Keywords: mrtcal-prog-manual

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## Computing the pixel coordinates

Given the actual position of the derotator, MRTCAL can compute the pixel position on sky from the current telescope position. This position has been pre-computed using the radio projection (`LONGOFF` and `LATOFF` columns in the IMB-FITS), only the per-pixel offsets are to be added. MRTCAL checks first if it knows the offsets for the current receiver (as found in the column `RECNAME` in the frontend table). The pixels offsets are not self-described in the IMBFITS: they must be hardcoded in the program, with a correct correspondance between the pixel number found in the backend table and the hardcoded offsets3.3. Once the offset of the current pixel is found, the strategy is the following.

- The current position (reference + position offset) is unprojected to absolute spherical coordinates.
- The pixel position angle on sky is computed, given the pixel position in the array and the derotator angle.
- The spherical distance from the array center is added using an inverse Haversine formula, i.e., the pixel position in absolute spherical coordinates is found by adding an arc of great-circle towards the correct direction.
- The resulting position is reprojected again as a reference + position offset in the current projection system.

The deprojection, offset in spherical coordinates, and reprojection are done by the Gildas internal engines.

Next: Description of the radio Up: Offsets of multi-pixel receiver Previous: Handling the derotator Contents