greenglass
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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greenglass
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glass in water with flourescent light normal color, pretty faint rings, same picture as the purple one above
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/02/09 04:27 PM)
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Ed Jones
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A regular fluorescent is far from ideal. Do you know how flat your mirror really is? How much trouble was it for you to find the fringes?
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Ed Jones
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greenglass
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I never made a flat mirror yet because I couldn't test it. If I can get a collimated light source going it should be easier. The flat in the pictures is not done polishing but with a window glass on it it seems 5 waves concave and convex near the edge. I never seen fringes in the water test two previous tries, probably the course threaded bolts and not getting it level. I focused my eyes on the bulb reflection instead of the water and the fringes were seen. Also I could see them by focusing on the water. It took about half an hour to see the fringes, I used the reflection of an LED merging together, the glass under water reflection's about 20 times dimmer than the water one. Not much fun and not a useful test either. Needs a better setup.
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10" f/5 dob unf.
7x50 Tasco binos
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Ed Jones
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Actually the reflectivity of glass is only 5 times dimmer. I've made a number of flats using this test and no it's not as nice as using a reference flat in air. However if you lack a reference flat I think it is easier than making 3 flats for the 3 flat method IMHO. With the right light source, collimating lens and set up it is a useful test and kind of cool. It's biggest drawback is finding the required collimating lens but then Dick used a mirror for his collimator.
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Ed Jones
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Mark Harry
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Why can't you specify a long distance between source and flat tested? it would minimise the effect, and negate the necessity of the lens altogether. Can't remember, but isn't it around a 20:1 ratio of flat diameter?
I still say it would be easier to shine a compact flourescent at the ceiling with a glare shield. It's 10 times better than a hanging light illuminating the flats as the reflection off your pan of water shows. The bands will be very evident.
Mark
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Ed Jones
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That's a fun math exercise Mark. For the worse case 1/8 inch water thickness or 1/4 inch of path length at a sodium wavelength of .441 in water gives 14,399 waves path length (my earlier math was off I think). If we want no more than a 1/10 wave error then cos(14399/14399.1)= .999993 or 0.21 degrees half angle. That's about 45 feet or 134:1. Of course 1/8 inch is a bit much but this distance is proportional to the water thickness and mirror size. I guess you could set up a telescope at the right distance away with an ordinary mirror folding the path down to eliminate the collimating lens. Might work.
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Ed Jones
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Dick Parker
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Ed -
Question. Since we are discussing diffuse light, is it the distance of the light source or the person viewing the test that is important. Consider the old standby flat tester pictured in all the telescope making texts (attachment) where a diffuse monochromatic light is housed above the piece being tested. The diffuse light shines down through a pane of glass at 45 degrees angle to the person viewing the test. This pane of glass acts as a beam splitter, so the light, after producing fringes, reflects out to the person viewing the test. Would stepping back several feet from the test set up accomplish the same thing?? I often do that when I want a quick sanity check of something I'm working on. For critical evaluation I use my Fizeau interferometer that provides collimated light.
Thanks
Dick Parker
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DAVIDG
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Quote:
I guess you could set up a telescope at the right distance away with an ordinary mirror folding the path down to eliminate the collimating lens. Might work.
That is almost exactly what Dick Parker did and was written up in his ATMJ article on doing the test. He hang a Netwonian off the ceiling facing down and put a neon bulb at the focus. Dick came to the Delmarva Mirror Class this year with beautiful autocollimation setup in which he made the 12" flat via the water test.
- 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
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Stepping back 45 feet would work. A collimating lens is much simpler. Many projection TV screens have fresnel lenses which work nicely, you can sometimes find them on ebay cheap. I have several that I cut the center area out for a collimator lens.
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Ed Jones
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tim53
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I thought I read somewhere (maybe an old S&T Gleanings article?) about adding liquid detergent to the water test to change the surface tension and allow the water layer to be thinner above the flat being tested?
