As others have said, the most likely explanation is the attenuation of the air scatter by the sample holder. There is no sharp edge to the shadow which indicates the source of scatter is along an extended path. Ed Berry pointed out that he didn't see spots in the shadow. This might be because the threshold you used to illustrate this brought the spots plus background too low to see but you can check this. As Ed says, the data might actually be better in this region.
The fun thing to do is to print out the image on transparent sheet at approximately the same size as the image on the plate itself. Move the detector back and put the sheet in its place at the correct distance. If there is room to get your head in, you can then view from the far side of the image through to the crystal mount, collimator etc. All should then become clear (provided that is you can still see through the sheet).
From: CCP4 bulletin board [mailto:[log in to unmask]]On Behalf Of
Sent: 03 May 2007 23:52
To: [log in to unmask]
Subject: [ccp4bb] image plate shadow
Dear ccp4bb users,
I have a question that concerns a problem with data collection (not
directly related to CCP4). It might be a really stupid mistake I'm making...
I'm attaching a diffraction frame where, as you can easily see, there
is an annoying shadow (on purpose the contrast on the image has been
forced to highlight the problem). The shadow doesn't correspond to the
cryo nozzle, since the nozzle is set coming in from the same direction
as the attachment of the beamstop, i.e. right opposite to the shadow.
The detector is an image plate (Mar345), and this problem appears when
collecting at high-ish resolution (say better than 1.8).
I wonder if someone has encountered a similar problem...I've been
playing around for a while now, unfortunately not finding a way out.
could it be associated to the metallic pin of the cryo-loop on which the
xtal is actually mounted? I thought if the loop is rather short, at high
diffraction angles this could actually be a shadow generated by the tip
of the metallic rod....they're pretty much conventional pins though, and
I didn't step into this type of problem before. In any case how could I
handle this, if it were the actual cause of this anomaly?
I tried to adjust the collimator end, or even change it trying other
sizes as well, with no success.
I'll certainly appreciate any help / suggestions!
Alejandro Buschiazzo, PhD
Laboratory of Structural Biology
Pasteur Institute of Montevideo
Phone: +5982 5220910 int. 120
Fax: +5982 5220910 int. 111