CHANDRA ELECTRONIC ANNOUNCEMENT NUMBER 38


One announcement :

1. Updated Default Aimpoint and Offsets for ACIS


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Item 1.  Updated Default Aimpoint and Offsets for ACIS
==========================================================================


This notice, concerning the recent (19 Feb 2007) update of the default
aimpoint on the ACIS detectors, will be available tomorrow, along with the refe
renced 
figures, on our website (http://cxc.harvard.edu/) linked from the 
Announcements box and from the Proposer page.

Proposers should note that the updated default aimpoint position for
the ACIS detectors supersceeds information in the Cycle 9 documentation
and software. 



Dr. Belinda Wilkes  
Assistant Director       
Chandra X-ray Center (CXC)      

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Updated ACIS aimpoint and default offsets
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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:
http://cxc.harvard.edu/cal/Letg/ACIS_params/for recommended offsets
for 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. 

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.


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