[ccp4bb]: Saturated pixels on Mar345 scanners

["Leslie A." <andrew@mrc-lmb.cam.ac.uk>]

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In response to Laurent Maveyraud's query:


Saturated pixels on Mar345 scanners
====================================

    It might be better if the response came from Mar Research, but
this is how I understand the situation for Mar345 scanners.

First, I should point out that (in spite of what it says in the
mosflm.hlp file !!) the cutoff applied by MOSFLM for Mar345 images is
150000 when using the 150 micron pixel size, and 65000 when using the
100 micron pixel size. The correct value is reflected in the MOSFLM
logfile.

The dynamic range depends on the pixel size because, when in 150
micron mode, each cartesian pixel is made up of two spiral pixels,
each 150x75 microns, each with a dynamic range of 65535. Because
correction factors are also applied during the spiral to cartesian
conversion, an actual cartesian pixel can have values larger than
131070 (2x65535) , so values up to 250000 are "allowed" by the Mar
software without being classified as overloads. Overloaded pixels are
assigned very large values in the spiral pixels so that ALL pixels in
the Cartesian frame to which they contribute significantly will be
flagged as overloads (I don't know what value is actually assigned).

I do not have enough data to say whether or not the response is indeed
linear up to pixel values of 150000.


          Andrew Leslie