I think that's what I did when I tried this test on a Meade 2.6" 2ndary mirror that I suspected was the culprit in my 8" Newt not delivering sharp planetary views (I was right, the mirror seemed to be lousy!). I still have the mirror, so maybe someday I'll try this again to make sure I did everything right.
-Tim.
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"We`re just waiting looking skyward as the days come down.
Someone promised there`d be answers, if we stayed around."
-Orchestral Maneuvers in the Dark, "The Romance of the Telescope"
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Ed Jones
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It occurs to me that another way to do this without a collimator lens is to photograph the fringes in a well defined setup. Knowing the water thickness accurately and the distance to the camera you could calculate how many rings of power you would expect to see then using a fringe analysis program like Fringe XP subtract them out to get the true wavefront. You would need to make a fixture sort of like a spherometer but with a needle tip to measure the water thickness exactly. It might be worth contacting Dave Rowe to see what it would take to do this. You could then use a light box like Dave built. What do you think?
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Ed Jones
Edited by Ed Jones (07/03/09 12:31 PM)
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DAVIDG
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It seems that the major hang up on the water test is making collimated light, especially if you want to test a big flats since you need a big lens. I had to dig around for a couple of months before I found 8" plano convex lens to use. I couldn't find a fresnel lens with a long enough focal length. I'm wondering if a simple solution is inexpensive spherical reflector, the type that is used in commerical and house hold lighting ? The quality of the collimated light doesn't need to be perfect. A small neon blub is placed at the focus it is hung above the water container and then one uses a simple 45 degree window glass beam splitter to view the test.
One thing I thought to try is to add a water soluble polymer to the water. The polymer would increase the vicosity of the water but won't change the refractive index by much. The higher viscosity would dampen vibration much quicker. I have an article on doing the test from the British Ast. Society in which they add Gum of Aerobic to the water which is doing the same thing but not as well as high molecular weight polymer would.
- 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
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Dave,
I really don't have that much problem with vibrations that I would want to put any additives in the water. I found that below a certain water thickness the vibratons are dampened automatically perhaps due to friction with the glass. Maybe Dave or greenglass have seen this effect? The only thing that I do add to the water sometimes is a strong dye (food coloring) when I test aluminized mirrors. It cuts down the reflectivity of the aluminum to enhance the fringe contrast.
BTY I posed my idea to the Yahoo interferometry group to get their input.
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Ed Jones
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DAVIDG
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Ed,
I havene't had a problem with vibration either put just wondering if the polymer would make things even better. Like I said, the other article I have says the Gum of Aerobic helps and people like Tywmann used it.
I think your idea would work fine, just wondering about the added complexity of the setup vs finding a simple method to make large diameter collimated light.
Here's an idea, what if you placed in the water a small flat of know quality as an internal standard along with the flat being tested. If you photograph it and the flat under test at the same time, you should be able to back out the error introduced by the thickness of the water and angle of the light by looking at how much error was introduced into the fringes on the good flat, then apply that correction to the one under test ?
- 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
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Dave,
I think a thinner water level works best, adding a thickener would require a thicker water level.
My idea wouldn't be a more complex setup, just a typical light box and a simple water thickness checker (I'll make one up to prove it out). I could write a simple Excel program to calculate the fringe pattern given the viewing distance and water thickness. This would keep water as the reference instead of your smaller flat. It would be nice if this could be incorporated into a fringe analysis program.
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Ed Jones
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Gordon Rayner
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I found my copy of D. F. Horne's book, OPTICAL PRODUCTION TECHNOLOGY, Crane, Russak & Company, New York, 1972 which is aimed at the professional optician, with many machines illustrated.
The pentaprism moving across the mirror on a track, together with an autocollimating telescope, is described in the context of measuring radii of curvature, so it is only peripherally related to the topic of this thread.
