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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.
Next: Computing the pixel coordinates
Up: Offsets of multi-pixel receiver
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Gildas manager
2023-06-01