[3dem] Set value and actual magnification differ

[Subangstrom]

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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