[ATM] Testing Techniques

[Ken Hunter]

Mitch...
   
  The biggest piece of advice I gave all my students over the years involved setting up for the Focault test.
   
   At the minimum, you need to:
   
  1. Place the tester in front of the mirror with the knife edge in it's center of travel position (crosswise to the optical axis and lengthwise along the optical axis).
   
  2. Move the tester about until you see the center of the mirror "Null out". This is done while keeping the KE centered as in step 1.
   
  3. Move the KE (Fwd or Back) to it's extreme limit along the optical axis and verify that the KE reflection in the mirror bisects the disk exactly at the top and bottom. If it doesnt, ROTATE the tester base until the above condition is met.
   
  4. Move the KE to the other extreme of it's travel (Fwd or Back) and verify that the disk is still bisected exactly in half but this time the reflected shadow is on the other half of the mirror. If it's not exactly bisected, ROTATE the tester base until the shadow is half way from it's offset position towards the desired bisected condition and then center it up (bisected) by moving the tester base (L-R).
   
  5. Following this setup (which may take a couple of iterations of the above) you should be able to watch the zones roll nicely across the face of the mirror by simply moving the KE along the optical axis.
   
  What you have just accomplished is to align the tester axis to the optical axis.
   
  Once this is done, you are able to perform the Focault test simply by moving the KE along the optical axis and take your measurements with possibly very little sideways motion needed. If the tester is not aligned this way,  the geometry of the test is not proper and errors are introduced especially if you are doing the poor mans caustic test where the measurements are made on one side of the optical axis and the position of the caustic null is assumed to be equal on both sides of the axis.
   
  Hope this tip helps.
   
  Ken Hunter
  

Mitchell R <funnybone101 at embarqmail.com> wrote:
  Hey Everyone!



With the figuring on my 10" continuing I thought I just might stimulate
some conversation by seeing if anyone has any tips or advice on getting
accurate Foucault readings.



I find practice helps, as well as my low power (2.5X) spotting scope. 



I've found it is good to take readings going from center to edge then
edge to center, and that the edge zone is a real bugger to get good. There
is more diffraction around mask zones opposite the knife edge and that
sometimes tricks me into nulling differently.



No specific question, just looking for tips, advice, commentary, etc.



Mitch 





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