Troubles with Tomography » History » Revision 3
Revision 2 (Amber Herold, 04/27/2010 02:19 PM) → Revision 3/7 (Eric Hou, 06/24/2010 02:56 PM)
h1. Troubles with Tomography * <link linkend="tomo_intensity">Image appears oversaturated with overflow to negative values</link> * <link linkend="tomo_intensity">Images acquired in tomography have different intensity range from that acquired from other nodes with the same preset and exposure time.</link> * <link linkend="tomo_large_move">Images move by a large amount between tilts.</link> h2. Unusual intensity range in the tomography image Two possible reasons: # * The exposure time has been modified by the tomography node to spread the total dose to all images in the tomogram. Solution: Reduce the total dose or reduce preset beam intensity. You can find the real exposure time from the log panel of the tomography node or through the information page of the image in the web image viewers. # * The image intensity of images collected by the Tomography class node does not have the same meaning as in other images in Leginoon. The flat-field corrected intensity values are multiplied by 10 before the image is saved and displayed as MRC image as signed 16-bit integer. Therefore any original CCD count of 3276.8 or larger will overflow. All other images in Leginon are saved without manipulation as float. Therefore, the images obtained through tomography node and another acquisition nodes will appear differently in the both the web image viewer and within Leginon. Solution: Reduce the total dose. Find out what exposure time corresponds to the fractionated dose from your tilt angle step and range and total dose and take an image at tomo preset with such an exposure in Navigation node. You will need to reduce the total dose if a good fraction of the counts are larger than 3200 even though it would not appear to be saturated in the float scale without the 10x factor. h2. Large movement between tilted images Find out how smooth the movement is by comparing the images. If transition is smooth and linear: # * Magnification too high Solution: Lower the magnification or perform <link linkend="Tomo_low_mag_fitting" >low-magnification rough model fitting</link>. # * Optical axis offset model is too different from the initial guess for the non-linear least-square fitting. Solution: Perform <link linkend="Tomo_low_mag_fitting">low-magnification rough model fitting</link>. If transition is not smooth nor linear: # * Goniometer behavior unpredictable Solution: Service the goniometer. ______ [[Troubles with Calibrations - Troubleshooting|< Troubles with Calibrations - Troubleshooting]] ______