However, in the section just preceding the spherometry discussion in Horne, he has an illustrated discussion of Fizeau equipment and techniques. The 45 cm aperture liquid reference Fizeau interferometer at the U. K. National Physical Laboratory at Teddington is shown in cross-section.
I bought the book from the estate of A.S. DeVany, author of Master Optical Techniques (Wiley?) My copy is in storage, so I cannot comment on what is in there about this thread topic.
I have a 5 inch Davidson Fizeau planinterferometer which I am not using, as well as a professionally made Newton's rings viewing box. I am in southern California.
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greenglass
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Been using this Rayleigh water tester last evening and it works ok, the magnifying lens is 90 mm and 11 3/4" f.l. I put it that far from the light ( white compact flourescent ) there's a 1" opening for the light and white paper to diffuse it. Using the flashlight to level the flat. Once I lined up the reflections and looked in the splitter window and was lucky to see fringes. Now the round flat looks like 5 fringes concave, not 20 from the no lens test. I tilted the lens to move the 2 reflections of the bulb off the flat. I think plain water works ok, didn't try an aluminumized flat and food color yet. They are glued to the spiders. An interesting tester. Thanks for info.
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/04/09 07:08 PM)
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greenglass
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Rayleigh water tester, needs a neon bulb instead.
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/04/09 06:47 PM)
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Ed Jones
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Congratulations greenglass! It's not the easiest test to master.
I made this water thickness tester for testing water thickness (only 10 minutes work). I bored a 1/2 inch hole in a block of maple, nailed in 3 aluminum nails, cut off the heads and filed them smooth. I can zero it out on a flat glass. The micrometer barrel has a ball tip so I back it out, set it on the glass under water and slowly screw it in. When it touches the water it is easy to see repeatable to a few thousandths. I set up an 8 inch flat and measured water thicknesses to see which was best. Here are the results in inches:
.110 fringes too wigly and not much dampening
.052 better dampening but still a bit wigly, useable
.035 best overall, good dampening took about 15 seconds
.020 not useable, leveling took way too long
Looks like around a millimeter was the best, this may vary with size though. This flat was pretty good except for a rolled edge and I counted about 8 rings without the collimator lens so I think it might be possible to work up a method without the lens.
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Ed Jones
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DAVIDG
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GreenGlass,
Try using a red filter. You don't need to filter the source if you don't have a filter large enough to cover the opening for the bulb. Simply looking through the filter will isolate the red emmission line from CFL bulb and make the fringes much easier to see.
From the design of your light box, it looks like the articles were useful.
- 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
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I was able to model the water test (without using a lens) in Zemax. I measured my viewing distance at 49 inches and knowing the water thickness Zemax computed 8.3 fringes, good agreement with my fringe count. The picture below is the computed interferogram at 49 inches on the left and at 10 feet away on the right for my 8 inch mirror. If you wanted to make a big flat for autocollimation and didn't have a big lens for testing you could use Zemax to compute what the fringe pattern should be.
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Ed Jones
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Gordon Rayner
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Someone , somewhere has pointed out that autocollimation "flats" do not need to be perfectly flat, just smooth mirrors of very long radius of curvature.
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Dick Parker
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Yes, it has been documented true that an autocollimation flat does not have to be flat. Malacara, in Optical Shop Testing, appendix 4 gives the following formula d=128*(F/D)^2*e, where d is the depth of the sagetta of a non flat autocollimation flat in fringes and e is the error you will get in your tested mirror in fringes. Essentially, if your flat is not flat, you will, in effect, be testing with a light source closer to or further away than infinity, so you will get an ellipse or a hyperbola. You can be hundreds of fringes off (depending on the focal ratio of the mirror you are testing and the final error in the parabloid you desire)
I suggest you consider that "being smooth" means on hills, holes, zones, edge problems, aspheric surfaces etc. As a practical matter, using collimated light against a good reference flat gives intereference bands that are straight and parallel when the optic being tested exactly matches the reference, in this case a known flat. This becomes a null and errors are easy to detect.
