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Inexpensive interferometers for measurement of fast spherical surfaces

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#26 BGRE

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Posted 21 June 2019 - 12:55 AM

I've used both SLR and DSLR cameras as well as industrial vision cameras. I merely take a sequence of exposures starting at 1s or so and halve each successive exposure, a couple of them will straddle the optimum exposure. One can then quickly home in on the optimum with the next exposure sequence using exposures that subdivide the interval between the pair in the previous sequence that straddle the optimum value.
Once you've done this a few times you can estimate the required exposure by eye.

Alternatively one can calculate the ballpark exposure and quickly home in.
Industrial vision cameras allow the exposure to adjusted in steps of a few tens of microsec.
Once one has a pair of exposures (with exposures that differ by a factor of 2) that straddle the optimum the optimum exposure is easily calculated.

#27 gr5org

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Posted 21 June 2019 - 11:55 AM

>I doubt that the test results would be accurate...

 

If DFTFringe has a problem with your igram it will be obvious - there will be huge 1 wave errors in areas that are too dark or some similar issue that is obvious.  It won't be some subtle 1/10 wave error.

 

You'd be amazed what kind of crappy igrams DFTFringe can handle.  You can get 1/50th wave accuracy (as compared to a zygo psi) if you average enough pictures but crappy looking igrams will probably be the least of your issues.  More importantly one has to worry about things like air currents and air stratification and induced astigmatism.  I talk about that stuff in my videos.  DFTFringe has a "test stand removal" procedure that involves taking at least one igram of your mirror, then rotating the mirror 90 degrees and taking a second and inputting those to the "test stand removal" wizard.  Not doing that means DFTFringe may show your mirror having astig when it doesn't.  Other than astig, bath interferomater plus DFTFringe is easier, cheaper and more accurate than ronchi.  At least from prices I've seen and what I've learned so far.


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#28 MKV

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Posted 21 June 2019 - 01:15 PM

I was able to get excellent 2017 sun eclipse pictures in RAW mode with my DSLR but with  these Igrams seem to be  different ...

I had similar issues with a SONY DSC H300 years ago. I then purchased a Samsung mirrorless digital camera and replaced its zoom lens with an old 35 mm film fixed focal length camera lens. I highly suggest mirrorless cameras (they are much lighter than the b ulky and heavy DSLRs) and old fixed-focal length lenses. And a camera with a manual override is a must. You probably also need to turn down your laser brightness with a variable potentiometer to a non-lasing mode, and if that doesn't work your var. potentiometer is inappropriate for the particular laser source used. For example I can't use my DPSS green laser with the same potentiometer that I use for my red laser diode.


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#29 gr5org

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Posted 21 June 2019 - 03:39 PM

My Bath IF works fine with the 5mw laser at full brightness.  It's important for the F/stop to be wide open so I have to control that on the camera with ISO down to the minimum (ISO 100) and shutter speed up at 1/1000 to 1/3000 of a second.  But it works fine.  It's really not that hard to use a Bath.  At least I didn't find any major gotchas.  Well the lens has to be close to the IF - so close for some people that it's almost touching the IF.


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#30 MKV

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Posted 21 June 2019 - 08:23 PM

It's really not that hard to use a Bath.  At least I didn't find any major gotchas.  Well the lens has to be close to the IF - so close for some people that it's almost touching the IF.

Compared to other IFs the Bath is the easiest to set up and operate; also definitely the cheapest and the simplest to build. However, the camera lens most definitely does not have to be close to the interferometer.

 

You can (and should) use (preferably) a small 2 to 3 power Keplerian relay telescope (because it's more compact than a Galilean version). Build one from suprlus lenses such as a 30 mm f/2 achromat, and a 25  mm eyepiece or a 10-power Hastigns triplet magnifier. 

 

Years ago, I originally used a large relay scope because I conveniently had two 50 mm achromatic objective lenses of approximately 2:1 focal lengths.

 

A relay scope lets you observe and record test results of any of the commonly available ATMs tests -- form the Ronchi and Foucault to IF.

 

Mladen

 

IMAG0644a (2).jpg


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#31 BGRE

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Posted 21 June 2019 - 09:21 PM

Since the "objective" lens forms a real image of the test surface and the incident light is divergent a doublet may give better performance if reversed, Raytracing can be used to explore this and other options. The "eyepiece" works with the camera lens as a a finite conjugate relay to reimage the test surface onto the image sensor.
If the real image formed by the "objective" is adjusted so that it lies in the focal plane of the "eyepiece" then the camera lens can be focused on infinity.
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