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What technique will be used to determine the geometric distortion?

Geometric Distortion:
Is there a decision yet on the technique which will be used to
determine the geometric distortion?  Will it follow a similar
method as that used by  Holtzman et al. for WFPC2?
If there has been a decision, could you give us information on the
field (omega Cen?), filter (F555W?), and grid spacing/pattern which
will be used? This will be useful for us to decide on our astrometric
calibration plan.
Is there now a decision on when this geometric calibration will
be done and when the results will be made available?

Reply
We will go into the mission with ground based measurements of the
geometric distortion from thermal vac testing. These are encapsulated
in a reference file called the IDCTAB which basically codes it as a
set of polynomial coefficients. We thought this was better than the
older methods that hardwired it into the software. If you are
interested in that, take a look at ACS/ISR 01-08 which you can get to
from our web page (HST, instruments, ACS, ISRs is the sequence).
Once the instrument is up there, there is a verification/calibration
period, SMOV. There will be an initial on-orbit geometric distortion
calibration which uses 47 Tuc and the F475W filter. Proposal number is
9028, PI Gerhardt Meurer who is at Johns Hopkins University (this part
is an ACS IDT responsibility). You can find the proposal on the web,
start at:  http://www.stsci.edu:8083/public/propinfo.html and type in
9028 for proposal ID.  Gerhardt intends to follow Casertano's methods 
in getting to a solution. 
Once Cycle 11 begins properly (July), there will be a full calibration
program developed. It will include additional work on the geometry.
Also in Cycle 11 the calibration outsourcing program 9443 was approved
(look on the web starting at the same url as above). This is Ivan King
applying the same techniques that he did for WFPC2, see PASP 112,
1360.  This will probably be of significant interest to you, but of
course the proposal isn't in yet. Ivan aims to get far higher accuracy
than we try to achieve in the normal calibration program.
We will be working hard to get the inital calibration done and
delivered in a timely manner.