Re: [ATM] Sphereometer accuracy

[Stan Truitt <s.truitt@verizon.net>]

Aloha,

>Yup, +/- a mm or so... I have thus far survived. I have not
>yet worked a scope that had specs that could not tolerate
>an actual ROC that differed from the specs by this amount.
>Have a lot of (any) ATMs encountered such extreme requirements?
Yes, in two arenas:

One, in lensmaking.  e.g. Maksutov shells, and large N.A. transmissive domes.

Two, in matching or pre-fabricating RC "Cass" secondaries to ensure 
the hyperboloidal conjugate separation matches the prescription, and 
bestows a correct aspheric coefficient, and also provides coma-free 
correction while "zeroing" the system spherical aberration. All this 
being done while keeping the "Cass" back focus within range of the 
focuser.

>Regarding convexes, I simply measure the tool, which
>should be a very close replica.
I also use a very long working distance autocollimating microscope 
mounted on a measuring translating stage. The operator focuses on the 
flash polished vertex of the optic (taking a reading), and then 
moving until the microscope focus comes into coincidence with the 
center of curvature of that surface, and returns the reflected beam 
through the microscope (then taking a second reading). The difference 
between readings being a precisely measured Rc.  With this technique, 
large departures from sphericity of the test surface requires a 
modified setup.  Of course, no female test plate is required; and a 
very sensitive, and direct, measure of astig. is provided without 
complex fringe, or poorly understood, Ronchi analysis.


>  > It also takes more time, and unless you set
>>  up some equipment it can be tricky to get an accurate
>>  measurement when
>
>
>Maybe depends on what you have handy. I started doing
>this because it was really fast and seemed at least as
>good as a good spherometer used properly at the optical
>surface.
Well put!


>The only one I have seen seemed to have similar precision as
>the spherometers we are more familiar with. I would appreciate
>if someone here can tell me what this device might be called
>and what precision is typical.
The ones with which I am familiar were called profilometers, and were 
not specific to opthalmics.
Precisions on the order of ten millimicrometers are attainable, and 
accuracies better than 100 millimicrometers are not uncommon, 
depending on the basic curvature and finish of the surface.


Fascinating devices!

Stan Truitt

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