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More on the immersion null

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#1 Ed Jones

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Posted 22 July 2019 - 02:53 PM

  After reading the April 1964 article in Sky I decided i would use this test for my 12 inch F/4, well actually F/4.3.  The biggest drawback is testing vertical and this put me 8.6 feet up from the mirror on a ladder in my garage where I had over 9 foot ceilings but it really wasn't a big problem.  This was back before fiber optics were common so I used a micro flashlight bulb, frosted glass and a slit measuring about 1/4 inch.  However a light bulb in the light path generates heat causing heat currents in the light path fairly quickly but I learned to quickly evaluate it once I turned it on or weight till it cooled.  A single fiber optic solves this problem and is much smaller, almost no obstruction and cheap.

 

  Water has too low an index to use the Ronchi test (with the Ronchi and light source together).  You would end up with a -0.77 conic; maybe good for a Dall-Kirkham primary but not a parabola.  A fluid with a higher index of 1.426 would be right for a parabola but (1) there are no common liquids with that index and (2) even if you make one the Fresnel equation gives the reflectivity of Pyrex immersed in it as .027% almost 10 times worse than water at 0.25%.  The solution is to separate light source and knife edge.  If you bring the KE out a little past the ROC and move the fiber back to bring the image out to the KE it will be a good null for a parabola.  By changing these conjugate distances you can get other conics from DK primaries to hyperbolics.  

 

  Alignment is pretty simple since you have only one optic to align.  I marked the center of my mirror with a magic marker then used a plumb bob (a piece of string with a fishing weight) hanging from the KE to the center mark on the mirror.  Then I adjust the mirror to do a KE test (dry) at the ROC.  I then would move the KE out the short distance difference and then cover the mirror in water in an oversized pan to the right depth.  Then I move the light source (fiber) to bring the image back to the KE where it is a null.  A null test is very sensitive and is about 1/3 more sensitive than the Ross because the reflection occurs in water.  The residual test error is very small less than 1/40 wave.

 

 This test is not expensive to do; all you need is a single mode fiber and a pan of water.  A fiber optic is pretty cheap but it takes some patients to use.  You have to remove the cladding then scratch the fiber and bend it to where it breaks at the scratch.  You then project it onto a white screen to see if the output is an evenly illuminated disk.  More often you'll see light coming out the side of the end and a distorted output on the screen.  It usually takes many tries to get a good break.  I use a freshly broken piece of plate glass but they make a clever for doing this with a sapphire cutter but sort of expensive.  A red LED will work well for illumination.  Being an inline test you can replace the KE and use a high power eyepiece and examine the fiber optic end for sharpness and astigmatism, like looking at a small planet.

 

 Of course the test is sensitive to vibrations and air currents but those shouldn't be much of a problem.   To test a parabola you can use this example and scale it to your ROC.  Just divide your ROC by 100 and scale all the spacings. 

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#2 BGRE

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Posted 22 July 2019 - 03:06 PM

The fiber in the link isn't actually single mode in the visible.

This fiber is:

https://www.thorlabs...artnumber=SM450

However its NA is too small to fully illuminate the test surface.



#3 DAVIDG

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Posted 22 July 2019 - 04:31 PM

"A fluid with a higher index of 1.426 would be right for a parabola but (1) there are no common liquids with that index and (2) even if you make one the Fresnel equation gives the reflectivity of Pyrex immersed in it as .027% almost 10 times worse than water at 0.25%."

   If you did want  to do the test with a  liquid that has a refractive index of 1.426 you can make it by dissolving a known  amount of sucrose ( white sugar ) in water  http://www.refractom...tasheet-sucrose

 

             - Dave 


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#4 Ed Jones

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Posted 22 July 2019 - 04:42 PM

Its mode isn't important here but the NA is.  At F/2.5 this fiber is almost twice the Airy disk diameter but still works fine.  



#5 Ed Jones

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Posted 22 July 2019 - 04:47 PM

If the mirror is aluminized then the correct index fluid could be used but uncoated the reflection would be very faint.



#6 BGRE

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Posted 22 July 2019 - 05:56 PM

Its mode isn't important here but the NA is.  At F/2.5 this fiber is almost twice the Airy disk diameter but still works fine.  

Just dont use a laser source or multimode operation will result in "interesting" effects.

There are ways to circumvent these but fiber shakers and other mode stirrers increase the complexity of the setup.

Even with a non laser source its important to "fill" all the modes.



#7 BGRE

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Posted 22 July 2019 - 06:06 PM

Another solution is to double pass the setup by adding a small return sphere whose CoC  is located at the short conjugate and testing is performed at the long conjugate where the test and return beams are around F/5 for the example given.

A suitable high accuracy small diameter return sphere is required.

It may be feasible (with an interferometer) to use a variant of the random ball technique where the measurements are averaged for a number of random orientations of the return sphere. Readily available grade 5 silicon nitride balls may be suitable for the return sphere if this method is used.



#8 ccaissie

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Posted 23 July 2019 - 11:25 AM

Since reading of this null test in Malacara "Optical Shop testing" p.454, I worked to find a reasonably simple way to calculate it and set it up, but never used it.

 

Described by Holloran, the test can be used for convex surfaces as described by Purvayev.

 

Here's the page.

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#9 Ed Jones

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Posted 23 July 2019 - 12:14 PM

Testing through the back you will have to deal with the SA if you need a sphere. A hyperbolic cass secondary OTOH is a good test.  It's interesting that Malacara got only a 1/4 wave PV for a 1/2 meter f/2.5 parabola but I got 1/42 wave.  That's quite a difference, I'm pretty sure I'm correct but maybe someone else can weigh in.



#10 BGRE

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Posted 23 July 2019 - 10:19 PM

Ed

 

The section in Malacara on the immersion null only considered equal test conjugates, not the unequal test conjugates that you used. Meinel and Meinel found improved correction when using unequal test conjugates and testing a convex hyperboloid from the rear through the substrate.

 

I've added the unequal conjugate liquid immersion lens compensator to the list of things to do (along with using an evacuated test chamber). 

 

Perfect compensation isn't necessary with an interferometer as long as the residual is small and accurately known.




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