CHANDRA ELECTRONIC ANNOUNCEMENT NUMBER 75

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Contents:

1. Change to Chandra Aimpoint 

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Item 1: Change to Chandra Aimpoint
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We have noted a shift in the aimpoint of about 8 arcseconds  which
impacts observations made after July 11, 2011.  While there are no
permanent science impacts,  all aimpoints are affected. In particular
the direction of the shift moves targets positioned at the ACIS-S
default aimpoint closer to the node boundary, increasing the risk of
apparent loss of events at that boundary. 

We have made a change in internal software to return all observations
made after August 29 close to the previous aimpoint. (See Chandra
Electronic announcement #38:
http://cxc.harvard.edu/announcements/announce_38.html)  

We will continue to evaluate the situation over the next few weeks in
order to more completely characterize and quantify the change to the
aimpoint and update the database parameters needed for planning
observations. Until then: 

1-  No observer action is required.

2-  A change to all Y-offsets of +0.15' will be noticeable in the
OBSCAT  entries for all observation not observed as of August 29.  

3- The Observation Visualizer is not yet updated. If you desire a
specific offset be made to your observation, you must add an
additional Y-offset (+0.15 arcmin) to that shown by ObsVis in order to
correctly represent the location of the source on the detector. 

4- For observations made from July 11 through August 29,  spectral
analyses should be unaffected (except for a slightly lowered effective
exposure time if the source was on the node boundary).  However, we
recommend the observers be careful with their timing analysis. 

The easiest approach is to use Level 1 event list  (*.evt1* in the
"./secondary" directory).  No filtering has been applied to this data
so the timing is unaffected by the default bad pixel mask.  

One can create a custom bad pixel mask following the directions on the
CXC website: 

http://cxc.harvard.edu/ciao/threads/acisbadpixels/

with the node boundary mask (step B).  Then follow the "Reprocessing
Data to Create a New Level=2 Event File" thread to apply the new bad
pixel file to your data: 

http://cxc.harvard.edu/ciao/threads/createL2/

Other data analysis options for data affected by the offset are
possible and are located on threads within the CIAO data analysis
website. For any questions or problems please contact the CXC
HelpDesk. 

We will release a further bulletin once we have completed our analysis
of the aimpoint shift and updated all relevant software.

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