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Raster Scan (Electron Microscope) Rotation Algorithm?

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The trace on my Tektronix scope is rotated (to align it with the internal graticule) by varying the DC in an axial solenoid that the beam passes through. I don't know how fat that technique can twist the scan without distorting it, but if it can carry you to +/-22.5 degrees, that might be enough.

My preferred way: The deflection currents are analog. Matrixing the nominal X and Y drives will swing the beam to any angle you want. What you need to do that is a pair of digital volume controls -- one fed with X and a digital gain, the other with Y and a different digital gain -- between the DACs and each deflection driver. Computing the gains is a one-time thing. If you don't like the math, you can use a LUT.

The software to rotate the scan with the drive coils uses the same algorithm used for drawing slanted lines on stepper-motor plotters. Once thought through, implementation is straightforward.

45 degrees is trivial, but there is a sqrt(2) scale factor that you might need to correct.

Think Moire. In any case, if your step size exceeds your resolution limit, the image will be (non alcoholically) pixellated.

Clear as is.

See above. Great minds and all that.

I hope I helped.

Jerry

--
Engineering is the art of making what you want from things you can get.
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Reply to
Jerry Avins
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There's a lot of difference - the S.250 was the older of the two machines, rather more up-market and built with a wire-wrapped backplane. The S.200 was a reworking of the over-cheap S.100, with a a bunch of biggish printed circuit boards hooked togheter with ribbon cable, which made it a bit of swine to work on.

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DAC errors bigger than about 1/4 LSB meant that some lines overlap, making them brigher, while the under-lapped section were dimmer - so instead of a uniform white blank screen you got a blank screen with a visible line structure, which the human eye detects remarkably effectively.

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Sorry, none. Your microscopes should have come with complete circuit diagrams. Cambridge Instruments - or their successor organisations - will have all the circuit diagrams on microfilm, and should be able to supply a replacement set against the serial number of your machine. I've no idea how much they'd charge, or who you would ask these days, since Cambridge Instruments is now part of Karl Zeiss.

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You'd presumably want to talk to the Coldhams Lane site in Cambridge.

---------- Bill Sloman, Nijmegen

Reply to
bill.sloman

John, you're simplifying it a bit. The size of the cathode is one point that limits the resolution. Best in terms of resolution would be a cold field emitting cathode, but this doesn't supply a sufficiently big current. The cathode is more than just a cathode of an electron tube. I admit I forgot the difference between a SEM and normal EM in terms of the finer technical details.

Rene

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Ing.Buero R.Tschaggelar - http://www.ibrtses.com
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Reply to
Rene Tschaggelar

I was once tasked to design a filament current regulator for a hot FEC. LaB6 sounds terribly, terribly familiar, albeit this was almost 30 years ago. I saw one in an optical microscope, and it was hard to get it to focus properly because the light kept bouncing off the crystal facets. They were really endeavouring to get a one-atom point. And, of course, trying to accellerate this one-electron-wide beam to 30 KEV. I learned a lot about Hall effect current sensors when there's a 5 or 10 KV arc in the electron gun. When someone came up with TransZorbs, our MOV budget hit the floor. ;-)

Cheers! Rich

Reply to
Rich Grise

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