To: atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


Mark Holm wrote:

>Aditya Munshi wrote:
>
>>Is testing with an intermediate mask equivalent to testing with a mask
>>of 7-8 zones, what I mean to say is that because I'm not an expert at
>>Foucault testing ; testing with a 8 zone mask will no doubt have
>>doubtful readings.
>
>Yes, that is exactly why I made the intermediate mask feature in CouderMask.
>It is difficult to use a mask with narrow zones.  Two masks with nice width
>zones, offset to different radii is the way to go.  Recently I even used three
>masks on an 8 inch mirror because of edge trouble.  I had the normal 4 zone
>main mask, the 4 zone intermediate mask and a special mask with two 1/2 width
>zones right at the edge.  Those narrow ones were hard to read, but the results
>were very useful guiding me to bring up a bad edge.


I forgot to mention one important thing.  In order for the two mask
technique to work, you need to be able to change masks without disturbing
the Foucault test setup.  For most of us, this means that the mirror and
tester must both be on sturdy supports so that one can get up from the
testing position, walk over to the mirror, change the masks, walk back to
the mirror and resume testing without the distance between tester and
mirror changing more than about 0.001 inch (0.02 mm).  If both the mirror
and knife edge are supported on sturdy stands this isn't too tight a
requirement.  I use two old bedroom dressers, one holds the mirror stand,
the other holds the tester.  The one holding the tester has six inch long
legs about an inch in diameter.  These are the weak link in the system.  I
have to be careful not to lean on the dresser at all while testing.  Those
legs are too flexible.  However, I can put my paper for recording readings
on the dresser.  After each reading I turn on a light and write down the
result.  The dresser wobbles a little when I write, but seems to come back
to it's original position at least within my uncertainty of reading the
Foucault shadows.  Before my next mirror, I should probably come up with a
stiffer stand.


Mark Holm
mdholm{at}telerama.com

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