Image Shift Conventions

[John Donovan]

OK, I know I'm not the "brightest bulb in the pack", but I can't under stand why Cameca has determined the image shift polarities the way they have. For example when one has a non-zero image shift, and one switches to spot mode, the beam position moves as expected by the number of microns specified in the image shift.

Specifically, let's say that one sets the image shift to X = 10, Y = 10 (using the PeakSight software).  The instrument moves the beam relative to the centered (0,0 shift) 10 microns to the right in X as expected, but it then moves the beam 10 micron downwards (negative).  Quite unexpected.

So I took it upon myself to flip the polarity of the Y dimension for image shift so an image shift of 10,10 set the beam right 10 um and up 10 microns. Of course the Cameca PeakSight Shift display shows this as 10, -10.

Can anyone explain why Cameca inverts the sense of the Y image shift?

[Gseward]

John,

I haven't measured the scan voltage values within a time scale that I can remember them, but I'd gamble that the deflection voltages are -xV to +xV and -yV to +yV with 0,0, being no deflection (in the centre of the scan/column). If this is correct, then the SX100 beam-shift deflection polarity from 0,0, of +x to the right and +y down, doesn't seem so unreasonable.

Gareth

[John Donovan]

I get that, but the GUI is supposed to reflect the sample, not the instrument. At least in my "sample-centric" perspective:

http://probesoftware.com/download/Instru-vs-sample-centric.ppsx

Now engineering is great, but I think the GUI shouldn't constantly remind the operator that the upper left of an image is the 0,0 pixel, just because 50 years ago someone decided that the CRT electron gun should start "drawing" in that corner!

I'm working at PS today, so if you get a chance please check if the PeakSight Marker display reads Y inversely like the Shift display. Just curious.

[Gseward]

Marker has 0,0, at the centre of the screen, -y is down (i.e. opposite to Shift!) -x left.

[Probeman]

Marker has 0,0, at the centre of the screen, -y is down (i.e. opposite to Shift!) -x left.

Thank-you!  So now I'm asking why are they different!?

[Gseward]

Je ne sais pas.

I also notice there is some strange behaviour setting a shift when in analog scan mode, even when above the 1267x minimum mag

N.B. this is also the mag at which there are range-change relays activated (there are 4 ranges and shift is only available in ranges 2-4, I believe).

The following is an unusual set of circumstances, but an oddity nontheless; try this test:

Set instrument mag to minimum using the Cameca software (e.g. FOV ~ 2550), and leave in scanning mode.
Go into PFE analytical conditions, set 'analog scan' and set the magnification to say 4000x with any appropriate shift (e.g. 0.1,0 for Mag Analytical = 400000x; Mag Imaging can be any value) click OK.

on my instrument I hear the mag range relays click to range 2 and Mag is set to 4000x (FOV ~95um)  BUT PFE returns:
 "cannot set image shift (x,y) when mag is below 1267"
inspection of the shift  in the 'Roller' window of peaksite confirms that no shift has been set.

Now, open 'Analytical Conditions' again and click OK (don't change any values).

On my instrument, after this second attempt, the shift is now set and PFE does not generate any warning message.

If you repeat the above test with 4000x but start with the Cameca at high mag (Say 60um FOV) , then no warning is generated and the shift is set correctly, first time of asking.

So it seems there is a predictable yet undesirable behaviour when using analog scan - if the instrument is in 1st mag range then no shift is set; if already in the 2nd mag range the shift is applied. Interestingly however, if the instrument is in scanning 'off' mode, to start with, non of this happens!

So, I ask: is this behaviour consistent on other SX100 instruments? Perhaps this is because of the order you are sending commands?

I know these are little, mundane things, that don't apply to most people in most cases, but they can still catch people out, especially at 2100 when you just want to leave!!!

Cheers, and thanks for all the hard work.

Gareth

[Probeman]

Good one Gareth!

I see the problem. If one reads the scan status too quickly after setting the mag, the FOV parameter is still set to the previous mag.  But if you add a 200 msec delay after setting the mag but before reading the scan status, the correct mag (and FOV) are read, and the problem goes away!

I'll fix this tonight.
john

[John Donovan]

This subtle bug has been fixed and the PFE update is ready to download from the various sites:

http://probesoftware.com/smf/index.php?topic=40.0

Thanks, Gareth, for catching this!