Re: [ATM] No fringes

[Mike Lockwood <melockwo@uiuc.edu>]

Chris,

Chris Dalla Piazza wrote:
> I think my spacing must have been off because I did eventually see fringes
> with an ordinary fluorescent.  Once I used newspaper instead of the leftover
> specs of shellac they were apparent.  I think the newspaper actually
> increased the air gap.

Be careful with the increased air gap - the wider the air gap, the 
greater the effect of having a non-collimated light beam introduces 
errors into the results.

I use NO spacers, just the air gap, and my rule of thumb is to view 
the fringes from at least 10 times the diameter of the mirror under 
test away from the test setup.  For a wider gap you may need to go 
farther back than that, and you should use a beamsplitter to enable 
viewing the test pieces from directly overhead (without your head 
blocking the light source).

Once you have the mirror and test surface matching to a few fringes, 
the changes in the fringe shape are evident as you watch the fringes 
while backing away from the tester.

	Mike Lockwood


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