Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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"informal" flat test picture- not bad!
#2896323 - 01/29/09 06:15 PM Attachment (81 downloads)
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I was killing some time before supper, and was fooling around with an LED flashlight as a monochromatic light source to view the interference pattern. Wasn't having much success with the LEDs, so I was tilting the compact flourescent pole light to see how well I could see the pattern, Looked good enough to take a really good pic, and this is what I got. Darn good for being handheld, with no flash! pretty easy to see!
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rwiederrich
Goldfinger
Reged: 11/17/05
Loc: Always Dark skies of Belfair W...
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Re: "informal" flat test picture- not bad!
[Re: Mark Harry]
#2896345 - 01/29/09 06:23 PM
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I was killing some time before supper, and was fooling around with an LED flashlight as a monochromatic light source to view the interference pattern. Wasn't having much success with the LEDs, so I was tilting the compact flourescent pole light to see how well I could see the pattern, Looked good enough to take a really good pic, and this is what I got. Darn good foe being handheld, with no flash! pretty easy to see!
What mirror is that Mark.......is it the 20"?
Looks too small fer that one.
Interesting capture.
Rob
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: "informal" flat test picture- not bad!
[Re: Mark Harry]
#2896349 - 01/29/09 06:28 PM
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The 4.25" pyrex "shop" or "working" flat is on the bottom sitting on black felt. The top piece is a fuzed quartz reference with "validated" 1/20th wave surfaces; both sides. Real easy to see the colors. If you'll notice the purple fringe on the right, these 2 flats were getting real close. I cleaned them with a swipe on the soft skin of my forearm, believe it or not.
*******
This is what I'd call good typical spacing on the bands. Most you see on vendor websites has too many showing, which makes them appear deceptively straighter.
The tilt of the camera from a nominal vertical line also makes the lines appear straighter- The less tilt, the more accurate the test.
The pyrex piece is just for getting in the ballpark on other worked pieces, and I generally consider it about 1/8th wave surface error. Not ultra-precise, but good enough to use for casual tests and saving the reference flats.
They may not have been totally equilibrated, for these were in the cellar moments prior to taking the pic.
Light had a shield and shown the light directly at the ceiling only. That made a -LOT- of difference in the visibility of the bands!
Mark
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mark cowan
Vendor (Veritas Optics)
Reged: 06/03/05
Loc: salem, OR
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Re: "informal" flat test picture- not bad!
[Re: Mark Harry]
#2899094 - 01/31/09 06:56 AM
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Interesting, you can see the different fringe spacing depending on the wavelength. Many (but not all) fluorescents have 3 very narrow emission bands which you can see with a diffraction grating or simple lens. If you put a simple transmission filter on it for one color you'd have an inexpensive monochromatic source, no?
Best,
Mark
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Dick Parker
professor emeritus
Reged: 08/17/07
Loc: Tolland, CT and Chiefland, FL
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Re: "informal" flat test picture- not bad!
[Re: mark cowan]
#2899117 - 01/31/09 07:17 AM
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"...put a filter on it.." That is exactly what you get when you use a buglight bulb (white tube blacklight) and wrap it with green cellophane. There are two prominent frequencies from the bulb, but using the green cellophane filters out one of then. You end up with a nice, inexpensive, diffuse, monochromatic source. Fringes show up very nicely.
Dick Parker
Edited by Dick Parker (01/31/09 07:18 AM)
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Loc: Boechout, Belgium
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Re: "informal" flat test picture- not bad!
[Re: Dick Parker]
#2899125 - 01/31/09 07:27 AM
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Quote:
"...put a filter on it.." That is exactly what you get when you use a buglight bulb (white tube blacklight) and wrap it with green cellophane. There are two prominent frequencies from the bulb, but using the green cellophane filters out one of then. You end up with a nice, inexpensive, diffuse, monochromatic source. Fringes show up very nicely.
Dick Parker
Thanks for the tip, didn't know that... 
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DAVIDG
Post Laureate
Reged: 12/02/04
Loc: Hockessin, De
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Re: "informal" flat test picture- not bad!
[Re: Chriske]
#2899486 - 01/31/09 11:22 AM
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Here is link that discussed much of this awhile back.
