Beam Moving a Lot over Time
Added by Anonymous almost 15 years ago
We've had the recurring problem of the beam moving over the course of using Leginon, so much so that we normally have to reposition it over the CCD every 30 minutes or so. We followed the advice in the troubleshooting part of the manual of leaving the scope at high mag for ~30 minutes to eliminate the effects of hysterisis, but this didn't eliminate the problem.
This happens regardless of whether we try to focus or not, so I'm not sure that the other suggestion in the troubleshooting (Focus correction has taken the absolute focus point far away from UC focus) applies to this case (although it might).
Replies (3)
Re: Beam Moving a Lot over Time - Added by Anchi Cheng almost 15 years ago
Is this a problem before in the days when you collect data manually? Did you have to recenter the beam very often back then? If so, you may need to consult your service engineer. The lens normalization must not function well. When you talked about beam moving a lot, what is the mag where this happened? Just the high mag, all mags etc.? What was done during the time the beam was drifted, were you collecting data or was it idle? Was
the column valve closed? Are the presets being cycled every time a preset is sent? How much bigger is your beam relative to the CCD size?
We have little problem with beam moving now a day with our scope room and water chiller maintained at constant temperature.
The details helps.
Anchi
Re: Beam Moving a Lot over Time - Added by Anonymous almost 15 years ago
The beam usually moves the most at higher magnifications (=>50,000x), such as those found in the exposure and focus presets, although there has been some movement in the lower mags (though not as much). Usually, the beam does not move as much when the presets are constantly cycling (when it's fully automated) but in some cases when we are adjusting the collection parameters (e.g. hole targetting, exposure targetting, etc.) the beam will move over time. Still, there is some movement. We have only been closing the column valve when we leave the room and Leginon isn't collecting data--otherwise, they are open.
The presets are being cycled when one of the presets is sent; the beam coverage size is equal to the one given in page 96 of part 1 if the full Leginon 1.6 manual (1-2x the size of CCD).
I am not sure about the normalization of the lens, but I will find out about it; I think the water chiller is working properly too, as the other personnel using the microscope manually haven't had problems with the beam moving, but I can ask.
I'll let you know what I find out!
Thank you for the help,
~Carlos
Re: Beam Moving a Lot over Time - Added by Anchi Cheng almost 15 years ago
Assuming that manual users don't see this and lens normalization is on ( you will notice that it takes a little longer to go from a very low mag to a very high mag with the beam contract and move if you leave the screen down), the best advice I can give you since you have less problem when you are in full automation is to "park" the microscope at high mag (50,000x) preset while you are adjusting targeting parameters. We know that leaving the lens in LM mode ( < ~1800x ) means the objective lens is cooled down which can cause the type of the problem you have, although we see more in image shift alignment between presets than with beam shift.
Parking means that once the image is acquired and the microscope is idle, you manually send the desired preset to the microscope first before you evaluate the image, picking targets on it etc. We hope to automate this in the future but it needs some intelligence in the system that it doesn't have now.
Hope this helps.
Anchi