Updated ACIS aimpoint and default offsets

In late 2006 a 10" shift in the alignment of the Aspect Camera occurred when its primary focal plane temperature was cooled from -15C to -19C. In combination with the long-term alignment drift, the ACIS aimpoints using the previous default Y,Z offsets, have shifted. In order to place the aimpoint so that targets are at the optimal positions with respect to CHIP geometry and telescope focus, we have updated the default offsets for both ACIS-I and ACIS-S detectors to the following, effective 19 Feb 2007.

         Y       Z
---------------------
 ACIS-S   0     -0.25'
 ACIS-I  -0.2'  -0.25'

The figure indicates the direction of a positive offset in Y or Z. N.B. a negative offset moves the aimpoint in the positive direction on the Y and/or Z axes. The ACIS plate scale is 0.492 arcsec/pixel.

For most observers/proposers, who select the default offsets desiring that their target be close to the optical axis, this change is transparent. All Cycle 9 observations (and all remaining Cycle 8 observations) requesting the default offsets (i.e. with no values entered into RPS) will be made using the new default offsets.

Those proposers/observers who require specific offsets from the default aimpoint should enter into RPS Y,Z offsets relative to the new aimpoint listed below. This is most likely to affect gratings observers who wish to optimize the position of chip gaps along the spectrum of their source. please refer to the webpage: ACIS-S Parameters for recommended offsets for LETG/ACIS-S gratings observations.

Please note that, given the timing of this event, no Cycle 9 documentation or software will be updated to reflect the change in default aimpoint. In particular OBSVIS indicates the aimpoint and offsets based on the the previous location and default offsets (see below).

As of 19 Feb 2007, position of the aimpoints with 0 offsets are (labelled ``0 offset aimpoint" in the figures):

	chipx	chipy
---------------------
ACIS-S   222     491
ACIS-I   942     991

Given the new default Y,Z offsets listed above, a source using these offsets will be located at (labelled ``Default Aimpoint" on the figures):

	chipx	chipy
---------------------
ACIS-S   222     521
ACIS-I   972     967

The attached figures show the positions of the default aimpoints, with offsets, as a function of time, the position of the optical axis as a function of time, the default aimpoint with and without offsets, relevant chip edges/node boundaries, the width of the dither pattern and the direction of the spacecraft Y, Z axes for both ACIS-S and ACIS-I.

Figure 1 (ACIS-S3)

Figure 2 (ACIS-I3)

Previous aimpoint position, 0 offsets:

	chipx	chipy
---------------------
ACIS-S   232     501
ACIS-I   954     984

The previous default offsets were all 0 apart from ACIS-S Y offset of +10", which yielded a position on the detector of: 211, 501. Please see the POG Section 6.9 for a discussion of the previous aimpoints and offsets.

Last modified: 03/28/07

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