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For each spectrum, the Class Data Format expects a reference (usually
source) position. In MRTCAL, this reference is taken from the LONGOBJ and LATOBJ (Source longitude and latitude in
basis frame) in the Scan HDU.
From this reference, the IMB-FITS format can define an intermediate
reference position, through the SYSOFF, XOFFSET and YOFFSET columns in the Scan table:
- if only the Nasmyth row is present, an implicit
projection row is added at read time with (0,0) offsets,
- if the projection row is available, the associated
offsets are added to the antenna offsets (see next section) and used
into the CLASS Data Format,
- if something else than projection is available, e.g. horizontalTrue, a specific support is not yet available in
MRTCAL and an error is raised. However, if the associated offsets
are (0,0)3, Mrtcal will tolerate this system of offsets. This is a
temporary solution, waiting for a full support of all systems.
Gildas manager
2015-03-19