siriusandthepup
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Reged: 02/14/06
Loc: Central Texas, USA
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: siriusandthepup]
#5657307 - 02/01/13 10:44 PM
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I guess the moral of this story is to do your primary first on a wsDK and measure its correction carefully. Then you have tons of latitude with making your secondary.
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MKV
Carpal Tunnel
Reged: 01/20/11
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: siriusandthepup]
#5657763 - 02/02/13 09:26 AM
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In the interest of full disclosure, the tolerances are actually more generous than I have stated. I was just being conservative with the +-5 inches.
Ok, thanks for clarifying this. But since we're on the design again, I looked at your original chart and noticed that you list your primary as having a positive curvature. Could you clarify that?'
Using standard optical annotation convention, both mirrors will have the curvatures of the same sign.
Mladen
Edited by MKV (02/02/13 01:10 PM)
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MKV
Carpal Tunnel
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: siriusandthepup]
#5657826 - 02/02/13 10:03 AM
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I guess the moral of this story is to do your primary first on a wsDK and measure its correction carefully. Then you have tons of latitude with making your secondary.
Ed the way it's done is:
1. make your primary as close as possible to the required figure
2. make your secondary spherical
3. assemble the telescope
4. test it at the focus (preferably by autocllimation rather than stars)
5. touch up the secondary until you have a perfect null
Hint 1: make your primary very, very accurately to the desired radius of curvature. Readjust your design parameters to match the final radius of curvature, then -- if you can -- perform a knife-edge test at the conjugate foci until you get a clean null. At that point you have the exact figure on your mirror. Otherwise get a good Foucault tester and try to get as close to the figure as possible. BW, conjugate foci testing requires lots of windless space! It's a nice theoretical method for small mirrors but not for anything over 10 inches in diameter. A 12-inch f/4 may require as much as 30 feet!
Hint 2: proceed to make the secondary to match the new adjusted figures based on the primary. Polish it and figure it spherical using a test plate. At this point the two optics should be close, which is why you still need to "tweak" the primary until you have a perfect null at the focus of the complete scope.
Mladen
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siriusandthepup
super member
Reged: 02/14/06
Loc: Central Texas, USA
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: MKV]
#5657914 - 02/02/13 11:04 AM
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That's my lack of optical design experience showing. I could have easily flopped the sign. In my way of thinking (wrong BTW) a concave mirror (primary) has a positive sign and a concave mirror (secondary) a negative sign. That is reflected in my spreadsheet. I understand now that in the conventions used in the optical design software the direction of the curves is the deciding factor and since the curves of both the primary and the secondary are in the same direction left to right then the signs should be the same. Please correct my rookie mistakes and I will try to learn and readjust my thinking.
Also (again my lack of experience) the way I thought about primary mirror production is referencing a parabola at conic -1. In using the Foucalt test I was thinking that it was useful to generate target numbers reference the parabola full correction numbers. Hence 70.4% of the full parabola reference numbers for the DK ellipse correction targets.
I am listening to your comments with the intent to learn the proper way to do optical design. At least that way I can learn and be on the same page with the rest of the optical design community.
Thanks for all your help!
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Mike I. Jones
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Reged: 07/02/06
Loc: Fort Worth TX
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: MKV]
#5657919 - 02/02/13 11:07 AM
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MAJOR correction to replied post, lest mirrors be made incorrectly!
Directly per Malacara, "Optical Shop Testing", Foucault/caustic test figuring strength is the negative of the conic constant!
Figuring strength is NOT eccentricity!
If the DK formulas calculate a primary eccentricity e=0.83904708, the conic constant is -e^2 = -0.704. Tested at COC with Foucault or caustic methods, the mirror figuring strength is 0.704, or 70.4% the correction an equivalent paraboloid would require.
The mirror correction is NOT 83.9%!!
All, please be careful when posting formulations here, and double-check with references and for accuracies and typos. A mistaken formula can mislead someone into spending a lot of time figuring a mirror incorrectly, only to find it out after expensive coatings have been applied, and during final assembly and initial star testing.
Thanks,
Mike
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siriusandthepup
super member
Reged: 02/14/06
Loc: Central Texas, USA
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: Mike I. Jones]
#5657966 - 02/02/13 11:33 AM
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My primary is completed and coated at 70.4% of a parabola.
I sure hope that is right.
