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Jens.Jacobsen
journeyman
Reged: 02/13/08
Posts: 8
Loc: Denmark
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Hi All.
I'm currently "re-finishing" my secondary for a Groski type schiefspiegler. With re-finishing, I mean that I'm redoing the secondary. I had the two mirrors matching pretty good, but then, in an attack of not being happy with the primary, i re-figured the sphere. This turned out to be a difficult job, doing long-focus spheres is much different from ordinary paraboloids.
Well, the primary is now really smooth, with a very nice figure. This is good and well. But the focal length has shortened some 10 mm and the secondary is now too flat. Today i worjed for two hours doing long W's with the tool on top, and did not really see a change in the inteference pattern. This is now a bullseye with about 10 circles.
What shoud I do? Can I leave the secondary about 10 mm longer that the primary, providing that it is a nice sphere?
Suggestions?
Sincerely,
Jens
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jg3
sage
Reged: 05/27/07
Posts: 209
Loc: near Auburn, CA
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If you have a WinSpot model of your planned scope, try it out with the deviant focal length. Maybe tweek the separations to see if that helps.
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1444
Loc: Hockessin, De
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I don't think the secondary is 10mm longer in radius then the primary if you are seeing only 10 rings. Did you regrind the primary with the tool that will become the secondary ? Even if the radius is slightly different then the design it is important that both the primary and secondary are closely matched. Being short by 1" or less in radius from the design won't hurt but both the primary and secondary should be a close match.
Making a convex surface more convex is not easy. What has worked for me is to make another lap and press into it a star the same size as the lap. This leaves only triangles of pitch near the edge of the lap that will make contact with the glass. These will wear down only the edge of the convex piece. Once you go from rings to a few arcs you'll need to go back to the standard square grid lap to fine tune the figure.
- 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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Jens.Jacobsen
journeyman
Reged: 02/13/08
Posts: 8
Loc: Denmark
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A Starlap! Of course. Must have been away on vacation too long.
I will make a star lap today. Thanks, Dave
Sincerely, Jens
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2488
Loc: Northeast
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"I don't think the secondary is 10mm longer in radius then the primary if you are seeing only 10 rings. ..."
***
Just my opinion, at long ROCs this is possible. But what I understood about the curve specs, is that the curve could be off about 5%, and you'd still be fine.What's the ROC now?
Assuming that 10 rings is while the bullseye is centered. If tilted so that the bullseye is at the edge there will be roughly 80 fringes, or 40 wavelengths observed across the aperture. I figured about .0008" difference at this position. I know there's a way to calculate the radii difference, but I'll have to look it up.
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"Can I leave the secondary about 10 mm longer that the primary, providing that it is a nice sphere?"
I'm pretty sure you can. In any case, I don't think you're exceeding the 5% allowance.
M.
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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
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gnabgib
sage
Reged: 06/05/05
Posts: 309
Loc: Fall River MIlls Ca.
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Jens;
I have found that long spheres like those used in the
"sheepsprinklers" are much easier to achieve if the pitch lap is very hard (think granite hard) and you stop and cold press about every 15-20 minutes to keep good contact. I once had to replace the secondary on a 4.25 inch schief and the new secondary was 1 inch longer in radius. The scope still was an amazing performer!!
Kevin
Kevin
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1444
Loc: Hockessin, De
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Here is a spot diagram of the center of the field with the primary having a ROC that is 1" shorter then the secondary for my design. The result is that the coma is just starting to peek outside the airy disk.
- 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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Jens.Jacobsen
journeyman
Reged: 02/13/08
Posts: 8
Loc: Denmark
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Hi Mark.
The ROC on the secondary is very close to -3500 mm. The primary was figured to very close to 3500 (concave). Whit this combination I got almost straight lines over much over the surface of the secondary. The edges where curved, but since they are going to be thrown away that dosen't matter much. Then I refigured the primary, and the ROC shortened somewhat, to around 3485 mm. After that, lots of circles. I followed David's advice, and is now 30 minutes into a refiguring with a starlap, where the center is cut out. This helps, I can see the lines straightening across the center of the secondary, and moving towards the edge. As soon as i get sufficiently few lines 5-8 or so, across the mirror, I will start to evaluate the straightness of the lines and determine when they are sufficiently straight.
When you do have a difference between the primary and secondary, eq. circles or at least curved lines, how do you evaluate the quality of the secondary? No hooks on the lines? Gentle curves with no humps??
Sincerely
Jens
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Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2488
Loc: Northeast
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"how do you evaluate the quality ..."
If the radii are slightly different, it's best to have the secondary indicating 1-2 fringe lines "longer" ROC, if you get my drift. Use the back of the secondary, which is at least
-flash polished-, and observe the lines as nice, even circles. Displace the bullseye toward the edge. You want the lines to be essentially sections of nice round circle sectors.
Don't test through the back of the concave mirror for it will distort the lines and give false indications. Can be quite disturbing to the newcomer.
You can also see the edge of the primary and check it's quality by doing the test near the edge.
M.
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Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
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