Dose limitation in MSI-T
Added by Jason van Rooyen almost 14 years ago
Hi,
Is the "wait X seconds" option in the exposure dialogue the only way to limit the pre-exposure dose during final acquisition? I am trying to collect defocus pairs of very thin ice but the holes tend to "pop" during either exposure and/or I lose significant contrast from the particles even at -3.5 um.
Can the beam be blanked during the publication and other processing of the exposure images. I have 6 exposures in total per hole so the dose accumulates quite rapidly.
Thanks,
Jason
Replies (4)
RE: Dose limitation in MSI-T - Added by Anchi Cheng almost 14 years ago
When ccd camera is used, the shutter is controlled by ccd either automatically (Gatan,Eagle) or by a switch box (Tietz). The shutter is closed when the camera is not taking the exposure. If your shutters are set up for low-dose imaging, i.e. camera shuttered with pre-specimen not post-specimen shutter, the sample should only see the beam during the exposure time you set for the image. To test if your shuttering works right, make the beam intense enough that you can see the burn mark on the viewing screen if exposed for 10 seconds. Go to a fresh area and leave it there without taking an image for 10 seconds, then lower the screen to see if a burn mark appears.
There is a pre-exposure option in preset parameter. That pre-exposure is done by lowering the viewing screen, which, in low-dose kit, it normally means that the scope takes control of the shutter, and that is normally beam-not-blanked.
Wait X seconds before exposure in the exposure dialogue is not for pre-exposure but to wait for the lens or stage to become stable after the move. During the wait, publication, and other processing of the exposure images, the shutter is not triggered so unless you are using post-specimen shutter to shutter camera, there is no penalty for dose accumulation there.
If your shutter is set up right, I don't know any way to solve your problem except not to take that many exposures at one hole.
RE: Dose limitation in MSI-T - Added by Jason van Rooyen almost 14 years ago
After much staring at blinking lights I think I have the answer. It appears that the shutter cables have been switched at the back of our Gatan controller box. Shutter 1 (what must have been the pre-specimen shutter) stays open for 5 seconds after shutter 2 opens (for the given exposure time). The exposure time for the second focal pair was therefore 150 ms + 5 seconds!
Thanks.
RE: Dose limitation in MSI-T - Added by Anchi Cheng almost 14 years ago
Just want to reference another thread where the whole shutter business for Gatan camera is described;
http://emg.nysbc.org/boards/6/topics/528?r=529#message-529
I am curious where the 5 second pre-exposure with your shutter 1 comes from. Leginon never specifies which shutter to open and when for an exposure. It is in DM software where this is set. From what you describes, some how it is set to pre-expose for 5 second in DM, probably by someone doing plastic section imaging.
RE: Dose limitation in MSI-T - Added by Jason van Rooyen almost 14 years ago
Found it! The "use alternate shutter" check-box was selected in all three search,focus, and exposure configurations in DM. In addition, the idle state of both shutters is set to closed. In order to achieve an exposure DM opens shutter 2 for 5 seconds slightly before the actual exposure is achieved with the pre-specimen shutter. In your configuration shutter 2 is kept open in the idle state. The combined effect in our case was that the pre-specimen and not the camera shutter was being kept open for 5 seconds. Having unticked the boxes it works as proven with a beam-sensitive sample.
Thanks for your help.