditional DK optic set.
Care in figuring to smooth and zone free high tolerance is still needed of course so as not to impact the high system design Strehl capability. Actual Strehl will be determined by how well the optics are executed naturally.
I notice too that system Strehl remains above 0.999 from plus or minus 6" deviation on target secondary radius. That is plus or minus 3" on the secondary test plate focal length. That's not too hard a target to hit: from 88.7" to 94.7" on the test plate fl.
Thank you! Thank you 
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Mike I. Jones
Post Laureate
Reged: 07/02/06
Loc: Fort Worth TX
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Re: Tolerance on DK Cass secondary radius
[Re: siriusandthepup]
#5661166 - 02/04/13 06:17 AM
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You are most welcome! Now let's start wrangling them photons through the chute!
The low secondary amplification and resulting long ROC is indeed the reason for the loose radius tolerance. The limiting case is, of course, a flat secondary mirror!
Notice the asymmetry of the WFE curves going from too steep to too flat. They suggest that it is slightly better to err from nominal toward a longer rather than shorter radius, if you can stand the shorter system EFL.
Notice also that I pulled your foldout tertiary mirror away from your secondary by 3", reducing the path to focus from 28" to 25". I don't know if you were aware of it, but your tertiary was blocking a large lune of light headed from the lower side of the primary to the secondary.
Jus' sayin,
Mike
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siriusandthepup
super member
Reged: 02/14/06
Loc: Central Texas, USA
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Re: Tolerance on DK Cass secondary radius
[Re: Mike I. Jones]
#5661492 - 02/04/13 10:44 AM
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The low secondary amplification and resulting long ROC is indeed the reason for the loose radius tolerance. The limiting case is, of course, a flat secondary mirror!
Notice the asymmetry of the WFE curves going from too steep to too flat. They suggest that it is slightly better to err from nominal toward a longer rather than shorter radius, if you can stand the shorter system EFL.
Notice also that I pulled your foldout tertiary mirror away from your secondary by 3", reducing the path to focus from 28" to 25". I don't know if you were aware of it, but your tertiary was blocking a large lune of light headed from the lower side of the primary to the secondary.
Jus' sayin,
Mike
Mike,
In many ways the wsDK is more like a folded Newt than a traditional DK, without the drawback of the super sized secondary.
I did do a full size drawing of the optical layout and specifically made sure that the tertiary was completely hiding in the secondary shadow and that the light cone from the primary did not get vignetted. The full size drawing lets me see/design the largest/longest baffles I can get away with. The tertiary baffle will come very close to the primary light beam, but not intrude. On my spreadsheet I'm running a 3.5" tertiary and the distance from the flat to the focal plane is 17" - I am going to move the tertiary back to where I had it originally I think, which was 3" higher which will allow the focal plane to extend 6" from the side of the OTA. That is to accommodate binoviewing and an Ed Jones coma corrector. At this location the tertiary will be 20" from the focal plane. Again - 3.5" diagonal size. I am not sure where the 28" number came from, but it would require a 4" diagonal to intercept the light cone at that point, and in turn you are correct that the light from the primary would suffer vignetting.
Regarding the asymmetry - absolutely correct. I did run many scenarios and there is a mathematical non-linearity which kicks in on that side and causes things to begin to change quickly. I tried to give myself a little room from that side, while still giving me the the 8 to 8.3 F ratio I wanted. If this one was being designed for photo applications then you would want to go to that side for the faster F ratio and the flatter part of the curve. Any time you design a wsDK you should analyze where the "non-linearity" starts kicking in and move away from it some to the flatter part of the curve. This is the spot that gives you the most leeway on your as-built tolerances.
Thanks again for the wonderful charts and graphs,
Edited by siriusandthepup (02/04/13 11:00 AM)
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