greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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Does anyone know of other tests for flat mirrors than using a master flat which I don't have and the water test which I can't get to work and a foucault test?
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10" f/5 dob unf.
7x50 Tasco binos
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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If you have a small refractor one can use it. The flat is placed on the ground and starlight is reflected off of it into the small refractor. You observe the stars at high power and move the refractor from about 45 degree to the shallowest angle possible. The stars should stay round and tightly focused if the flat is flat. If the flat is larger then the objective in the refractor you need to do the test over a couple of different cords on the flat.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 965
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If you have a pentaprism and a track or slide ( which does not need to be ultra-precise), the pentaprism or pentamirror can be moved across various diameters of the flat under test. An autocollimating telescope is aimed at the prism from outside the rim of the mirror. The image of the reticle in its focal plane is bent 90 deg. by the prism, reflects off the mirror, and returns to the focal plane via the prism. The variations of position of the returned image are a measure of the slope of the mirror surface at several points across that diameter. Thus , a map of the mirror surface can be constructed.
This seems laborious and repetitive, but I recall its being shown in the optical production book by D. F. Horne. or possibly elsewhere. Perhaps it can be semi-automated.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3123
Loc: Northeast
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Use it in a Newt, with a known good primary. If it shows the typical 'stig pattern when viewing a star in and out of focus, it's no good. Suiter's book shows what certain amounts look like, so you might be able to quantify the error.
Mark
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So many projects, so little time!
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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Heres my water setup, I still can't see fringes, I'm using a fluorescent light which works for glass on glass fringes but after several attempts I can't get this water test working, The water is a couple mm above the glass and I'm getting the surfaces parallel with the bulb reflections by turning the allen keys then waiting till the water levels. Is the light source too weak?
Edited by greenglass (07/01/09 02:47 PM)
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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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Greenglass -
I don't see a link or attachment to your last note, so I can not see your set up, however you did refer to a flourescent light. To get fringes from the water test you should have a collimated light source. Diffuse monochromatic light (such as from a flourescent bulb) produces fringes, often referred to as "Newton's fringes", with two glass surfaces in contact.
Dick Parker
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3123
Loc: Northeast
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Don't let the light shine directly down onto the water surface, Bounce it off a white ceiling first, and keep the pan out of the direct glare. See if that makes a difference. (?) A pic of the situation with 2 flats, and compact flourescent:(I know not exactly the same, but you can see the bands are starkly visible this way. Light shining directly on the flats is very poor. perhaps this will enhance the water test to be viewed better)
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So many projects, so little time!
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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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I think you will find that for diffuse light to work, the surfaces need to be in extremely close. The distance involved between two glass surfaces in contact is really small. But, the distance between the surface of water and the glass under it is too thick for diffuse light to form fringes. (at least with all the experiments I tried). You can get water thin enough to see fringes with diffuse light if the glass surface is simply wet, but capillary action of the water that thin will cause it to bead up and deform thus producing very erratic, therefore unreliable, fringes. With collimated light, the distance between the two surfaces can be much greater, therefore it works well with water. Don't forget that the fringes with water are going to be dimmer than they are between two glass plates because of the different indexes of refraction (reflection??) involved.
If you succeed in getting fringes with water and diffuse light, I would be interested to hear it.
Dick Parker
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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Dick,
I have done the water test with diffuse monochrome light as per Porter in ATM-I. I had to slowly reduce the thickness of the water with a eye dropper. It was no more then a 1/16" thick when the fringes appeared. They start out as a tiny circle and as the level of the water is reduced they spread out. They were the best seen just before I could see the surfaces of the water start to distort from the surface tension with it and the edge of the glass.