When you are a several fringes off, you have a bull's eye and rings. Although you can count the fringes, that is not all. It is the progression distance of each successive ring from its adjacent one and the shape of the bull's eye that will help determine if your deviation from flat is a true spherical deviation. Also, as you press on one side and the bull's eye moves off center to one side, that circle should remain round. If it changes to an oval or a pear or something else, you have zones or an aspheric deviation from a pure spherical deviation from flat.
I suggest that it is so much easier, in the long run, to make the investment in time and effort to make the slight effort to get collimated light, figure the flat as flat as you can. Don't accept excuses and therefore get results from your autocollimation flat you can be confident in.
My 2 cents
Dick Parker
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Mark Harry
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Reged: 09/05/05
Posts: 3086
Loc: Northeast
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In the formula above, I agree wholeheartedly with Dick's assessment of flat quality. Make it the best you can. In hypothetical situations, you'll see that a flat that's only 1-2 fringes off will allow you to very closely assess the quantification aspects of overall error -RELIABLY-.
For the guys who want just what may be "good enough", go ahead and use a flat that's just a long sphere. But don't expect everyone whom may be savvy enough to test reliably to believe it!
*************
A question for Ed- does that ZEMAX representation make the flat appear convex in fit?
Mark
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So many projects, so little time!
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Coletti
newbie
Reged: 06/12/09
Posts: 4
Loc: Brazil - Curitiba city
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If the intention is to make the collimation lens for Rayleigh test:
What is the accuracy needed in the surface?
Consider a soda-lime glass, 1 "thick, the bending of the lens causes problems in water test?
I need to make an 12" optical flat, so need a collimating lens of D=13" at least, right? The focus of these collimation lens can be short than F/D5?
Polishing a lens so should be very difficult if you need high-precision surface...
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Best regards,
Sandro Coletti - Brazil
http://sandrocoletti.multiply.com/
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Ed Jones
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Loc: Sin-sin-atti
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Mark,
Without a lens the fringes appear concave. By making the test flat a very long radius convex sphere I can make the fringes straight. If anyone wants this Zemax file I can share it. I'm convinced that this can be a workable method of making a flat with an ordinary light box if you allow enough viewing distance so as not to have too many fringes and use Zemax to compute the fringe pattern.
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Ed Jones
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Dick Parker
sage
Reged: 08/17/07
Posts: 241
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Sandro -
When I made my 12.5 inch autocollimation flat I (temporarily) suspended my 12.5 inch F/5 telescope from the ceiling pointing down as my collimation light source. That gave me full aperture water test with good null fringes at no cost and probably an aftenoons worth of fussing around.
Dick Parker
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Dick Parker
sage
Reged: 08/17/07
Posts: 241
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If I understand Ed's idea, knowing the water depth and viewing distance, if you produce a null (straight fringes), you will end up with a flat that is only a few rings convex which would be acceptable flatness for an autocollimation flat. By nulling in this condition, you assure that the surface is smooth. This sounds like a very interesting and workable idea.
My earlier discussion was aimed at the frequent comments that an autocollimation flat does not have to be flat, but does have to be smooth. I wanted to address the relative difficulty of assessing smoothness if you are off flat by several rings. (you could use the Ritchey - Common test to sort that out, but that is a bit OT).
Need not overestimate the difficulty of obtaining a collimated light source. All you need is a Newtonian telescope, which everybody has, and you have a Fizeau interferometer of adequate quality at no cost, and only a bit of ingenuity to get it to work.
Dick Parker
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Ed Jones
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Dick,
It might be more than a few fringes depending on how big the flat is and how close you view it. You can use Zemax to calculate the number of fringes you would see for a given distance away. For example a 30 inch flat with 1 mm of water would show 5.4 fringes viewed at 20 feet but 71 fringes viewed at 6 feet away! A 30 inch primary would be work fine as a collimator but would be hard to come by.
