CosmicDave
newbie
Reged: 11/13/08
Posts: 4
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This is a 50+ year-old mirror that I have installed in a homebuilt truss Newt. Its a 12.5 inch f/8 that performs well; it has made detailed images of Jupiter and Saturn and I'm happy with it visually, but then I haven't had many mirrors to compare. Star testing does show differences between the inside and outside of focus diffraction rings. Outside is clear concentric but irregular shaped rings while inside focus the rings are fuzzy but haven't done it in real good seeing yet. I'm planning to get the mirror recoated this winter but would like to know if the figure is good.
Please have a look and post your comments. Thanks!
More Foucault Images
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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CosmicDave,
Welcome to CN!
I've looked at the Foucault images and I wouldn't be able to tell if the figure was good just by looking at them - it looks reasonably smooth, however; it appears to have a bit deep center; and then there's some strange artifacts at the edge in at least two places and maybe more, possibly from mirror clips in years past. Has it been recoated recently? Coatings make Foucault harder to read as you can never get them as clean as the bare glass.
An f/8 has hardly any correction, so it looks OK in that respect, but there are some oddities that may just be air currents. Can you do these with really still air, and no illumination in the room so the contrast is better?
Best,
Mark
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2926
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I'm no expert but that looks pretty good to me for f/8 - those rectangular depressions are interesting - could that be where the disk was clamped in a handle for easy pushing about on the lap? A stop ring would eliminate any bad effect from that without really diminishing performance.
-drl
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 659
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KEEP IT
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CosmicDave
newbie
Reged: 11/13/08
Posts: 4
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Thanks for the feedback!
drl: Yeah, I was really surprised to see the rectangular depressions, the seller claimed this mirror had been in storage all those years and never installed in a scope but the beveled edge is darker under these spots, like it was clamped in a cell for a long time. I don't know, but the mirror was shipped to me packed in a very old box from the company who coated it, and their return address in LA had a city code, not a zip code, dating the box at least to before 1960, I tend to believe that the coating is original; the finish is in fairly decent condition except for some light rub marks.
Mark: Thanks for the comments and yes, I did have some air currents floating around. The uneven dark areas on the right side went away last night when I redid it in calmer air. I will try as you suggested and darken the room. I'm getting ready to do some zone testing with a mask and will post up some more images if it works out.
I'm trying to understand this testing also because I've got a couple of blanks, 6" and 10", one of them I may be grinding as a winter project. Thinking f/6 or so. This is cool stuff to work on...
Dave
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2926
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That pretty much cinches it - whoever made this mirror probably used a tool to push the mirror around on the lap - it pushed up some glass where it "bit" into the mirror and it got polished off! Again you can eliminate that with a small aperture ring.
-drl
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ed_turco
member
Reged: 08/29/09
Posts: 26
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I know it's a little late, but those edge "zones" are going to cause some problems. Edge errors are nasty ones, like the dreaded TDE.
OK. not a complete turned edge but the effect is sure going to be interesting, and in no way, good.
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neo
professor emeritus
Reged: 02/12/08
Posts: 610
Loc: Iasi, Romania
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I believe you can solve easily that edge problem and cheap too by placing a baffle in front of the mirror. It will cover the bad edge and because it will diminish the aperture only a bit it won't affect its performance.
--------------------
Russian 15x50 binos
Home made 8" f/5 Newton on eq mount
Home made 70mm f/6 (Rodenstock Rotelar lens) Apo refractor
www.astronomy.ro
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3121
Loc: Northeast
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Looks somewhat like an F/8 appears, but sounds like it's undercorrected somewhat. Just this pic alone without some numbers to quantify your error makes a real assessment inpossible.
Strange, those 2 rather 'deep' divots- Looks reasonably smooth for an old mirror.
I'll look at the other pics.
M.
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3121
Loc: Northeast
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Well, edge- and you got some funny looking "WOW's that ain't doing anything beneficial. Or, maybe currents affecting the pics?
M.
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So many projects, so little time!
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CosmicDave
newbie
Reged: 11/13/08
Posts: 4
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Thanks for the replies. I have since placed a 1/4" masking ring over the mirror to cover the outer edge, reducing aperture to 12". When I get back to planetary imaging, I should get a better idea if it helps, but visually it seems to help sharpen the focus.
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