Bright and Dark reference images¶

Bright and Dark reference images need to be acquired for every camera setting that will
be used. The camera settings include image dimension, bin size, and offset. Over time,
references may need to be repeatedly acquired.

Correction Channels¶

When two flat-field-corrected images are correlated, there is often an origin peak
derived from the common normalization image even if both image acquisition contains only
noise. In order to avoid this problem, two or more sets of bright/dark references, and hence
normalization images can be obtained per CCD camera configuration. When a correlation
between two images will be done, Leginon will check the channel of the correction the first
acquired image has used and then force the new image to be corrected by a different
channel.

To use this function, simple set the number of channels to 2 or larger in
Correction/Settings/Image Correction/Reference Creation> while creating the reference
images.

Acquire reference images¶

• scope> make sure that the CCD will be acquiring images in an area with uniform
  beam intensity such as an empty area with no specimen nor support. You may skip a trip
  to the scope room by sending one of the high mag preset to the scope from
  Leginon.

• Leginon/Node Selector> Select "Correction" node.

• Leginon/Correction/Toolbar> Open "Settings" window.

• Leginon/Correction/Toolbar/Settings> Select one of the Common Camera
  Configuration or select Custom mode and enter your own values based on the presets you
  created.

• Leginon/Correction/Settings/Camera Configuration> Enter the Exposure time. It
  should be chosen so that the image is not saturated and ideally close to the condition
  that will be used in the experiments. If unsure about the experimental condition, use an
  exposure time that gives high but not saturated counts.

• Leginon/Correction/Settings>By default, the corrector node is set to average 3
  images together to create one reference image and to despike the hot pixels with
  averaged neighbor hood values. These can be changed if desired.

• Leginon/Correction/Settings> It is recommended to use 2 <link
  linkend="Cal_cor_channel">correction channels</link> for the camera configuration used
  for correlation peak search.

• Leginon/Correction/Settings> Click OK to exit settings.

• Leginon/Correction/Toolbar> Select "Raw image " from the pull down list of
  acquisition modes and then click on "Acquire" button next to the selector to view an
  image that is not corrected.

• Leginon/Correction/Toolbar> Select "Dark reference" in the acquisition mode
  selector and then click "Acquire" to acquire the Dark reference image for this
  particular camera configuration.

• Leginon/Correction/Toolbar> Select "Bright reference" and repeat the acquisition
  to obtain the Bright reference.

• Leginon/Correction/Toolbar> Select "Corrected image" and then "Acquire" to view
  the corrected image. A corrected image should be free of artifacts and have smaller
  standard deviation than the raw image, in general.

• Repeat steps 3-11 for all the images and bin sizes that will be used:

• If <link linkend="Cal_cor_plan">a pixel, a column/row</link> or a <link
  linkend="Cal_cor_region">region</link> gives bad values in the bright or dark image
  after a few trials, it may be excluded in all corrected images.

Dimension after binning</filename> </entry>	<filename moreinfo="none">Bin	number of correction channels	<entry>512	8	1*
512 (centered)	1	1

Dimension after binning</filename> </entry>	<filename moreinfo="none">Bin	number of correction channels	<entry>4096	1	1 or 2 if used for tomo preset
1024	4	2
512	8	2*
512 (centered)	1	1

• Only one channel is required for image shift alignment purpose. If the same camera
  configuration is used for presets that involves in correlation, two channels are strongly
  recommended.

Image Despike¶

The Despike feature removes random bright or hot pixels from the acquired images. This
hot pixel is assigned the average intensity of the surrounding area, a circle of the radius
which is entered in Neighborhood Size. The Despike Threshold is the number of standard
deviations away from the mean that qualifies a pixel for despike correction. The despike
affects the flat-field corrected image saved on the disk and can not be recovered.
Therefore, use a minimal neighborhood size to avoid artifact and set the threshold high to
avoid over-dispiking.

Correction Plan¶

Bad Pixel, Rows and Bad Cols are used to crop portions of the image that do not read
well off of the CCD. The values entered into here are determined empirically for each
instrument that Leginon operates on. If one column or row of the images is incorrect,
measure the location of the row and column that need to be removed from this image. These
values should then be entered as a sequence of values separated by commas by editing the
Plan. Click Save after adjusting.

Individual bad pixel can also be corrected by its surrounding pixels. Choose these
pixels with the selection tool on the image and then click on "Grab From Image".

Find A Single Bad Pixel¶

When a single pixel is defected, it may not be easy to find it on a large image, even if
it changes the stats dramatically. A tool is available to help finding these pixels:

• Leginon/Correction> Acquire either a corrected image that shows the bad
  stats.

• Leginon/Correction/Toolbar> Left-click on the <inlinegraphic
  fileref="http://emg.nysbc.org/software/leginon/images/icons/stagelocations.png"/>
  button to "Add extreme points to bad pixel list". There

• Leginon/Correction/Tools> Left-click on the "Add Region" tool that looks like
  "+". This adds the selected bad region to the bad pixel plan.

• Leginon/Corrections> Acquire a corrected image in the same configuration to
  check if the apearance improves.

Bad Region Correction¶

When a large region is covered by a fallen chip, image correction through bright/dark
reference may not be sufficient to produce a spike-free image since the bright and dark
values in the region are almost identical. To add such a large region into bad pixel plan,
do the following:

• Leginon/Correction> Acquire either a bright or corrected image that shows the
  bad region clearly.

• Leginon/Correction> Use "Regions" target tool next to the image to enclose the
  bad region. The corners that the target tool identifies can be larger than the bad
  region but should be close to its size so that not too much is corrected.

• Leginon/Correction/Tools> Left-click on the "Add Region" tool that looks like
  "+". This adds the selected bad region to the bad pixel plan.

• Leginon/Corrections> Acquire a corrected image in the same configuration to
  check if the apearance improves.

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< Pixel Size Calibration | Image Shift matrix calibration >

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