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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Quote:
Mike's box with the beam splitter window is a very nice feature since it allows one to view the fringes straight down and on the optical axis. If you view the fringes on an angle there will be false curvature introduced.
A regular piece of 1/4" plate glass can be used for the beamsplitter, but the upper and lower faces have the same reflection brightness, creating two images of the fringes, one shifted above the other slightly. Using the 50/50 window glass with the coated side down essentially eliminates any ghost image visibility. It really makes a difference in fringe contrast. I got two pieces from the local PPG store in Fort Worth, one for me and one for Walt Goldys. Walt uses mercury lights and a green filter rather than the sodium light I'm using. Both Newtons give nice contrasty fringes.
Multicolored fringes have much lower contrast, as the interference periods overlap each other. Fluorescent lights can give very faint green fringes when no other light source is available. The wavelength used for analysis is simply whatever the dominant color of the fringes you're using. Low contrast multicolored fringes are OK for visual work but not for CCD-based fringe processing.
I like either the 589nm or 546nm lines best for the Newton interferometer. Without the green filter, Walt's Hg Newton does show the violet fringe patterns along with the more easily visible green fringes, but the green is so much brighter you don't notice them visually. A DSLR sees the violet fringes clearly though, and you have to use a green filter if you plan to photograph them. The Na fringes have very high contrast with no filter needed.
Mike
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Kevin, that window must be flat to within about 1/4 grain bin! 
I like the phase shift in the grainbin fringes where the window is cracked.
Mike
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Mike,
Those are really nice vivid contrasty fringes. And a lot safer, better and easier setup than a salt flame. Could you give us a link to the light and ballast?
Thanks,
Gary
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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I think I'll have to find a smaller light, smallest Ive come across is 150W. We have some of those at work but it appears it'd be miles too much.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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kb4mxo
member
Reged: 07/17/06
Posts: 58
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Is the glass you are using a LowE coating? It the glass 6mm thick? I think the place I work for makes a glass like it. What it will do is let light pass with a loss of 1% and will reflect it back if it hits the coating. I am using this in my indirect passive solar room.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3129
Loc: Northeast
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Dave if you'll pay a closer note, the 2 basic colors exibited are a greenish blue, and a violet. You can assign the matching values for the wavelength when doing the calcs. This doesn't apply with the true Newton's fringes, or the primary pattern that applies with near perfect or convex fits. This is where you'll see -ALL- the colors highly reinforced; and they can be used directly to assess errors in the surface fit. It takes practice/experience to do the latter accurately. When you can do this without scratching either piece, you might have enough!(and a -GOOD- thing, too!) 
Mark
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So many projects, so little time!
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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Mark,
The issue is that both the greenish blue and violet fringes are not pure monochrome wavelengths but are caused by overlapping fringes. The eye combines the two colors to make the colors you see in the fringes. The problem with this that the fringe spacing is no longer 1/2 the wavelength of light. The width of the fringe is now a combination of the two or more wavelengths. Fringe width at a given air gap varies with wavelength. So if you used 632nm red light you would only see a few fringes compared to the same air gap and you used 471nm blue light. You don't know what percentage of the width of the fringe is coming from one wavelength vs the other, so there is big uncertinity in the accuracy of the measurement. This is why an interference test is done in pure monochrome light with a known wavelength. CFL bulbs have around 7 to 15 different emmission lines which cause overlapping fringes.
My drawing hopefully illustrates my point that the blue-green fringe in your pictures can be made up of the emmission line at 546 and one at 471nm but the width of the line is not 1/2 wavelength at 546 or 471 and is not a 50/50 split between the two wavelengths. By simply filtering the light source so one only sees one wavelength all this uncertainty goes away.
- 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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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3129
Loc: Northeast
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I have a couple more pics of the last one in the series. This one's pretty good.
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3129
Loc: Northeast
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The last pic. Also, of the polisher that did the work on the three.
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3129
Loc: Northeast
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Nice, round bullseye, good bands out to the edge, etc. The way it should turn out! As a group, all are certainly easily passable. There is still a very slight edge issue, and 2 ROCs evident between the center and outer ring. One fringe convex fit.
Dave if you want to split hairs with calculating whether it's 5430mm or 5435.102mm, knock yourself out. I'll gladly leave it to you. In this case, it's not necessary, has to be about .5% difference of the primary's curvature, if I recall the mfr's specs correctly. (I'm within 1/2-2/10%~) You should -NOT- make the calculation on anything but the primary fringe pattern where the surfaces are closest for best results. If you learn what all those colors mean with CFLs, you'd add a handy tidbit to your testing bag of tricks!
Later, and enjoy,
Mark
Had to edit the tolerance,sorry. I think I'm getting the SWINE FLU! Actually, kissed the wife when she had a headacke a couple days ago. I'm paying for it now.
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So many projects, so little time!
Edited by Mark Harry (10/07/09 04:03 PM)
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3129
Loc: Northeast
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Been setting on the table about 2 hours as I left it this afternoon. Oh, yeah!
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So many projects, so little time!
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