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VIMOS User Manual
VLT-MAN-ESO-14610-3509
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• Offset coordinates?. This parameter defines the coordinate system used to define telescope
offsets. If the SKY option is selected, telescope offsets are in RA and DEC, if DETECTOR is selected,
telescope offsets are in X and Y on the detector. In both cases the offsets are given in arcsec.
• List of offsets (arcsec) in X or RA and List of offsets (arcsec) in Y or
DEC. List of offsets between exposures. Offsets are defined relative to the previous position. The offsets are defined in RA/DEC or X/Y on detector depending on which option is selected for the (Offset
coordinates) field. If the first image is to be taken at the preset coordinates, the first set of offsets
should be 0,0. If there are less offset values than the defined number of offset positions, the template
will return at the beginning of the list of offsets and apply them again.
IMPORTANT: We stress that, in contrast with other ESO instruments, the offsets defined by the
user in P2PP refer to the apparent movement of the target on the detector independently of the
choice of Offset coordinates (SKY or DETECTOR), and not to the offsets applied to the
telecope:
For example, if 5 telescopes offsets are defined (number of telescope offsets?) together with the following series of offsets (apparent movement of the target on the detector):
RA or X
DEC or Y
0
2
-10
7
-5
the offsets applied to the telescope will be:
RA or X
DEC or Y
0
-2
10
-7
5
-2
0
-7
10
-2
If SKY option for coordinates is selected, then after the preset the first offset will be applied to place
the target 2 arcsec North with respect to its previous position on the detector by moving the telescope 2
arcsec to the South (0 offset in RA). At this position the first exposure will be taken. After that the target
will be moved further 10 arcsec West and 7 arcsec North with respect to the position of the previous
exposure, etc. until the number of exposures is reached.
If DETECTOR option for coordinates is selected, then the directions of the offsets on sky depends on
the actual rotator angle. If we assume that the orientation is such that the positive x-axis points to the
East, and positive y-axis points to the North (see Figure 2.3) then: if the target falls at pixel x = 0,
y = 0, after the telescope preset then the first exposure will be taken with the target at pixels x = 0,
y = 10 (offset is 200 , which is approximately 10 pixels). In other words, the telescope was moved 10
pixels in the directions of the negative y axis of the detector. Then the target will move to the pixel
position x = −50 and y = 45 on the detector, which corresponds in this case (PA=0) to a movement
of the telescope of 1000 East and 700 South with respect to the previous position, etc., until the number of
exposures is reached.
If the rotator angle is different than zero, for example PA=-45, and the offset defined in P2PP are
RA or X
DEC or Y
0
0
2
0
0
5
-2
0
0
-5
0
-5
-2
0
0
5
then Figure 4.4 and Figure 4.5 give the performed offsets in the case of DETECTOR and SKY coordinates respectively.