From: Dale Eason 
To: Arthur , "ATM{at}shore.net"

Reply-To: Dale Eason 


That is because the image from the mask is not very interesting.  It is an
accurate test and should be used.  But to get an overall feel for the
mirror the Ronchi test can give you a quick qualitative feel.

The mask is used to help you get numbers to feed into a data reduction
program like TEX, Sixtest, or Figure45. But visually the images are not
very interesting and give little info to the outside observer.  You need to
know the longitudinal knife position relative to another zone to make any
sense out of it.  The observer usually cuts the knife into the image to get
two of the holes on each side of the mask to match in brightness.  Usually
you try to get them to wink out simultaneously.  It is the rate of dimming
that you look to be the same.  A still image cannot capture that.  So to
sum up a still image of the mask Foucault test needs more info than just
the image to let other people evaluate it.

Dale Eason

-+- Arthur  wrote:
>
> Hello everyone,
> I've enjoyed observing the images that are posted of
> mirrors being tested.
> It gives me a visual as to what I can look forward
> to when I start testing
> my work. I've mostly seen images of the KE and
> Ronchi tests. But, I'm
> curious as to why there are no images of the mask
> and zones tests, or have
> just missed them? This is test that is still used,
> isn't it?
>
> Arthur
>
>


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