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MKV
Carpal Tunnel
Reged: 01/20/11
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: Mike I. Jones]
#5658052 - 02/02/13 12:34 PM
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Ok, thanks Mike. Apologies for mistaking the 70.4% meant to mean deviation from the sphere (which is what eccentricity, e, is). Just another reason to stick to conventional terminology i.e. conic constants, instead of %.
Mladen
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MKV
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: siriusandthepup]
#5658083 - 02/02/13 12:49 PM
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Sorry I misunderstood your terms, Ed. Your mirror has a conic constant = -0.704 and that is correct, spot on. Apparently this is what some call "correction". I just never thought of the conic constant as a simple percent of a parabola (because it's not).
Mladen
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siriusandthepup
super member
Reged: 02/14/06
Loc: Central Texas, USA
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: MKV]
#5658118 - 02/02/13 01:06 PM
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Mladen,
It's just a sign of my old school upbringing. When I started this project I had no ATM friends that were into Optical Design programs. I used ATMOS demo for my initial explorations and then generated my DK design spreadsheet based on the DK equations I found in a book. Again, no training or experience on my part. I checked my spreadsheet outputs against ATMOS results and all seemed well, so I proceeded from that point. It certainly would have been nice to have the benefit of Cloudy Nights "group think" and experience back then.
I evolved this project in a virtual design knowledge and experience vacuum, hence my slow pickup on the proper terminology and conventions.
I am enjoying the interactions with folks who do know what they are doing in optical design. It's a good learning experience for me, just please be patient with my slow pickup. I am persistent and I will eventually come up to speed. I do appreciate all the criticisms, questions and suggestions from everyone. I know that they are honest attempts to help. 
Thanks All!
Edited by siriusandthepup (02/02/13 01:19 PM)
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MKV
Carpal Tunnel
Reged: 01/20/11
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: siriusandthepup]
#5658133 - 02/02/13 01:16 PM
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Ed, thanks. You are very kind. It was an honest mistake. And thanks to Mike, again, for catching it in time. I altered the relevant post so as not to confuse anyone else.
Mladen.
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mark1234
member
Reged: 04/15/08
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: siriusandthepup]
#5658607 - 02/02/13 05:53 PM
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Ed, my very old, maybe 20 years old? Dall Kirkham 16cm F3.8/13.5 was done from equations in James Muirden's book from the 1970's. He pointed out that a slight oblating figuring would be needed on the spherical secondary after assembly and he was spot on. I now can ray-trace retrospectively and understand that this corrects the higher orders which the basic conic equations and a slide rule omitted. I can also confirm that the tolerances on radius and spacing are very wide, relatively speaking, for diffraction limited optics.
This small scope is still going strong on the original coatings. Tried to post the Registax png of some webcam moon images - png file too big but I think you can visualise a 6" f13.5 image at or near diffraction without chromatic abberation.
Mark
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siriusandthepup
super member
Reged: 02/14/06
Loc: Central Texas, USA
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: mark1234]
#5658730 - 02/02/13 06:53 PM
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Mark,
Do you remember the name of the Muirden book? I thought I'd read them all. I would love to know more about the touch ups not covered by the basic equations. If I remember correctly I got the equations from Stephen Tonkin's book "Amateur Telescope Making".
Also, do you have a pic of your telescope? I'm sure we'd all love to see it. Thanks!
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JohnH
Carpal Tunnel
Reged: 10/04/05
Loc: Squamish BC Moved!!!!!
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: siriusandthepup]
#5659060 - 02/02/13 10:48 PM
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Someone was talking regarding centering the optics.
With a DK in mind, I gave a critical examination of the secondary cell/focuser and made these conclusions.
Since there is around 1 1/2" range from the stalk the secondary rides on, that spacing is not especially critical as it would help give a good range of back focus for various things like cameras, spectrographs etc.
There is NO provision for CENTERING the secondary, only adjusting its tilt, as the secondary is a spherical surface, it has no unique center of revolution
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Loc: Fort Worth TX
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: MKV]
#5659251 - 02/03/13 01:32 AM
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Thanks for correcting the error, Mladen.
However, figuring strength actually can be expressed as relative to a paraboloid in percentage of correction. The negative of the conic constant >> IS << the figuring strength. Figuring strength directly and linearly gives the scale factor to be multiplied by theoretical Foucault r^2/R or r^2/2R knife-edge positions for non-paraboloidal mirrors.
