Re: [ATM] How do I test a convex surface?

[Dominic-Luc Webb <dlwebb@canit.se>]

On Mon, 26 Mar 2007, Chris Dalla Piazza wrote:

> Do you need anything exotic for the tank glass or would a regular aquarium
> tank do the trick?
>
> Even if the aquarium glass compromises the quality of the wavefront, I am
> only interested in detecting errors at the edge or center.  I could pretty
> much ignore any striatic, rough appearing, or irregular errors since I don't
> have a history of that problem.  I never have trouble obtaining a sphere so
> even if I munge the solution slightly, I am guessing the result is slight
> spherical aberration which I can also ignore.
>
> Am I right in thinking that the only effect of the flat air/glass and
> glass/solution interfaces is to alter the apparent ROC of the lens?


> This test should also eliminate errors from the surface not being tested.
> This sounds pretty neat.  I think I'm going to be googling "Salt water index
> of refraction" in the near future.

The tank does not need to do anything more than hold the solution,
a tube with slightly larger diameter than the lens will due. The
tank material does not take part in the test. Just think of the
usual tester in which the space between the tester and the lens is
housed in a tube filled with liquid and the back side of the lens
is what gives rise to reflection that you will measure on the
tester. The surface closest to the tester is invisible and does not
contribute to the signal, so you test (and only see) the back
side of the lens as a concave.

The solution interface, with its matched RI, makes the lens invisible.
This is in fact how you prepare the solution. You keep mixing until
the lens disappears when submerged in this solution. ROC is not
affected.

Sugar might be more effective for you than salt. Common glass has an
RI around 1.52. Air is about 1.000. Water is about 1.333. Sugar (80%)
is about 1.49 (close, actually) while seawater is about 1.339. The
most straightforward solution would be microscope immersion oil, since
this is quite deliberately manufactured to have the same RI as optical
glass in microscope objectives. Obviously, this is expensive and not
made to perfectly match your particular glass, so cheap substitutes
are desirable. I believe Johnson's brand Baby Oil is common in the
USA (I am guessing you are there) and in many other countries. I
have read somewhere that it has a refractive index of 1.54, which is
quite close to optical glass (might be what I used long ago). This is
close enough that playing with wavelength or temperature might give a
perfect match (if needed). Otherwise, most other oils will have lower
RI than glass, so combine with one of these to get a perfect match.
Again, this is determined by disappearance of the lens when submerged.

Dominic-Luc Webb

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