Assigned ID: 104 ----- Attempting Submission 1 (Wed Feb 23 19:00:59 GMT 2011) ----- ----- Attempting Submission 1 (Mon Jul 11 01:26:05 GMT 2011) ----- We have an ongoing long-term program, renewed in Cycle 18 for 3 years, in which we are imaging several important visual binaries, including Sirius and Procyon. The aims of this astrometric program are to determine precise dynamical masses for the white-dwarf and main-sequence components of the binaries, and to search for (or set limits on) third bodies in the systems down to planetary masses. With the advent of WFC3 in our program, we are forced to saturate the images of Sirius A and Procyon A, and instead use the diffraction spikes to infer their centroid locations. In order to calibrate the use of the spikes against the conventional method of centroiding an unsaturated image, which is the method used for the much fainter white-dwarf companions, we request one orbit for imaging of a fainter A-type star. For this star it is possible to obtain both unsaturated images for conventional centroiding and deeper saturated images to which we can apply our diffraction-spike algorithm and derive the offsets. We can thus apply corrections to our diffraction-spike-based astrometry of the saturated Sirius A and Procyon A images, reducing the systematic errors to below 1 mas, and setting tight limits on third bodies. We will not request any additional funding if this supplemental request is approved, and we will waive the proprietary period.

These observations of an 8th-magnitude A5 V star are being made in order to calibrate the method of using diffraction spikes to centroid images of bright, overexposed stars in WFC3/UVIS images. We have a separate program of imaging of the bright stars Sirius and Procyon with the F953N filter. These images are necessarily overexposed because there is no way to take unsaturated WFC3 images of these very bright stars. To determine whether there are systematic offsets from the true centroids and centroids determined from the diffraction spikes, we will observe the 8th-magnitude star with the same filter and UVIS aperture used for Sirius and Procyon, and will combine unsaturated 5-sec exposures with saturated ~600 sec exposures. We will use the same 4-point dither pattern being used for the two bright stars. HOT STARS 0 false 0.0 0.0 false false false false SUPPORTED STAR A4-A9 V-IV J2000 J2000 A5 V WFC3/UVIS Imaging No No F953N POS-TARG DITHER 0.173 LINE 0.112 false true 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false 1310347051141 /user/bond/klingsor1/hstcycle19/diffspikes/OP-cache/12598-v01.xml The scheduling data was generated by CASM 19.3. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12598 :visit-id "01" :proposal-version -1 :trans-version "20110509" :date "10-Jul-2011 21:17:30.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 36624 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12598 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12598 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2916 :al-overhead-at-end 38 :internal-dur-at-end 79 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) true 1 0