[3dem] Set value and actual magnification differ

[Rado Danev]

Hi Sergej,

 

Yet another (very unpleasant) possibility is an intermittent short in the
coil of a projector lens. You won't be able to easily detect this by
monitoring the lens current because the lens power supplies are constant
current. Instead, check the voltage across the lens coil for intermittent
jumps. The magnification change spikes will also be visible on the
fluorescent screen.

 

I would put my bet on a communication/software issue though because the few
spurious images had higher magnification than what was requested. Is the
50kX mag one of the standard mags? In other words, were the spurious images
at a different mag from the standard mag range?

 

All the best,

 

Rado

 

 

From: 3dem [mailto:3dem-bounces at ncmir.ucsd.edu] On Behalf Of Subangstrom
Sent: Tuesday, February 20, 2018 23:29
To: 3dem at ncmir.ucsd.edu
Subject: Re: [3dem] Set value and actual magnification differ

 

Hi Dr. Masich,

There are a few points to make here and two things that could cause
magnification problems in general.

Some points:

The magnification you read out in the user interface is the magnification at
the "flu screen." The magnification you read out on the user interface when
you lift the screen is the magnification at the film plane. The actual
magnification (or as we commonly refer to it: pixel size) at the camera
plane will depend on the camera's position along the path of the beam. The
further along the beam path, the greater the magnification. 

It is expected that you will have a higher mag at the camera plane than what
is read out on the user interface. From your description however, it sounds
like the mag that is read out on the screen is fixed but the effective
magnification at the camera plane is changing (from 50kX to some other
value). If this is indeed the case, I would check on the following:

The magnification is determined by the excitation values of the projection
lenses. For any given magnification, each lens of the projection system
should have a specified current flowing through it that would achieve that
magnification at the given high tension value. For CM systems, if you open
up the service documentation that is delivered with the system, a table of
these currents is available for reference. You can open up the "display
currents" page on the interface and compare them. For a fully functional
system the current values won't be exact, but very close. In my experience,
these values are trustworthy. In theory, it is also possible to pull out the
lens cable, put a current meter in series, and measure the 'true current' in
the event you need to get into deeper troubleshooting.

If the currents are correct sometimes and not other times, then you have a
problem to troubleshoot. 

The problem can be:

1) The HT circuit- there are several boards that govern the HT and how it
ties in to the lenses, any one of them could be defective

2) The lens circuit- there are several boards that amplify the signal and
produce the current for each lens, any one of them could be defective

3) Both circuits- this is less likely, but it could be more than one board
on each circuit causing this issue; if that were the case I'd expect a more
dramatic symptom

I hope this was helpful.

Best,

Mark Ebrahim
Subangstrom LLC

On 2/20/2018 2:04 PM, Sergej Masich wrote:

Dear 3D-EM community, 

 

We have observed a strange phenomenon in our old FEI CM200 microscope. For
example, among many images acquired at the set value of 38 kX, there were a
few images having actual magnification of 50 kX. The similar behaviour was
observed for other magnifications as well. Our very experienced engineer
could not explain the discrepancy. Can somebody advise what can have caused
the problem?

 

Best regards,

 

Sergej   

=============================
Dr. Sergej Masich
Dept. of Cell and Molecular Biology
Karolinska Institutet
171 77 Stockholm, Sweden
tel: (+)46 - 8 - 524 873 61
mobile: (+)46 - 736 - 833 693
e-mail: Sergej.Masich at ki.se <mailto:Sergej.Masich at ki.se> 

 

 

 






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