Keywords: mrtcal-prog-manual

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## Handling the derotator

If the subscan has no derotator table, the current pixel (obviously the unique pixel) is left centered on the current position (reference + position offset) computed in the section . On the other hand, if a derotator table is present, MRTCAL will apply a per-pixel offset.

First of all, MRTCAL tries to guess the actual derotator angle on sky. The strategy is the following:

- angle on sky will be chosen from the column `sAct` (``sky Actual''), as opposed to `fAct` (``frame Actual'') and `hAct` (``horizontal Actual''). This assumes `sAct` values are reliable even if the current derotator mode is not sky.
- if the column `sAct` is empty (this can happen around midnight), a warning is raised and the returned value is the commanded value as found in the frontend table header. In this case, the derotator mode must be sky.
- if the column `sAct` provides no value within the subscan range, a warning is raised.
- if the column `sAct` has a single row, or if the chunk MJD is beyond the table limits, the unique/nearest value is used.
- if the chunk MJD lies within the column `sAct` limits, the derotator angle is interpolated according the associated `MJD` column and to the chunk MJD computed earlier.
- if the chunk MJD is further than 5 seconds3.2 from the nearest point in the column `sAct`, an additional warning is raised.
- finally, if the actual angle is more than 0.5 degrees away from the commanded value (e.g. derotator has reached its rotating limit), a warning is raised.

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