Position of Rotation Axis

   

Next the file selection utility asks you to SELECT 180 DEGREE ROTATION IMAGE FILE . This image is used together with the first image to try to determine the position of the rotation axis to sub-pixel accuracy. This works by reversing one the two images (in the appropriate axis), and finding the maximum cross-correlation    position for each row (or column). This is performed to single pixel resolution, and the pixels next to the maximum correlation position are used to fit a parabola  and find its centre to give the best correlation to sub-pixel accuracy.

Normally, the 180 degree rotation image is in the next file after the last projection file in the series. The image is displayed and you must confirm that the correct projection data has been found. You must then SELECT RANGE OF IMAGE TO USE. Here you defined graphically a rectangle, but only one-dimensional limits are used. For vertical rotation axis data, the vertical range is defined, for horizontal rotation axis data, the horizontal range is defined.

Only rows (or columns) in the defined range are used to calculate the position of the rotation axis. If the object does not fill all of the range, it's best to select only the range where the object is present.

The average rotation axis position calculated and its standard deviation are output in the terminal window. Normally this should be close to the centre of the detector, and the value of the standard deviation should be small (a fraction of a pixel). The average value appears as the default value in the PIXEL COORDINATE OF ROTATION AXIS input form. You may select the default by clicking on O.K., or over-ride the default with a value which you have obtained by other means.

This value is measured in units of the detector image pixels, where the left-hand side of the first pixel is defined as 0.0 and the right hand side of the first pixel is defined as 1.0. Thus if the image has 1024 pixels then the right-hand edge of the last pixel is pixel coordinate 1024.0 and the centre of the image is pixel coordinate 512.0.

  

Figure 9: Pixel Coordinate of Rotation Axis