[3DEM] Question about Thon rings

[Shi, Dan (NIH/NCI) [C]]

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Dear Eduardo,
Besides the differences of EM settings for film and CCD camera mentioned
by Drs Tivol and Rachel, CCD camera may be better detector than film
except the resolution limitation by pixel size for following reasons. 
a) One can set CCD dynamic rang to fit the dosage, which would optimize
the linear range, the DQE and PSF are two factors to determine your CCD
quality, and film's may depend on grain sizes and developer. 
b) The Thon rings from film image need to be either digitized with
scanner or diffracted by laser diffract-meter, in which fog levels of
films would be induced into digitized images or diffractions as
background noise, and the Thon rings from dark current subtracted CCD
image is directly from FFT. This would be even more important for film
at low dose situation because of low signal level.
c) If you have an old style of film holder (solid back), the
backscattering electrons would add more noise to the image.

You may want to synchronize imaging conditions for both CCD and film as
Bill and Reinhard suggested, and another few conditions you also could
try with thin carbon film, different HT(such as 120 kv and 200 kv),
series magnifications(x39k to x110k) and series dosage. If the Thon
rings of the CCD camera always reach the edge of the FFT frame, it may
mean you are very lucky to have an excellent CCD camera with very good
DQE and small PSF. Good luck,

Regards,

Dan Shi, PhD
Contract researcher of Aerotek Co on LCB/NCI/NIH

-----Original Message-----
From: Eduardo Sanz-Garcia [mailto:esanzgar at chem.byu.edu] 
Sent: Thursday, January 25, 2007 5:43 PM
To: 3dem at ncmir.ucsd.edu
Subject: [3DEM] Question about Thon rings

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I have a question about Thon rings. I have realized recently that when
I take pictures using the CCD camera I get better Thon rings than when
I take the image using film (less rings and more diffuse). The specimen
is the same in both cases and the intensity of the beam is adjusted
automatically in both cases. I am using a Technai  F30.
Any clue why the film is giving me worse result than CCD?
Thank you in advance for your suggestions. I will appreciate them a lot.
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