- Dave Interference testing
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Loc: Boechout, Belgium
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Re: "informal" flat test picture- not bad!
[Re: DAVIDG]
#2899506 - 01/31/09 11:29 AM
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Thanks...!...
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: "informal" flat test picture- not bad!
[Re: Chriske]
#2899914 - 01/31/09 03:03 PM
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One error in that thread, DavidG, when the camera or your eye is tilted away from right above the pieces, you will see STRAIGHTER lines. At about a 45 degree angle, you can make -any- flat look good!
In testing curves, I know this works also when viewing thru a convex piece of glass, but you get weird indications at the edge of a concave piece. Concaves are unreliable to view through.
M.
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DAVIDG
Post Laureate
Reged: 12/02/04
Loc: Hockessin, De
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Re: "informal" flat test picture- not bad!
[Re: Mark Harry]
#2900234 - 01/31/09 05:44 PM Attachment (30 downloads)
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The curving of the fringes can be a problem for high precession work when they are not viewed straight on or without the use a Fizeau interferometer, that is why Gary built his testing box with the semi reflective mirror at 45 degrees which is pictured in the thread I pointed to. This removes the problem. Here is the outline of the basic design that Gary built. There is a nice discussion in Amateur Telescope Making book 2 on making and testing flats.
- Dave
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neo
Carpal Tunnel
Reged: 02/12/08
Loc: Iasi, Romania
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Re: "informal" flat test picture- not bad!
[Re: DAVIDG]
#2900356 - 01/31/09 06:41 PM
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By the way if you are talking interferometry tests I have a newbie question about testing flats. What would be the differences between testing with a simple Fizeau interferometer and a more complex one like the laser interfermeters? It's about precision, sensitivity?
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GlennLeDrew
Postmaster
Reged: 06/18/08
Loc: Ottawa, Ontario, Canada
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Re: "informal" flat test picture- not bad!
[Re: neo]
#2900756 - 01/31/09 10:44 PM
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Neo,
No fundamental difference, really. Both techniques rely on the interaction between a 'reference' wavefront with that produced by the piece under test. Both allow similar precision, too. Where laser interferometry offers a significant advantage is in the testing of really large optics where a test plate could be quite expensive and/or difficult to fabricate.
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neo
Carpal Tunnel
Reged: 02/12/08
Loc: Iasi, Romania
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Re: "informal" flat test picture- not bad!
[Re: GlennLeDrew]
#2901114 - 02/01/09 05:40 AM
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Ok. Now I got it  . So I can make my own flats ( secondarys for medium size newts) and test them by Fizeau method and still I'll be able to make them as accurate as possible.
Thanks Glenn for Reply!
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kfrederick
Carpal Tunnel
Reged: 02/01/08
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Re: "informal" flat test picture- not bad!
[Re: neo]
#2901183 - 02/01/09 07:52 AM
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hi mark great pic // was looking at my mirror at a low angle and could see fringes nice arcs have you seen this ?? the light was reflecting off the snow the mirror has a RC 320 inches kevin
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Mark Harry
Vendor
Reged: 09/05/05
Loc: Northeast USA
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Re: "informal" flat test picture- not bad!
[Re: kfrederick]
#2902289 - 02/01/09 06:02 PM
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It looks like there's yellow, green, blue, and magenta colors present. I wonder what kind of filter you'd use- Perhaps a red to darken the blue-green-magenta bands to improve the "contrast"?
M.
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DAVIDG
Post Laureate
Reged: 12/02/04
Loc: Hockessin, De
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Re: "informal" flat test picture- not bad!
[Re: Mark Harry]
#2902455 - 02/01/09 08:08 PM Attachment (30 downloads)
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The engery saver bulbs are gas discharge types. They have a number of defined emission lines. The green and blue are from mecury, the red is most likely hydrogen and the yellow sodium. Many inexpensive plastic filters will do a great job in filtering the light so only one wavelength is transmitted. Clear "gift wrap" of either red or green will usually do a good the job.
Here is a picture taken using my desk lamp and holding a a piece of red plastic in front of the camera. By the way you don't have to filter the light source, you can just look through a small filter like the ones used on an eyepiece.
- Dave
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