This command produces an estimate of the bulk charge transfer efficiency in the horizontal (HCTE) and vertical direction (VCTE) (by the EPER method (Janesick et al., 1987)). For the HCTE the command first averages the rows given as the second parameter. The command uses the number of image pixels, the last image pixel and the first bias overscan pixel is obtained and computes the HCTE according to the formula:
where: bs are the counts above the bias level in the first overscan pixel in a row; ic are the count above the bias level in the last image pixel in a row; ni are the number of image pixels in a row.
The values for the bias offset is extracted from the keyword BIASMEAN, and in computed by the command TESTB/CCD. To determine the image section of the CCD and the overscan regions one can use the commands READ/IMAGE, PLOT/COLUMN and PLOT/ROW.
Note that the last column of a row is often slightly brighter than the rest of the row (because the pixel is slightly larger). The vertical charge transfer efficiency is computed in a similar way.