Description¶

The guider_cds step computes countrate images from the Correlated Double Sampling (CDS) detector readouts used in FGS guiding mode data. The exact way in which the countrate images are computed depends on the guiding mode (ID, ACQ1, ACQ2, TRACK, FineGuide) in use.

ID mode¶

The ID mode uses 2 integrations (NINTS=2) with 2 groups per integration (NGROUPS=2). For this mode the guider_cds step first computes a difference image for each integration by subtracting group 1 from group 2. A final difference image is then computed by taking the minimum value at each pixel from the 2 integrations. The minimum difference image is then divided by the group time to produce a countrate image. The output data array is 3D, with dimensions of (ncols x nrows x 1).

ACQ1, ACQ2, and TRACK modes¶

These modes use multiple integrations (NINTS>1) with 2 groups per integration (NGROUPS=2). For these modes the guider_cds step computes a countrate image for each integration, by subtracting group 1 from group 2 and dividing by the group time. The output data array is 3D, with dimensions of (ncols x nrows x nints).

FineGuide mode¶

The FineGuide mode uses many integrations (NINTS>>1) with 4 groups at the beginning and 4 groups at the end of each integration. The guider_cds step computes a countrate image for each integration by subtracting the average of the first 4 groups from the average of the last 4 groups and dividing by the group time. The output data array is 3D, with dimensions of (ncols x nrows x nints).

After successful completion of the step, the “BUNIT” keyword in the output data is updated to “DN/s” and the “S_GUICDS” keyword is set to “COMPLETE”.