Offset Pointing (Y/Z Detector Target Offset )
The user may wish to offset-point the observatory pointing for a
number of reasons, e.g., to reduce the effects of ACIS pile-up or to
avoid HRC pore limits by blurring the image. Offset pointing is
discussed in Chapter 3 of the
Proposer's Observatory Guide(POG) .
In general however, most observers/proposers should leave BLANK all detector offsets in CPS if they wish their target to be close to the optical axis (best focus position). For a detailed discussion about the position of the optical axis and the default aimpoint please refer to section 4.5 in the POG .
A positive Z offset moves the target from ACIS-I towards ACIS-S. A
positive Y offset moves the target from chip S3 towards chip S2. (see
POG Figure 3.1). Note the 180 degree difference between the Y/Z
coordinates and the Delta-Y/Delta-Z coordinates.
Note that the RA and Dec of the optical axis will depend upon the
spacecraft roll angle at the time of the observation. If the observer
wants the optical axis at a particular RA and Dec with a YZ target
offset, then the observer must specify a roll constraint.
All units are arcmins. The overall valid range is -50.0 to +50.0 for
(y,z) offsets. Thus it is possible to specify an offset so large that
the target is not imaged anywhere on the detector, so exercise
caution. Furthermore, if the target is offset by more than 15 arcmins,
then ghost images (see POG Chapter 4) from the target may be visible.
Finally, offsets are implemented in software by redefining the
pointing position. This means that roll constraints apply at the new
position.
Y Pointing Offset:
Sense: negative Y offset moves the target away from the
aimpoint in the direction of S4 on ACIS-S (ie. further onto the S3 chip)
Z Pointing Offset:
Sense: positive Z offset moves the target towards the readouts in
ACIS-S (i.e. away from ACIS-I)
Recommendations:
Any numeric value entered in the box will override the default values.
Proposers/observers who require specific offsets from the default aimpoint
should enter into CPS Y,Z offsets in arcmin relative to the new aimpoint .
This is most likely to affect gratings observers who wish to
optimize the position of chip gaps along the
spectrum of their source.
WARNING: It is quite possible to specify an offset so large that the
target is not imaged anywhere on the detector, so exercise caution. To make sure your
target is placed as desired, we recommend use of the Observation
Visualizer(ObsVis) available in the latest CIAO downloadable package at
http://cxc.harvard.edu/ciao/download.
Note that CPS software forbids entry of offsets larger than 10/30/20/50arcmin
for ACIS-I/ACIS-S/HRC-I/HRC-S.
SIM Translation Offset
Offset from nominal translation position.
This is a motion of the SIM and thus the aimpoint away from the
default position on the detector along the z-axis (which is along the
axis from ACIS-S to ACIS-I).
Warning: The standard sub-arrays DO NOT take into account these recommended
offsets. Thus if you wish to use a sub-array in conjunction with one of these
offsets, you will need to specify it explicitly as a custom sub-array. For
recommended values please see the web pages:
http://cxc.harvard.edu/cal/Letg/ACIS_params
for recommended offsets for LETG/ACIS-S gratings observations and
http://space.mit.edu/ASC/calib/hetgsubarray.html
for the HETG.
Units are in millimeters.
Sense: a negative motion moves the aimpoint (and the target) towards the
readouts on ACIS-S (ie. away from ACIS-I)
Units: mm (scale 2.93mm/arcmin)
Valid values by detector are:
Detector Minimum Maximum
-----------------------------
HRC-S -12.5439 61.3518
HRC-I -61.3518 126.621
ACIS-S -190.500 22.5685
ACIS-I -22.5685 27.4739
