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VLT-MAN-ESO-14650-4942
P96
24.06.2015
66 of 161
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
FITS header keywords HIERARCH ESO SEQ AG XCEN and YCEN record the location of
the centre of the SLIT or IFU in the image.
3.2.2 Blind offset precisions
For targets fainter than ~22mag (see Table 3) we recommend to perform a blind offset from a
reference star. For a blind offset acquisition, the coordinates of the reference target
(reasonably bright star) needs to be given in the target field in the OB, and not the
coordinates of the target itself.
This reference target is then centered on the slit/IFU during the acquisition (see 3.2.1).
The offset to the science target is specified in the acquisition template as 'Offset RA' and
'Offset DEC'. These offsets are given in arcsec, and denote offsets on sky. This means that
e.g. an offset of “offset RA=10.5" and "offset DEC = -5.0" will move the slit to a target that is
10.5arcsec East and 5.0arcsec South of the reference target. Both the reference target as
well as the science target must be clearly indicated on the finding chart.
It is important to note that the offsets in all templates are offsets on sky as in other Paranal
instruments. They are computed as follows:
the “offset RA” corresponds to (delta RA)xcos(DEC),
and the “offset DEC” is the difference delta DEC.
It is also important to take into account the proper motions of the objects.
For the reference-acquisition star it has to be directly in BOB pmra and pmdec parameters.
Moreover, it is necessary for the blind offsets between the acquisition star and the target to
take into account the proper motions of both objects in the computation of the offsets.
The accuracy of the blind offset is better than a few mas. However, if the offset is very large
(much larger than the fov of the Cassegrain focus of few arcmins), then the telescope
operator may have to choose another VLT guide star which could introduce an offset in the
coordinates and positioning (depending on the quality of the guide star coordinates).
Note: the VLT guide star in the acquisition template should not be confused with the
reference star for the blind offset. Usually, the guide star is taken from a catalogue, and
the coordinates can be set to 0 in the OB, this is true for direct and blind offset acquisition.