K2 Summit introduction¶

The main use of K2 Summit is in Counted or Super-resolution mode in combination with dose fractionation (frame saving). The counting detection is achieved by very fast internal frame rate and hardware image processing that locates the the single incident electron event in that low dose image, and the addition of these electron counts at given pixel over the output frame time.

Recommended dose rate and exposure time for different K2 modes¶

This recommendation is based on our experience.

As a general rule, the dose rate corresponds to the value used during DM gain reference acquisition. You can put down the small viewing screen once the intensity is adjusted at the beginning of DM procedure to get the values. You will notice that Linear mode calibration uses much higher beam intensity than the calibration for counted/super-resolution modes. For our Tecnai F20 at 200 kV, the former reading of exposure time by the small viewing screen is 0.25 s while the value for latter is 1.6 s. You should use similar intensity during Leginon calibration and data collection.

At 1.2 A/pixel, we use the following for final exposure that has binning of 1: |mode|small viewing screen exp time (s)|detector dose rate (e/physical pixel)|acquisition exposure time for ~ 20 e/A^2 specimen dose with 1.2 A/pixel pixel size (s)| |Linear|0.25|64|0.5| |Counted/Super-resolution*|1.6|10|5.0|
*Most literature recommend at even lower dose in the range of 6-8 e/s/pixel. This is just a number we have had success with.

Preset Recommendation¶

See Pre-MSI_Set-up specific for K2 camera using counted/super-resolution mode.