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.