Greenglass, the thickness of the water and the use of reflections of the bulb I bet are your problems. Try shining a laser at the surfaces and looking at the reflection on the ceiling. You need to adjust the level until all of them are on top of each other. Also trying viewing thru a red filter. The fluorescent light isn't monochrome enough and I find a simple red plastic filter good enough to isolate the red line from hydrogen. As Dick has stated they are fainter then viewing with two pieces of glass. Also remember that when you do see fingers, they will appeared curved even if the surfaces is flat. You need the collimate light and the ablity to view the fringers on axis to get them to appear without distortion.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Here are a few tricks I've learned:
Use much finer screws, yours look like 1/4-20 which make it hard to make adjustments on the order of microns.
Use a laser pointer reflected vertical some distance to help line up the two reflections.
Get rid of your air bubble to keep the background dark.
You will likely have tightly spaced fringes at first that are hard to see so use a magnifier to look for the fringes in a small area, open up the fringes then go to a larger lens.
Use a thin water layer; overfill it then see how much water you can remove before leveling takes too long.
I use a F4T5 BL flourescent blacklight bulb used in bug zappers.
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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I put finer thread bolts on, got the air out from behind the disc and hooray found some fringes, the water is shallow, readjusts in about 5 sec. How is the light setup? I can only see fringes in the reflection of the tube light and when I level it more to get less lines they fade. They're visible but didn't photograph.
Edited by greenglass (07/01/09 09:21 PM)
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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glass on glass with same light (plate glass not figured)
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/01/09 08:25 PM)
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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If you see fringes your just about there, 5 seconds is perfect. I suspect the fluorescent bulb doesn't have enough contrast. Even the glass on glass isn't that contrasty. I'd find a better light source. Try a neon lamp at the focus of a collimating lens like Texereau used. An unphosphored mercury lamp works great if you can find one.
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Ed Jones
Edited by Ed Jones (07/01/09 10:51 PM)
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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I see that Surplus Shed has a blacklight bulb #M2138
for $1.50! It will need a ballast to run it however.
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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Flourescent light enhanced with photo studio same glass in pan of water, I think the disc is around 20 waves concave and couple waves TDE. The fringes seem to be concentric around the point I'm viewing from and get finer when viewed away from perpendicular? Very touchy to get a few fringes.
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/02/09 04:29 PM)
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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Just keep in mind that you can't make an evaluation of flatness without using a collimating lens. I can't stress this point enough.
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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Is that from the refraction of water? And the same glass with a piece of window glass on it read 2 waves. The water test the way I have it is making the glass look more curved. 
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/02/09 01:18 PM)
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3123
Loc: Northeast
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You need the lens to correct abberations from the angle difference the light path follows to your eye. At an angle, the light path is longer than what's exactly normal, or straight on. Hence, the fringe is "in the wrong place", or appears so. I guess that if you practice enough with a transmission flat, the experience can be aquired without resorting to a lens every time you check a flat.
This becomes a nasty aspect if you check fringes for straightness through the back of a concave test reference. It's extremely unreliable in this particular situation.
Mark
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So many projects, so little time!
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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It looks like you got the test to work ! The need for the collimated light is caused by the thickness of the water film. When you test glass on glass, the interference takes place in the air gap between the two surfaces. This gap is very thin. With the water test, the film of water is many times thicker.
Ed Jones' Sky and Telescope article is an excellent reference on doing the test and he goes into the exact details of why the collimated light is critical. As Ed has stressed you can't make a critical judgment of flatness unless you use collimated light. Without it thou you can tell if the surface is smooth and what the condition of the edge is like.
You might try testing your small flat since you have a better idea of what it's surface quality is and compare them to what your seeing now.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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It's not the refraction, it's just that without a lens the light goes through at different angles without a collimator. Different angles equate to rings of fringes that aren't real, a result of the long path length in water. I had to learn this lesson the hard way so I want to be sure others avoid this mistake.
If you have a flat that you know is pretty good try the water test without a lens and you'll see many rings of power to demonstrate this effect. Up close many rings, fewer when you are farther away.
But you got fringes so you've mastered the hard part of this test.
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Ed Jones
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