[3DEM] thon rings

[Reinhard Rachel]

**** Messages to this list are automatically archived ***
**** Please limit quoting of previous postings to the bare minimum ****

Dear Eduardo,
to your question about Thon rings:

1) do you use exactly the same exposure time and C1 and C2 settings? if 
not, use exactly the same settings and exposure time (specimen drift 
etc ...?)
-- 'automatic exposure' may be 'user-friendly' but will result in not 
comparable settings. For this reason, we try NOT to use 'automatic' for 
determining the exposure time, but rather use a fixed value. Shorter 
exposure times are often ok for the CCD cameras, but this also may 
'eliminate' problems associated with specimen drift (or instabilities 
of lenses / currents? not very likely with a Tecnai F30, but may 
happen)
- on the other side, longer exposure times MAY give an image with 
higher statistical quality, provided EVERYTHING in the scope is stable, 
but ... it also can result in worse images. (specimen??)

2) when looking at the film: do you look for Thon rings in exactly the 
same (small) specimen area as visualized with the CCD camera? 
- in larger specimen areas, you can have problems arising from various 
sources, like specimen tilt, "beam-induced movement", etc etc.
( here, the question arises again: what type of specimen? and: cryo?)

3) which shutter did you use? the same in both cases? that depends on 
the setup of the microscope and camera. (at least in 'my' older FEI TEM 
- a CM12 - you have the choice to control the illumination either with 
the beam blanker (above the specimen) and the 'exposure shutter' (below 
the specimen). IF the two images (film vs. CCD) were taken not with the 
same shutter control, this may influence the quality of the image (this 
also depends on the exposure time), as the behaviour (speed!) of the 
two shutters is different. 

and Bill Tivol's remark on the coherence / crossover is (most) 
important, too. 

regards, Reinhard

---------------------
PD Dr.Reinhard Rachel
Universität Regensburg
Centre for EM at the Faculty III/Biology
(Chair for Anatomy)
D-93053 Regensburg - Germany
fon +49 941 943 2837, 1720
fax +49 941 943 2868

--
For information on how to subscribe/unsubscribe/etc., send mail to
3dem-request at 3dem.ucsd.edu with the text "help" in the body of the
message.