[3dem] Low Dose Focus/Exposure

[Tommi White]

Thank you to everyone that has replied...

My LD focus magnification is the same as the LD exposure...a common
question.

Some suggestions I will try include...

1.  "Image Shifts SA" alignment applies an automatic correction to each SA
magnification to make focus difference small between all SA magnifications:

2.  There is an option in low dose to take a defocus series.  Make sure
that that is not activated.  It is designed to take an exposure at a
certain defocus from where you set it in focus mode.

3.  check beam shift pivot point alignment to make sure the track ball is
shifting the beam instead of tilting the beam.

4.   redo your parfocal alignment.

5.  check that the Alignment parameter “Image Shift and Focus” at the
magnifications

6.  Is the eucentric focus preset at real "zero" focus as well? If you do
leave the normalization "on" for the lenses when switching form focus to
exposure, and the eucentric focus preset is off by, say, two microns, then
that's the cause of what you're observing ...

7.   in the alignment procedures there is a magnification dependent defocus
step which often is neglected.

I will report back on what worked (probably later this week)...thanks again
for all the suggestions.

Sincerely,
Tommi


On Monday, April 27, 2015, Tommi White <whitetommi at gmail.com> wrote:

> Dear 3DEM.
>
> Perhaps the community can assist with this issue we are struggling with.
> In Low Dose on our Tecnai F30, the Low Dose focus image has a different
> defocus value (using FFT) versus Low Dose Exposure.
>
> I have been checking this on a relatively flat cross-grating (not cryo)
> specimen.  I set LD exposure to be zero defocus (with focus knob), but when
> I switch over to LD focus (~ 3 micron away on tilt axis) the image is
> defocused (~ -2 micron).
>
> I believe there is some 200 kV procedure alignment that must not be set
> properly - but am not quite sure which one...any suggestions on what to
> tweak to get this exposure/focus area reflecting the same defocus?
>
> Thanks for any suggestions...
> Tommi
>
> Tommi A. White, Ph.D.
> Associate Director, Electron Microscopy Core Facility
> Assistant Professor of Biochemistry
> University of Missouri
> W125 Veterinary Medicine
> Columbia, MO 65211
> 573-882-8304
> http://emc.missouri.edu
>
>
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