K2 Summit and K2 Base Direct Detection Camera User's Guide
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electronics is able to recognize each electron event (at 400 frames per second) and find the
center of that event with sub-pixel precision.
The net effect is a quadrupling of the effective number of pixels (pushing beyond the Nyquist
information limit to even higher resolution), as well as a further improvement of the DQE
(and MTF). Practically, this means that the field of view can be increased for the same end
resolution allowing the researcher to capture much more data per image.
As a point of interest, Counted mode uses the same processing algorithm as Super-
Resolution, but bins pixels 2x2 in the camera to support faster live viewing speeds and a more
compact data format. The decision of which mode to use should be based on user criteria and
experiments on an application by application basis. This fact is the reason that a single Super-
Resolution acquisition is all that is needed to produce software gain references for both
Super-Resolution and Counted modes.
The Need for Speed:
Electron counting is only possible if the camera can read-out images
fast enough to “see” the individual electrons “raining” down on the sensor. K2 was
specifically designed to count at a typical Cryo EM dose rate of 10-20 e
-
/pixel/s allowing a
20 e
-
/Å2 low dose image to be recorded in 1-2 s (at a magnification equivalent to 1 Å/pixel).
To do so the K2 Summit reads out at a rate of 400 full frames/s (5.7 Gigapixels per second!)
using a highly parallelized, high-speed architecture able to process image data at 80 Gb/s.
Much of the discussion surrounding Direct Detection cameras focuses on only the
performance improvements of the sensor relatively to traditional CCD cameras. What this
misses is that it is not possible to routinely acquire images in Counted mode unless the data
processing pipeline is in place to handle this massive volume of data in real time.
We like to say that K2 is “The One That Counts” because counting is what gives the biggest
performance boost to the K2 camera. K2 is the only Direct Detection EM camera specifically
designed for counting and able to do so in a useful way.
Counting (K2 Summit only):
Individual primary electrons are counted in-line in the Summit processor on a pixel-by-
pixel, frame-by-frame basis.
The electron Counted mode of K2 Summit replaces the analogue signal from each
primary electron with a discrete count.
The benefit of counting is that it completely rejects the read noise and dramatically lifts
the Detective Quantum Efficiency (DQE) of the detector across all spatial frequencies.
