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after today's analysis&calibration meeting in the "green area", we internally sat together and generated following plan which I would like to share with you.
As most of you know, the timing scheme of the current source dynamic range scan does not cope very well with the XFEL timing. Therefore, it is currently not possible to acquire current source scans at the beam line. (To give you a better understanding of how such a scan should look like, I attached a scan of an arbitrary pixel to this email.) In order to correct the raw images in medium and low gain, the dynamic range scan is essential. Therefore, we decided to fall back to the dynamic range scans we previously obtained at FS-DS labs.
In principle, from the dynamic range scan one obtains both, gains and offsets of medium and low gain. Unlike the gain, the offsets depend very strongly on temperature (and other things), so, while it makes at least some sense to use the gains we obtained in our lab, it makes basically no sense to use the offsets. (Please note that the current source scans we took in our lab failed for some pixels and sometimes even whole asics, even though the pixels and ASICs are actually good. To explain why would exceed the scope of this email but we are happy to explain it in detail if you like.)
We decided to split the calibration into following tasks:
Manuela and Torsten are working hard to combine the current source data from our lab with x-ray gain data taken before in our lab. This step gives us the possibility to have calibration parameters in place asap. Once this is working and the calibration parameters can be applied to raw data properly, in a second step they will feed in the Cu x-ray data we obtained at SPB in 30 bunch mode.
Offset calibration in three gain stages
In order to provide reliable offset data for medium and low gain, we plan following approach:
We (Davide) prepared two additional patterns which in principle take dark data as usual. The difference is that they switch gain during the integration period and by that we get the offset of the medium and low gain, respectively. After chats with Adrian and Jola we agreed that the best time to run these scans would be 1-2 hours before the start of the shift, especially as Software- and ITDM people should be around in the event of technical problems. Therefore, I suggest to meet at SPB around 6pm unless you have any objections.
We are convinced that this is currently the most reasonable way to determine the offsets of medium and low gain for the time being (and maybe even in future (tbc)). (Remember that the offset of the high gain is obtained from conventional dark data.)
We will provide you a block diagram of the new calibration approach tomorrow evening.
Aschkan (on behalf of the AGIPD calibration team)