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dougspeterson
Pooh-Bah
Reged: 09/01/04
Posts: 1041
Loc: Murrieta, CA
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I believe the Gregory F15 version was indeed aspherized at the corrector to remove a fifth order residual, and the F23 version left all spherical. That residual however visible in the star test may not be image destroying, like the lower order SA3 would be. Given the residual from Zemax, the Strehl could be calculated, it may well be high.
Also there is a small adjustment to the radii of the mak correctors to turn the tiny amount of primary color into secondary, discussed in Rutten, etc., Telescope design.
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18" Dob
12" SCT
8" TMB F6 + Chromacor
6" F6 APM/LZOS Fluorite triplet, 32", 32lbs
2ea. 6mm singlets, one blind
"--Granted, that's a worse case scenario. The destruction might in fact be ... limited to our own galaxy."
Edited by dougspeterson (09/23/09 02:13 PM)
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George Kiger
member
Reged: 05/19/06
Posts: 29
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I recall a response from John Gregory saying that the f/15 design did not need aspherizing, but that the need might arise when the curves did not match the design closely enough or the surfaces were not smooth enough. Also, I think the original spec and the spec modified for moving color correction to the 70% zone were for 6-inch scopes. Would the OSLO results change significantly if the scope was 6 rather than 8 inches.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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These are the spot comparisons for George's 8" f/15 Greg Mak at 0.25 and 0 deg. off axis and a wavelength range of 435-656nm. All-spherical performance is poor, and George must have had to aspherize either R1 or the primary to achieve good on-axis performance.
There is no real difference in the on-axis performance by aspherizing either R1 or the primary. Aspherizing R1 at 546nm gives slightly smaller off-axis spots, however. This makes sense as R1 is the system aperture stop.
The only way to know what George really did would be to test R1 and the primary and see which surface has the asphere.
The airspace between the corrector and primary I'm getting with R1 aspherized is 16.14" and BWD=9.25". Maximum R1 departure from sphere is about a half wavelength.
Mike
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lineman_16735
Tak-o-holic
Reged: 12/04/04
Posts: 2604
Loc: Central PA
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Thanks again Mike. I think I just need to send the entire scope to Jim at OMI and have him test it. Hopefully he will get back to me with a ball park quote soon so we can get this project moving in the right direction.
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Chris
A mount from Illinois
A scope from Japan
A camera from Cal-I-Fornia
A dog from Kentucky
A wife and kids from the "Twilight Zone"
The Geek Shed
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lineman_16735
Tak-o-holic
Reged: 12/04/04
Posts: 2604
Loc: Central PA
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Jim from OMI got back to me with a quote. $340 to recoat the mirrors, and an additional $280 to test and align the scope. I'm kind of at a crossroad here. I have no doubt OMI will do a bang up job on the scope. On the other hand I have no idea how well the scope is going to perform? I hate to spend too much money on the scope if it isn't going to be as good as it should be. On one hand I think I should just get the coatings done, and see how it goes. On the other hand I think that I may not get all the scope can offer if I neglect to have the testing and alignment done by a pro? The adapter that we made should be anodized soon and then the Moonlite focuser can be installed. I may have the meniscus and primary cells anodized or powder coated as well. The tube is some sort of plastic so I guess I will just have to paint it? On a side note, if you recall I mentioned earlier that the baffle tube had what appeared to be a 3 reference marks. I measured from the secondary spot to the center of the primary and adjusted the focuser until it was very close to 16.14 inches. This brought the the baffle tube to the center mark of the 3 reference marks. One of the other 2 marks would require the mirror to be moved towards the meniscus by about 1/4" and the other mark would require I move the primary away from the meniscus about 1/8" or so from the center mark. So I think these marks were made by George as he tested the scope. Which one is correct if any I dunno?
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Chris
A mount from Illinois
A scope from Japan
A camera from Cal-I-Fornia
A dog from Kentucky
A wife and kids from the "Twilight Zone"
The Geek Shed
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1989
Loc: Hockessin, De
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I would put the scope back together with mirror at the 16.14" position then using the new focuser on the back,at least a 15mm eyepiece, and star test it. Keep moving the mirror about 1/8" at a time until you hopefully find a place were the star test shows equal or nearly equal patterns on each side of focus indicating good correction. If you find this spot then you know the 'scope will perform well when recoated if not then it's time to do more testing to determine what the problem is before you spend the money.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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