From charley at snf.stanford.edu  Tue Oct 28 13:45:44 2003
From: charley at snf.stanford.edu (Charley)
Date: Tue, 28 Oct 2003 13:45:44 -0800
Subject: Upper aperture adjustment
Message-ID: <3F9EE388.A0676FB2@snf.stanford.edu>



    Remember, fans, the upper adjustable aperture doesn't affect the
image unless you
tune it way off it's proper place. It is for sampling the beam, for the
noise cancelling
system, and is NOT an imaged aperture: only the bottom one will cure
image shift
when wobbling the focus.
    They are easily mistaken for each other-I do it when I'm not paying
attention. I'll
never admit it public, nor do you guys. But it's better left alone. I
can reset it fairly
quickly, so if you do turn it by mistake (the only way...), let me know
and I'll put
it back to center.
    Also, I've changed the objective aperture, the lower one you DO
adjust, on the
S-800, yesterday. Astigmatism is way down. I did find the aperture set
to '3', which
is OK, if that's what you want. The rest of us don't, so put it BACK
when you leave
the machine. This aperture is of larger diameter than '4' and gives you
a bigger spot
on your specimen surface, which gives you more signal, but worse
resolution. It's
really not necessary under any circumstances here in our lab to use #3,
so if you
think it's needed, find me and show  me why. I may have a better plan.
Maybe
not, too!
   chw3