----- Submission 1 (Thu May 05 19:09:56 GMT 2005) ----- Phase II Proposal 10621 (omegacen) successfully submitted. ----- Submission 2 (Tue Jul 26 17:02:23 GMT 2005) ----- Assigned ID: 256 ----- Submission 1 (Fri Jan 21 17:41:39 GMT 2005) ----- Phase I Proposal 256 (omegacen) successfully submitted. ACS Zero Point Verification The uncertainties in the photometric zero points create a fundamental limit to the accuracy of photometry. The current state of the ACS calibration is surprisingly poor, with zero point uncertainties of 0.03 magnitudes. The reason for this is that the ACS calibrations are based primarily on semi-emprical synthetic zero points and observations of fields too crowded for accurate ground-based photometry. I propose to remedy this problem by obtaining ACS images of the omega Cen standard field with all nine broadband ACS/WFC filters. This will permit the direct determination of the ACS zero points by comparison with excellent ground-based photometry, and should reduce their uncertainties to less than 0.01 magnitudes. A second benefit is that it will facilitate the comparison of the WFPC2 and ACS photometric systems, which will be important as WFPC2 is phased out and ACS becomes HST's primary imager. Finally, three of the filters will be repeated from my Cycle 12 observations, allowing for a measurement of any change in sensitivity.
The goal of the proposed observations is to obtain accurate photometry of the omega Cen standard stars in the WFC in all nine broadband filters. For the optimal comparisons with WFPC2 observations, the exposure times will be 25 seconds, which is the median exposure time of the WFPC2 omega Cen observations. (This will also ensure signal-to-noise exceeding that of the ground-based standard data.) Based on my Cycle 12 observations of this field, I do not expect cosmic rays to be a significant problem. Thus I will again be obtaining two images per filter, one with the field centered on each chip. The resulting 18 exposures will fit into the three orbit allocation. STELLAR POPULATIONS 0 true 0.0 false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER J2000 13 25 37.0000 -47 35 38.00 J2000 ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F550M ACS/WFC Imaging No No F555W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F625W ACS/WFC Imaging No No F775W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F850LP /Users/andy/HST_C14/omegacen/OP-cache/omegacen-v01.xml (Tic-Visit :proposal-id 10621 :visit-id "01" :proposal-version -1 :trans-version "20050323" :date "05-May-2005 12:17:04.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 5116032 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10621 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10621 :visit-id "01" :orbit-number 1 :pcs-time 367 :si-vis-utilization 2997 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3398 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10621 :visit-id "01" :orbit-number 2 :pcs-time 367 :si-vis-utilization 2995 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3398 :pcs-gap 0 ) (Tic-Orbit :proposal-id 10621 :visit-id "01" :orbit-number 3 :pcs-time 367 :si-vis-utilization 2949 :al-overhead-at-end 0 :internal-dur-at-end 349 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3398 :pcs-gap 0 ) true 3 0 /Users/andy/HST_C14/omegacen/OP-cache/omegacen-v01.xml