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Hi Mark, we solved this problem some time ago for our remote crystal
centering system. Fixing the problem requires a somewhat tedious
calibration procedure and don't have the notes with me since I am
out of town. I'll get back to you with more details when I return.
The basic idea is this: on our Navatar system, the zoom does not
occur about the actual center of the image, but it does seem to stay
fixed on a given off-center pixel which you can determine. There is a
simple iterative procedure I developed to find this point. You also
need to calibrate your zoom scale. The zoom scale factor (microns per
pixel) does not change linearly with zoom. The formula for scale as
a function of zoom is obtained using an electron microscopy
calibration grid and a log-scale least-squares fitting procedure.
Once you know these parameters (which vary for each different
microscope), you can calculate the crosshair positions you need at
any zoom.
More later.
Richard Gillilan
MacCHESS
Cornell
On Jul 4, 2006, at 11:26 AM, Mark Agacan wrote:
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I have a question regarding our inverted-phi axis-mounted video
camera,
used to centre crystals in the path of the x-ray beam.
If I centre the crosshairs on a crystal so that it stays perfectly in
the middle with no precession - no problem, the beam hits and you get
diffraction. Thereafter, if I zoom in or out, the crosshairs will no
longer centre perfectly on the original part of the crystal, but
will be
significantly off to the left or right hand sides, with the result
that
centring on crosshairs means you'll miss the crystal with the beam.
I realize that this is because the video camera is not fully aligned
(as, say, the detector has to be aligned within a couple of pixels all
the forward and all the way back) and is designed to be in-focus at a
specific distance, matching the distance from the camera to the
crystal
in the beam.
My question is this: when using very small crystals, is there a
simple
way to get your video camera to zoom in to check if the crystal is
actually in the x-ray beam.
We use a Rigaku M007 HF and r-axis iV++ detector.
Hope this makes sense!
Best regards
Mark
_____________________________________
Dr Mark Agacan
Scientific Officer,
Division of Biological Chemistry
and Molecular Microbiology,
Wellcome Trust Biocentre,
School of Life Sciences,
Dow St.,
University of Dundee,
Dundee, DD1 5EH
Tel: +44 1382 348751
Fax: +44 1382 345764
_____________________________________
