greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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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
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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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
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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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
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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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
Post Laureate
Reged: 09/05/05
Posts: 3124
Loc: Northeast
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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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So many projects, so little time!
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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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
sage
Reged: 08/17/07
Posts: 247
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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
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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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
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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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
Pooh-Bah
Reged: 12/17/04
Posts: 1440
Loc: Highland Park, CA
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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
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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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
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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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
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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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
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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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
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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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
scholastic sledgehammer
Reged: 03/24/07
Posts: 965
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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
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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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
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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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
Pooh-Bah
Reged: 04/06/04
Posts: 1415
Loc: Sin-sin-atti
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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
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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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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