Hi Jacob, I'll take a first crack as I am sure many will follow. It is true that with CCD detectors one has to be careful how small an oscillation range to use for a frame before read noise starts to eat into the data quality. Pilatus offers two major new features - is fast and is photon counting as opposed to integrating detector. The speed allows to collect data without a shutter and it is very important as it can dramatically improve data quality. Now there are fast CCD detectors as well on the market. Being a photon counter, Pilatus has no "read" noise which, as you have pointed out, allows you to collect as thin a frame as you want. However, it is if you consider the detector only. In reality, if you go very thin and very fast, you may not have enough flux to record the data. Also, even once we get rid of the shutter, there are still other sources of instabilities and they do affect the fast data collection adversely. One could try going (very) thin sliced and somewhat slow but there is another gotcha there. Most rotation stages used for rotating the sample crystal, do not like going extremely slow which would be the case with thin frames and long exposure times. In this case the speed may not remain as constant as we would like during data collection. I think there was a publication from Diamond Synchrotron discussing strategies of data collection with Pilatus. We've done a little bit of systematic studies as well and while things may well be sample- and facility-dependent, ~0.2 degree frames with ~0.2 sec exposure time seemed to make good compromise between above-mentioned issues. Here I would like to emphasize again - there certainly will be samples which will benefit from somewhat different parameters. Hope it helps, Cheers, N.
Ruslan Sanishvili (Nukri) Macromolecular Crystallographer GM/CA@APS X-ray Science Division, ANL 9700 S. Cass Ave. Lemont, IL 60439 Tel: (630)252-0665 Fax: (630)252-0667 [email protected] ________________________________________ From: CCP4 bulletin board [[email protected]] on behalf of Keller, Jacob [[email protected]] Sent: Wednesday, April 30, 2014 7:49 AM To: [email protected] Subject: [ccp4bb] Pilatus and Strategy wrt Radiation Damage Dear Pilatus/Radiation Damage Cognoscenti, I read a few years ago, before the advent of Pilatus detectors, that the best strategy was a sort of compromise between number of images and detector readout noise "overhead." I have heard that Pilatus detectors, however, have essentially no readout noise, so I am wondering whether strategies have changed in light of this, i.e., is the best practice now to collect as many images as possible at lowest exposure possible? JPK ******************************************* Jacob Pearson Keller, PhD Looger Lab/HHMI Janelia Farms Research Campus 19700 Helix Dr, Ashburn, VA 20147 email: [email protected] *******************************************
