Re: [ccp4bb]: Problems with low mosaicity crystals

[Edward Berry <eaberry@lbl.gov>]

***  For details on how to be removed from this list visit the  ***
***          CCP4 home page http://www.ccp4.ac.uk         ***



Marc SCHILTZ wrote:
> 
... . .

> I do not think that turbulences in the cryo-stream can produce
> such an angular vibration. This would only be possible if the
> crystal has a highly anisotropic shape (plates for instance)
> so that the turbulences can exert an angular moment to the
> crystal.
> 
This was my first thought also, but actually I believe there would be 
a rotational component. If you think of the hair loop as a lever 1 mm 
long with a fulcrum at the end where it inserts into the pin, then a 
translation of 0.01 mm at the tip (crystal) will rotate the crystal by
something like asin(0.01/1) which is not insignificant (.57 deg). 
If the loop curves in bending, the effective fulcrum is closer and 
the rotation would be greater.

However I believe a large part of the "vibration" seen when observing
a crystal in the cold stream with visible light is due to turbulence
in the cold stream affecting the optical path- like the underwater
blade of an oar seems to jiggle when ripples disturb the surface of
the water.

Ed