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What the Foucault!?

Started by brucesdad13, 07/20/2015 03:15PM
Posted 07/20/2015 03:15PM Opening Post
Sorry, couldn't resist a go at WTF. I think I have everything I need to start grinding my first mirror. Well at least I have the Pyrex blank, plate glass tool, abrasives, and polishing agents. I will probably have to rustle up a spray bottle and etc.

The various books I have acquired suggest building a Foucault testing device. I think I've fashioned a crude knife-edge apparatus and mirror stand. I placed the mirror from my Criterion RV-6 into the holder and after much trial and error discovered the right placement of mirror and tester. They are approximately 100" apart which seems to confirm the 50" etched on the back of the primary.

I haven't added a Barr scale or anything for measuring yet. Do these photos seem to indicate I'm moving in the right direction? I can see bright green specks where the coating is pitted or missing. I also see what looks like a big smudge... and where the mirror cell marked the mirror edge on one side. I'm just pushing against the piece of wood with the razor blade to move it into the light. I had a white light in my first design and then read that white LEDs aren't a good monochromatic light source so I went to green. I need to grind the surface of the green LED flat... it has concentric grooves and they were visible at certain angles...

Attached Image:

brucesdad13's attachment for post 59551

~ Charlie Stevenson

8" f/5.7 String Telescope - 1st Scope Build; 2nd Place Stellafane 2016 Optical Award for Newtonians 12.5" and Smaller
10" f/4.5 Newtonian (June 2015) mirror refigured by Optic Wave Labs P-V WaveFront 1/14.24, Strehl Ratio 0.993 (Aug '15)
Criterion RV-6 seems to be circa 1973 (June 2015) [For Sale]
Celestron C8-A XLT (January 2015) [For Sale]
Celestron PowerSeeker 70AZ (Christmas 2014)
Aldrich Astronomical Society member since 2015
Posted 07/24/2015 06:28AM #1
Good luck, it looks like you have a great start. However, with your Foucault apparatus already built you ,may be tempted to keep looking at it. The adage, "grind more, measure less" is a good approach. Personally I use a once tester up until figuring starts. 100 or 150 lip is pretty good and they are relatively easy to find. If you haven't read David Harbor"s Understanding Foucault available from Amazon, you should grab a copy.

Posted 07/26/2015 03:21AM #2
A piece of frosted or invisible scotch tape over the hole on the lighthouse to diffuse the light may negate the need to grind down the LED.


Fujinon 10x70 FMT-SX binocs
Tak FS-78 ƒ/8.1 - 630mm fl.
6" ƒ/6.5 homeground newt. - 981mm fl.
10" ƒ/4.85 homeground dob. - 1231mm fl.

Posted 07/26/2015 07:23AM | Edited 07/26/2015 04:56PM #3
That's an admirable job, Charles! Few observations: A pinhole source (poked hole in Al foil) is less subject to problems because its orientation has no preference. I put a tiny piece of Opal Glass just behind the pinhole. I also added a tiny condensed alignment source just below the pinhole so I could align (and then even USE) the gizmo in normal room light. Completely eliminates the difficulty of fishing around for the feeble return. At work we developed QKE "Quantitative Knife Edge" that is every bit as accurate as interferometry. So the old adage that Focault is NOT quantitative has become...untrue. Now the Knife can quantify ALL the aberrations! Ummm...with your source so far to the side, you will get quite a lot of field astigmatism in your signature. You will notice that its axis is vertical and it does not rotate with the mirror. Can mitigate that by getting the source much closer to the viewing knife. Tom Dey ADDITION: I just realized that the X-astig will NOT manifest overtly with the vertical knife because of the knife orientation...which is GOOD, because it is artifactual anyway. Only error will be a slight difference in focus AKA you are testing at one of the focal lines...the "wrong" one... So the test becomes blissfully unaware of the astig induced by the test set. QKE proves its presence because two orthogonal, coplanar knives are used. The software is simple and retrieves wavefront. In this case it would of course (ideally) be the spherical ab characteristic of testing a parab at CC. Anyway - just go with the shadowgram signatures and zonal radii as the books suggest --- and you will end up with a GREAT mirror! Tom Dey

29-inch Dob in a dome
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