[3dem] VPP phase development curve

Radostin Danev danev at biochem.mpg.de
Fri Feb 24 00:01:48 PST 2017

Hi Jian,

If you see rapid jumps in the phase shift curve that usually means that some
of the CTF fits failed. Did you look at the diagnostic output? Do the CTFs
look reasonable? Try limiting the defocus search range to something like
3000 - 7000 A, assuming you collected with 500 nm target defocus. Also, set
the low resolution limit for the fits to 20 A.
In principle there is no way to stabilize the phase shift. It will gradually
increase with the dose on the VPP. There are two ways to slow down the phase
shift development: increase the phase plate temperature and/or reduce the
beam current. The maximum recommended heater settings are (somebody from FEI
please correct me if I'm wrong) 4.2 V and 25 mA. The current is the more
critical setting, please do not exceed 25 mA.
To reduce the beam current you could try to shrink the beam. We typically
use beam diameters ~1.2 um with beam currents 0.100 - 0.200 nA. The dose on
the phase plate per image is = (beam current in nA) x (exposure time in s).
The dose you mentioned is the dose on the sample and that is not
representative of the total dose on the phase plate.



-----Original Message-----
From: 3dem [mailto:3dem-bounces at ncmir.ucsd.edu] On Behalf Of jamon
Sent: 24 February 2017 03:27
To: 3dem at ncmir.ucsd.edu
Subject: [3dem] VPP phase development curve

Dear Lister,

Does anyone calibrated the VPP phase development curve during data
collection, using ctffind4.1.x or gCTF? I ran some test on Krios and notice
the phase changes rapidly from 0 degree to 90 degree with less than
<500e/A2, then reach 120 degree at 1000e/A2. Later the phase shift moves
much slower from 120 degree to 140 degree within 5000e/A2. It suggest that
the usable dose window is very narrow. Is there a way to control phase shift
stable at 90 degree instead of 120 degree? Your suggestions are appreciated
in advance.

Thanks and Regards,


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