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Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
Brian Engel
member
Reged: 08/25/09
Posts: 92
Loc: Cincinnati,Oh
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I guess this if for all of the optics guru's....
A friend of mine (a respected professional optician - not going to mention names) did me a nice favor and tested a 5" minor axis flat for a 27" f/4.3 telescope.
We have been "chasing" weird star image problems and after exhausting collimation,mirror support and other possible problems we decided to have the flat tested.
So, the flat was tested and shows 1/2 wave error on the mirror (fringe test). It appears to be smooth convex error to me.
Here is the image, my question is this.... What effect will this have on the wavefront? Being smooth and convex I am thinking that most of the error induced would be taken out by focusing?
Guess we are looking at having the secondary "refigured" just wondering what the impact of this bad flat is?
Thanks...
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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My limited knowledge guess is astigmatism error cause the lines are not parallel. Could make star tests ovals. Ronald
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10" f/5 dob unf.
7x50 Tasco binos
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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I don't think its possible to focus it out because its a 90 degree reflection. 180 degree reflection like a Cassegrain secondary would focus out a convex error better but not astigmatism either. Ronald
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10" f/5 dob unf.
7x50 Tasco binos
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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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Brian -
If the secondary is a spherical departure from flat, the aberration introduced to the final image is predominantly astigmatism, due to the mirror being tilted 45 degrees. At the levels of curvature you are apt to encounter, there is no significant contribution to spherical aberration. According to ZMAX, it takes 5/8 wave of surface curvature of the secondary for the rays in a spot diagram to fill the Airy disk.
Dick Parker
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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the worst case is the surface error to change the wavefront 1.4 times which is .56 waves. Ronald
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (10/04/09 09:08 PM)
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wh48gs
sage
Reged: 03/02/07
Posts: 458
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Quote:
So, the flat was tested and shows 1/2 wave error on the mirror (fringe test). It appears to be smooth convex error to me.
Here is the image, my question is this.... What effect will this have on the wavefront? Being smooth and convex I am thinking that most of the error induced would be taken out by focusing?
The error depends on diagonal's surface shape. If the surface is nearly spherical, the PV wavefront error is about 0.7 of the surface error, practically all astigmatism. The 0.4 wave PV surface would then make it 0.28 wave PV wavefront of astigmatism, which corresponds to 0.057 wave RMS. Still better than diffraction limited, but potentially noticeable degradation (0.88 Strehl degradation factor).
If the surface is cylindrical, then the surface error multiplies by a factor 1.4 in the wavefront. That would make it 0.56 wave PV wavefront of astigmatism. It is much different story now, with the corresponding wavefront RMS of 0.114, and Strehl degradatoon factor 0.6.
There are some illustrations on my site:
http://www.telescope-optics.net/newtonian.htm
Vla
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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I'd say you have more than spherical power, more likely warpage from how it was blocked. In any case it should be refigured.
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Ed Jones
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sixela
Postmaster
Reged: 12/23/04
Posts: 10855
Loc: Boechout, Belgium
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Quote:
Brian -
If the secondary is a spherical departure from flat, the aberration introduced to the final image is predominantly astigmatism, due to the mirror being tilted 45 degrees.
If it's spherical. I've got a secondary in my drawer with well matched curvatures (the radius of curvature is shorter on the minor axis by almost exactly sqrt(2)) and in a test setup that corresponds to a slow scope there is little astigmatism (even using a setup with a Foucault-tester, thus testing at the centre of curvature rather than the focal plane, is enough to make the problems with the thing much less apparent).
Installed in a very fast scope (where some parts of the wavefront hit at an angle quite different from other parts) it's still a bit ugly, though, despite the rather adventurous claims from the UK manufacturer.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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