NetNotes
the cause of the issue. 2. What is the age and batch of the tungsten filaments? If they are all from the same batch it could be that the batch is faulty itself. 3. Check the voltage board electronics. Tere could be a surge which might lead to an overload. Regan M
reganhll@gmail.com
I’m inclined to think something is grounding at the grid cap or
anode. Is the grid cap touching the filament tip? How many spacers are going into the base before screwing on the Wehnelt? Is the anode/grid cap discolored, dirty? How long had your filaments been lasting before this event? Bil Schneider
wfschneider@wisc.edu
Initial comments: Rebuilt? (uncommon today, but not unheard-
of) Solution: use new filaments, not rebuilts. Pre -centered? Solution: do not trust that they are really pre-centered. Critically center them if necessary. Make sure that you are setting the correct height in the Wehnelt cap. Do not trust that the height did not change when you took it off. The manual will tell you how to properly do this. The depth of the filament in the aperture will determine how much current it takes to cause emission and may cause premature failure. Important! Critically clean the Wehnelt aperture along with the whole anode and cathode cap every time (shiny caps). It is easy to get “in a hurry” and just change the filament. (Also, I know that you know this, but do not handle any of the components with bare fingers.) Let the chamber pump down extra time after a filament exchange to allow for any potential outgassing in the gun section. I have had a few bad batches of filaments that have uneven heights and were just poorly made. It is possible for the current control to be set way too high, or the current control circuit may have failed. While it is a great work horse SEM, the Hitachi S3500-N is getting a little long in the tooth. Dan Crane
crane.dan@dol.gov
I would suggest you do as Dan Crane already said. We had a
similar issue with our W-TEM. A perfect Wehnelt cleaning procedure helped us overcome that. We also vacuum-cleaned the gun assembly with a dedicated vacuum cleaner. Erico Freitas
ericotadeu@ufmg.br
Magnification Question 3DEM Listserver Does anyone know whether the magnifications reported by a Talos
Artica are supposed to be “mag on the viewing screen” or “mag at the film plane”? We have a serious issue with the calibrated magnifications on our microscope, but I just recalibrated it and got more or less the same lousy magnifications. I’m trying to understand our issues and knowing what the microscope thinks it is reporting would be helpful. David Gene Morgan
dagmorga@indiana.edu
Did you have a look at the calibration tables aſter re-calibration
(by the way, using a X-grating on the Orius CCD or Falcon or K2/ K3???) and check against the bluebook values? What matters at the end is the correct calibration of the mag with the detector in use. Te nominal mag shown on the TEM UI is of secondary importance. Sacha De Carlo
sacha.decarlo@
dectris.com
Te magnifications reported by a Talos Arctica user interface
should be “mag on the viewing screen” if the viewing screen is inserted. Tis will change to “mag on the CCD” when the viewing screen is extracted. But as Sacha pointed out, what matters is really the calibrated mag on the camera used for calibration and data collection, and this mag will not show up in the user interface, but is stored in the magnification table in Gatan GMS if you are using a Gatan CCD/K2/ K3 camera. Zongli Li
zongli.li@
gmail.com
72
Just a heads-up. I was shocked to see that the magnification
written into a file name by the Talos soſtware depends on the size of the monitor attached. Maybe this is STEM-specific? It’s very misleading. Altogether magnification is a slippery concept though. Te pixel sampling is much less ambiguous: what length in the specimen does a pixel in the recorded file represent? Multiply by the number of pixels for the field of view. It’s the same image whether you view on your phone or projected onto the wall. Michael Elbaum michael.elbaum@
weizmann.ac.il
In the old days with film cameras, when the screen went up, the
indicated magnification changed to reflect the magnification at the film camera plane. Any digital camera mounted below that plane would thus get a post-magnification, typically 1.4× for Philips/FEI/ TFS instruments. Without film cameras (Glacios, Arctica, Krios) this behavior has at some point been removed from the soſtware. As stated by others, magnification is the worst thing to use as a parameter, we should just stick to calibrated physical pixel size of the detector.Wim Hagen
hagen@embl.de
Yup, I also find myself many times saying “physical pixel size” only
to realize right aſterward the possibility for confusion. To add to that, when someone asks me about the “magnification,” I will oſten respond with the “physical pixel size” referring specifically to the image pixel size at the detector’s physical sampling rate (independent of image resampling/“binning”). Tis has been a recurring source of confusion, any suggestions for clarifying the nomenclature here? Craig Yoshioka
yoshiokc@ohsu.edu
I agree with Wim, magnification is not terribly useful by itself.
To clarify some terminology: detector pixel size—this is the size of the pixel on the chip. For example, 14 µm for a Falcon 3 camera, and 5 µm for a K3 image pixel size—the size of the pixel represented in the image. If I understand him correctly, this is the calibrated pixel size Wim is talking about. In TEM, the magnification is determined by the ratio of
the two. Mag = detector pixel size /
image pixel size. As a thought experiment—a microscope with a K3 mounted on the same plane as a Falcon 3 would give images of the same magnification, but would sample the image plane differently (Field of view is different, and sampling frequency is different). The term “physical pixel size,” although common, is sometimes used to describe the image pixel size (as Wim did previously) and sometimes the detector pixel size (the physical dimensions of the pixels on the sensor chip). It is usually context that dictates what is meant by physical pixel size, so be careful of this term. Mike Strauss
mike.strauss@
mcgill.ca
Yes, scientifically the pixel size is way more useful than the
magnification. A colleague from light microscopy is consequently using the term “dexel” (=detector pixel) for the physical pixel on the detector to distinguish it from the pixel in the image. Aſter this discussion I realize how useful it is to use “dexel” and “pixel” to clearly distinguish between them. Tobias Furstenhaupt
furstenh@mpi-cbg.de
For tomography I like to think of the field of view, which is
relevant and obviously on the specimen, and the number of pixels in the image file. TEM, STEM, binning, whatever, the recorded scale is just the ratio of those two. For single particle analysis, the scale in nm/pixel is unambiguously on the sample. Detector (or camera) pixel size is just a number, in microns. One wouldn’t really say the camera pixel size is 14 microns per pixel. Te semantics make sense like this. Michael Elbaum
michael.elbaum@weizmann.ac.il
www.microscopy-today.com • 2020 September
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