An ellipsoidal primary for a Dall-Kirkham might have an eccentricity of 0.8390471, giving a conic constant of -(e^2) = -0.704 and thus a figuring strength of +0.704. This implies that if the primary were zonally tested at center of curvature, the zonal r^2/R or r^2/2R values giving theoretical KE shifts for an equivalent paraboloid would be scaled by 0.704, or 70.4% of the zonal shifts for the paraboloid. The mirror can accurately be said to be 70.4% corrected relative to a paraboloid.
Likewise, a hyperboloidal primary for a Ritchey-Chretien Cassegrain might have a conic constant of -1.076, and thus a figuring strength of +1.076. Zonal KE shifts for the equivalent paraboloid are all multiplied by 1.076, and the mirror can be zonally tested at COC in exactly the same fashion as the equivalent paraboloid, just with slightly stronger KE shifts. This RC primary could be said to be 107.6% corrected.
A Rosin hyperboloidal astrograph primary is an extreme example of figuring strength, as Rosin primaries often have conic constants of down to -1.5 and even -2.0 or less, depending on the overall system design.
A sphere is the other extreme of figuring strength. A sphere has zero eccentricity, thus both the conic constant and figuring strength are zero as well. Zonal knife edge positions for the equivalent paraboloid would all be multiplied by zero, and this is of course exactly what we know to be the case for testing a sphere.
The same principle can be applied when making the primary mirror for, say, a Wright-Schmidt telescope, which typically has an oblate spheroid figure with positive conic constant and negative figuring strength. Equivalent paraboloid zonal settings would then be multiplied by a negative figuring strength, implying that the center of the mirror should focus long relative to the edge, which is indeed the case for an oblate spheroid.
Hope that helps,
Mike
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MKV
Carpal Tunnel
Reged: 01/20/11
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: mark1234]
#5659555 - 02/03/13 09:12 AM
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Dall Kirkham 16cm F3.8/13.5 was done from equations in James Muirden's book from the 1970's. He pointed out that a slight oblating figuring would be needed on the spherical secondary after assembly and he was spot on.
Mark, if you undercorrect the primary, then slight oblating of the secondary would indeed correct the figure. However, if the primary is overcorrected, just the opposite would be true, you'd be creating a slight ellipsoid on the secondary.
That may seem like an easy fix mathematically, but from the shop technique point of view it's much easier to make all the corrections on the primary and leave the spherical secondary alone.
Spherical secondary also makes DKs easier to collimate and maintain collimation, so why throw that advantage away by aspherzying it when all the correction can be on the already aspherized primary?
Mladen
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Ajohn
sage
Reged: 12/03/07
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: Mike I. Jones]
#5659631 - 02/03/13 09:53 AM
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The 14.5" seems familiar, but a lot of photons have passed through the exit pupil since that one. Seems like I was helping a guy out that was considering a modified DK. I got that so-called Baader design from a post over on the Yahoo Astrograph group. Still not sure about it, but it does work beautifully on my 16" f/6 Newt. I think when you get into wide fields and lower focal ratios, though, no generalized commercial corrector is going to work well, and a custom design is required.
Mike
Telescope services do one Mike which they say will work down to F3 but it's rather expensive and long so would only be of use on an astrography with the camera on axis.
http://www.teleskop-express.de/shop/product_info.php/info/p5903_TS-Optics-2-5...
There is also a Bader to F3.5 as well as the other one they do
http://www.teleskop-express.de/shop/product_info.php/language/en/info/p486_Ba...
There is a syntra / skywatcher one to F4 around on there as well. Long again which means huge central obstruction I suspect.
John
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Ajohn
sage
Reged: 12/03/07
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Re: wsDK - weak secondary Dall Kirkham telescope
[Re: Ajohn]
#5659898 - 02/03/13 12:30 PM Attachment (15 downloads)
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I came across this corrected DK by Wynne. He refers to it as a wide field cassegrain, spherical 2ndry plus corrector with a parabolic main mirror. UV to IR using ubk7 plus a rather flat field. Nice even spot size too.
It's as per his prescription other than I have left it in mms rather than cms and have set the 2ndry dia for 0 field angle which oddly according to oslo doesn't cause any light loss across the field.
John
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