[3dem] CTF-correction with phase plate

[Philip Köck]

Is the phase shift due to the PP treated as constant (relative to the unscattered beam) above 20 Angstrom resolution (and as unknown at very low resolution)?


Philip

________________________________
From: 3dem <3dem-bounces at ncmir.ucsd.edu> on behalf of Wim Hagen <hagen at embl.de>
Sent: 27 May 2019 15:49:33
To: microscopy at microscopy.com; 3dem at ncmir.ucsd.edu
Subject: Re: [3dem] CTF-correction with phase plate

Dear Philip,

I am not aware of any CTF-estimation package that applies the correct model for a Volta phase plate. The Volta patch to give the un-scattered beam its phase shift has a size I the back focal plane, meaning it gives a soft cut-on, see figure 2B in https://elifesciences.org/articles/23006 . The workaround for this is to start fitting CTF past 20Å. This then requires a nice thin particle-crowded sample to stil successfully fit enough Thon-rings to estimate defocus, astigmatism and the additional phase shift parameter.

Best,

Wim

On 27. May 2019, at 13:16, Philip Köck <Philip.Koeck at ki.se<mailto:Philip.Koeck at ki.se>> wrote:

Dear all.
I'm trying to find out how the contribution of a hole free or Volta phase plate to the PCTF is described mathematically in various packages that are used for cryo-EM of macromolecules.
The physics behind it would also be of interest.
Does anybody know a reference or have some material that can be shared?


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