testing a homemade Dob mirror

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bluemax
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testing a homemade Dob mirror

Unread post by bluemax » Fri May 03, 2019 5:47 am

Other than a star test, can anyone suggest a way to tell if my 6" F5 reflector mirror is spherical or parabolic? I made a Dob for the grandkids out of a yard sale type mirror and it has a really bad fuzzy view. The surface is crappy and I want to re-coat (about $60 to do it) but before I spend the loot want to be sure it's parabolic.
Frank N

12.5" F5 single arm Dob

"I'm a seeker too. But my dreams aren't like yours. I can't help thinking that somewhere in the universe there has to be something better than man. Has to be"

Taylor in "Planet of the Apes" 1968
Bruce D
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Re: testing mirror

Unread post by Bruce D » Sat May 04, 2019 4:11 pm

Frank I think a Foucault tester is the best way to tell what you have for a mirror- and it's shape. Someone in the club might have one. Was it a decent yardsale scope, or a real cheapie? A spherical 6" f5 would really be pushing it I think-
Bruce D
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mark.m
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Re: testing mirror

Unread post by mark.m » Sat May 04, 2019 4:45 pm

There's a formula in Texereau's How To Make A Telescope (page 19) that states the minimum f/ratio for a spherical mirror to achieve 1/4 wave accuracy (at the wavefront). For a 6-inch mirror, the formula works out to f/8.2 minimum. My own experience with 1/4 wave mirrors is that the overall degradation from what is possible with a better figure is quite noticeable with 1/4 wave error at the wavefront. So, a 6" f/5 spherical mirror would produce noticeably poor images.

Yes, a Foucault test of a 6" f/5 mirror will immediately show whether the mirror is spherical or not (even without any data collection and analysis). I've got a homemade Foucault tester, although it's a little beat up -- I last used it 15 years ago. Not sure I'd want to depend on it to actually measure the mirror's surface, but if all you're really looking for is a thumbs-up/down on whether it's been figured beyond a sphere into something resembling a parabloid, it should do fine.
Mark M
Portsmouth, RI
10" Meade SCT
SBIG ST-9E
GM2000 (10Micron)
American Association of Variable Star Observers (AAVSO) observer code: MMU
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bluemax
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Re: testing mirror

Unread post by bluemax » Sun May 05, 2019 5:25 am

Thanks Bruce and Mark! Awesome Mark, I would love to test this mirror with your home made device! Not looking at all for wavefront measurements, just is it parabolic or not. If it is, I'll get it re-coated and we're off to the stars! I would be happy to come to your location with the mirror for the test.
How long does it take to set up and test? Wondering if doing this at a meeting would be interesting to the members?
Frank N

12.5" F5 single arm Dob

"I'm a seeker too. But my dreams aren't like yours. I can't help thinking that somewhere in the universe there has to be something better than man. Has to be"

Taylor in "Planet of the Apes" 1968
Bruce D
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Re: testing mirror

Unread post by Bruce D » Sun May 05, 2019 11:08 am

A quick peek with a FT will also show if there are other disasterous issues with the mirror's figure- it may be parabolized but have a turned edge- which often can be easily corrected with an edge mask.

I would be interested in a FT demo at a meeting, it's been 50 years since I've used one.
Bruce D
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mark.m
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Re: testing mirror

Unread post by mark.m » Mon May 06, 2019 9:05 pm

Hmmm. I've been doing some experiments -- most of my prior experience has been testing polished glass mirrors prior to aluminizing. And for those mirrors, you really need a very dark room in order to see the Foucault shadows because the reflection from the uncoated naked glass is somewhat dim. However, with my 6" (f/6) mirror pulled out of my Dob (and which was recently re-aluminized), all it takes is a darkened room (after sunset). It doesn't need to be pitch black.

And so, yes, we could do this at a meeting (or after a meeting). I discovered earlier today that using a laser pointer during setup makes getting the initial optical alignment a breeze (5 minutes or so). Within about another 5 minutes I had shadows clearly visible well enough to distinguish between spherical and non-spherical surfaces.

I also tried to get a snapshot of the shadows with my DSLR, but exposure times were running a few seconds. Since I was hand-holding the camera, that didn't work well at all. However, it should be possible to rig up a camera mount that would enable the long exposure times needed to photograph the shadows, and that would help make this more of a group activity.

[Ah. Guess what! Despite all the jiggling during the exposure, I did manage to catch some sense of the shadows with the DSLR. This shadow pattern is typical of a parabloid.]
DSC_0017.jpg
DSC_0017.jpg (25.05 KiB) Viewed 5255 times
So, I'm thinking that this could turn into a multi-part activity. We could quickly answer your question at the end of the next meeting, and then we could do another session for the larger group to educate/illustrate the use of a Foucault Tester.

- Mark
Mark M
Portsmouth, RI
10" Meade SCT
SBIG ST-9E
GM2000 (10Micron)
American Association of Variable Star Observers (AAVSO) observer code: MMU
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bluemax
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Re: testing mirror

Unread post by bluemax » Mon May 06, 2019 10:22 pm

Mark, I will contact Andy to see if the full FT demo would be of interest to the group. As to my mirror, I am in Bristol and would be happy to come to your location to place the mirror on your rig for testing if that works.
Frank N

12.5" F5 single arm Dob

"I'm a seeker too. But my dreams aren't like yours. I can't help thinking that somewhere in the universe there has to be something better than man. Has to be"

Taylor in "Planet of the Apes" 1968
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