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Ed Jones
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 955
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Are there now diode lasers stable enough , and with sufficient coherence length, at low cost, to supplant/replace HeNe lasers in replacing the old light sources in the Davidson Fizeau planinterferometers?
I have seen diffusing adhesive tape, or ground plastic or groundglass transmission windows, some slowly rotating, as speckle eliminators for HeNe replacements of the original lightsources.
Would such lasers be applicable to the water test, (with pinholes, and/or beam expanders, beamsplitters, video cameras, or other appropriate items)?
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1394
Loc: Sin-sin-atti
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Many monochromatic light sources will show fringes but a point source would be harder to use than larger broader source IMHO.
Another useful application of this test is testing diagonal mirrors. However the fringes are very difficult to see with an aluminized mirror, 40 to 1 contrast. You can add a strong food coloring solution to absorb the reflection off the aluminum or spray on a weak mixture of cerium oxide which settles out on the mirror and acts like a filter. I look at the reflections of a green laser and try to get both reflections equally bright. You can keep spraying on more cerium to knock down the mirrors reflectivity. If you get both reflections the same the contrast is as good as in air. The cerium won't hurt the aluminum.
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Ed Jones
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refractory
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Reged: 02/05/05
Posts: 1226
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As some of you already know, I have a 26 inch optical comparator mirror from an old 50 inch Jones and Lamson unit. The surface has a shallow, narrow winding scratch on one side, and a couple of tiny nicks here and there, but otherwise nearly flawless at least to the eye. There is a deep conchoidal chip on the back starting at the edge, where there was a drilled hole that I guess originally held a pin to hold the mirror on its mount.
I did some star tests last year with a Celestron CR6 refractor, and was able to get pinpoint stars at many magnifications at high angles, though above/below focus the images were ellipsoidal- possibly due to the lack of proper support of the mirror?
Anyway, I don't have proper equipment here to do testing- could the water method work, or would I risk damage to the mirror surface? Or would this even work with a mirrored surface?
Thanks,
Jess Tauber
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3086
Loc: Northeast
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There's an easy way to see the IF pattern when dealing with a coated flat on an interferometer. I used to peel one layer of a Kleenex off, and drape it on the secondary. It's a bit mottled from the random density of fibers, but it makes the pattern easy to see, with no liquids or abrasives on the tested glass. (the same idea as the cerium on the glass)
Mark
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So many projects, so little time!
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Coletti
newbie
Reged: 06/12/09
Posts: 4
Loc: Brazil - Curitiba city
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Quote:
Sandro -
When I made my 12.5 inch autocollimation flat I (temporarily) suspended my 12.5 inch F/5 telescope from the ceiling pointing down as my collimation light source. That gave me full aperture water test with good null fringes at no cost and probably an aftenoons worth of fussing around.
Dick Parker
Hello Dick
This is what you did?
http://spie.org/x13539.xml?ArticleID=x13539
In the article, the author says to use an off-axis Parabolic mirror, If you used a common Parabolic mirror, then it is easier. Great tip, thanks.
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Best regards,
Sandro Coletti - Brazil
http://sandrocoletti.multiply.com/
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Ed Jones
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A big reflective mirror would be hard to learn on. I'd suggest learning on a small uncoated mirror first. Getting and even density of dye or cerium over a 26 in. mirror might be tricky.
Here's a diagonal I tested yesterday, you can see variations from the cerium nonuniformity. Still works though. I should have used a green filter too.
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 539
Loc: Ontario, Canada
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How is this for flatness? It's 3.5" of a 4" flat. Ronald
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/08/09 04:31 PM)
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Ed Jones
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Its astigmatic. Looks like you clipped it a little too. Did you use the water test? If so each fringe is 3/8 wave. Fringes on a good flat are straight, parallel and spaced the same.
BTY I found a fluorescent light fixture at Wally World for $5 and the F4T5/BL bulb at replacementlightbulbs.com.
