From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


A stable surface is indeed required. Remember that you are measuring angles
that are giving a different crossing of thousandths of an inch at a typical
100" of length.  Those are very small angles and you need to have
something fairly stable in order to keep the measuring end positioned
accurately. The use of seperate tables for the mirror and tester tend not
to work well at all on a wood floor or even a home concrete second floor,
especially if the tables rock on their legs.
The best thing is to do your work on a concrete floor that is laid on the
ground.  Construction of solid tables is also helpful and even better, the
construction of a single sturdy table that sits firmly on the floor or
ground is the best.
I have to do my work in a university lab room on the second floor and the
tabletop that I do the testing on will detect when the elevator starts
moving or the air conditioning is running, a problem in the summertime.
Also, even as solid as the bench tops are, leaning on one will change the
result a lot.
If nothing else, some short platforms that sit on the garage floor without
rocking will do you fine and generally those floors are laid on the ground.
If not that, then the bottom floor of the house, apartment building or even
at a friend's house if necessary and you will find a good bit of
improvement for the testing.
All of this is modifyable as needed as I have done work in a mobile home
across a room.  The thing here is to keep the vibrations of the mobile home
with its twisting and other such motions to a minimum by being the only
moving thing in there when testing.  In other words, flexible places can be
used but that means that you have to stop moving about when testing or the
movements will move the ROC point about.  Even a medium sized cat makes a
difference there.
I will note that Peter Ceravolo displayed his interfrometer some years ago
at RTMC and he had it on a piece of plywood on top of two sawhorses and,
even though the whole rig was done on a piece of 1/4" or so aluminum,
the flexure of somebody touching the sawhorse was enough to make the
fringes move on the interfrometer.  A decent breeze also affected the
results. You can do work on a flexible support but that just means that you
have to minimize the changes in forces around the support. Hope I got all
of this through to you in an understandable form. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

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