[ATM] 2d hartman test review

[Dale Eason <atmpob@yahoo.com>]

I recently wrote on this list that I had tried the 2D
hartman test and software the Jim Burrows wrote.  It
is well documented if you open the program and click
on the help button.

I tried it to measure my 16 x1 inch F5 plate glass
mirror.

It was harder to setup and execute than a Foucault
test.  It requires a point source and some sort of
camera preferably digital with no lens to capture the
test images. You also have to make a mask that is much
more complex than that used with Foucault.  I found
that part vary tedious.  It took me 2 days to make it
for my 16" mirror.  It is susceptible to poor seeing
in the test area.  Air currents cause it problems like
they do with an interferometer.  It is best to average
several images before analysis.  The analysis requires
a little user input to guide it and take 10 to 15
minutes on a fast machine.

It has a learning curve about like Foucault so it
takes awhile to get good results.  You have to invest
time to learn any mirror test method.

While I was trying out the 2d Hartman test I learned
how to make a Bath Interferometer.  It too is more
complex and also more expensive than the Foucault
setup.  However it is easier to execute once you have
built it.  Much more expensive than the Hartman but
easier to use.

Today I went back and compared the results from the
two methods.  I had never done that after I learned
how to use the Bath Interferometer.  The test results
are similar. Both gave similar strehls both saw the
major defects of the mirror in 2d, which were asitg,
turned up edge, high middle. 

Conclusion.
If you are willing to invest the time and have little
money then the 2d Hartman test is a good method.  It
is not easier to use than Foucault however.  If you
like to discover and learn new things it can be a lot
of fun.  The mask is not so much of a problem with
smaller mirrors.  

PS
Anyone interested in more info about the Bath
interferometer and its use is welcome to join the
Yahoo Interferometry group.

Dale Eason




		
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