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Ed Jones
Edited by Ed Jones (07/08/09 05:30 PM)
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greenglass
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Reged: 01/22/06
Posts: 539
Loc: Ontario, Canada
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Yes it's the same water tester, so that's why the fringes fan out, astigmatism, that would explain the straight fringes and curved when they're at another angle across it. Ronald
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/08/09 07:18 PM)
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 955
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I want to test some aluminized diagonal flat mirrors. I have never used my Davidson Optronics 5 inch planinterferometer. My limited knowledge about Fizeau testing has been dormant for over five years.
It is inconvenient to unpack the Davidson at this time to experiment. I had determined to sell it, and probably shall do so, but this thread has re-interested me .
Please confirm/refute/comment:
Using a laser source increases the coherence length, so fringes can be obtained at much greater spacing, between the reference transmission window with one flat surface and the element being tested(ff. ZYGO (?))?. Diffuse filtering screens can be placed in this conveniently increased gap(?).
Accordingly, to avoid draping light filtering Kleenex on the aluminized mirror(which is probably not a problem),to reduce the reflected light and thus increase fringe contrast, could one use one or more mosquito netting screens above, and not touching, the flat mirror being tested? Any Moire patterns caused by using two or more screens(whose screen patterns would be not parallel, unless great pains were taken to make them not rotated with respect to each other) for this filtration could be ignored (?).
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Ed Jones
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Gordon,
Yes a HeNe laser has has a long coherence length, many meters. Mosquito netting might work, I was told nylon panty hose was one solution. I tried it many years ago and I believe it worked OK, not as nice as your typical interferogram, I don't remember any Moire problems but there is a lot of scattered light. Other solutions would be to put a partially transmissive coating on your reference flat, say 30% if you check mostly coated mirrors. Another slightly more expensive option is a partially transmittiong pellicle, they are fragile but work great.
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 539
Loc: Ontario, Canada
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This is what I had in my 5" dob, 1 fringe or 1/2 wave surface.
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/09/09 03:36 PM)
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 539
Loc: Ontario, Canada
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Same one another direction, well now I know it's a bad one.
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/09/09 03:34 PM)
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Ed Jones
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Quote:
1 fringe or 1/2 wave surface
I agree it looks like 1 fringe but it would be 1/2 wave in air only. Under water however 1 fringe is only 3/8 wave because the wavelength in water is shorter than in air. It's a more sensitive test in water.
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 539
Loc: Ontario, Canada
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2.25" minor axis seems flat
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/11/09 01:14 AM)
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Ed Jones
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Posts: 1394
Loc: Sin-sin-atti
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I think you're right but the contrast is a bit low. Can you elaborate on how you did it? How difficult is it for you to do?
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Ed Jones
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 539
Loc: Ontario, Canada
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I used the water Rayleigh tester I made last week, I took about 20 pictures and took an hour to find the fringes and used red and green food color cause it has an old silver coating that's 1/2 gone. Sometimes it takes over an hour to find the fringes, or rarely a couple minutes, mostly about 20 minutes, the pictures came out very yellow for some reason, that's why the contrast is poor. Also I noticed the water moves and gives false readings, my dog walking upstairs shakes the water a couple fringes and I'm on a cement basement, window closed, back about 2 feet from the tester so I don't breathe on it, and don't move around in the chair and wait 10 seconds and take a picture. Still some pictures show bent fringes which bend another direction in the next photo without touching anything. But most are simular.
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (07/11/09 08:57 PM)
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 620
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stoped by ed/s and he showed me the water test /ed makes it easy .cool test
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 539
Loc: Ontario, Canada
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I found a neon night light at dollarama
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10" f/5 dob unf.
7x50 Tasco binos
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1394
Loc: Sin-sin-atti
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If you use food coloring or better yet a cerium mixture try to get the reflection from the aluminized flat equal in brightness to the reflection off the water surface. If you do that before you bring the 2 laser dots together the fringes will be very easy to see.
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Ed Jones
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