Comet ISON (C/2012 S1), potentially on its first sojourn into the inner-solar system, will pass within two solar radii of the Sun's surface at perihelion. It presents us with a unique opportunity to study the properties of an Oort Cloud comet and to characterize evolutionary changes in the nucleus as it experiences the extraordinary and rapid change in its thermal and dynamical environment. Measuring the pre-perihelion size of the nucleus is fundamental to characterizing these changes. We propose to measure the nuclear size of Comet ISON as early as possible, particularly before the onset of water sublimation in early summer. An early nuclear size measurement enables or enhances the science returns of many future observations of this comet, and improves our predictions for its survivability and possible outcomes from the perihelion passage, greatly benefiting observation planning. Potentially contemporaneous observations with Herschel Space Telescope allow for the measurement of the nuclear albedo, further strengthening our proposed observations. The goal of this observation is to measure the size of the nucleus of Comet C/2012 S1 (ISON) before its activity is expected to dramatically increase when water sublimation is turned on at ~3 AU heliocentric distance in this fall. A small geocentric distance is essential for the goal. Since the geocentric distance of this comet is currently increasing until after it enters the HST exclusion zone in mid May until early October, we need to execute the observations as early as possible. We plan to use three orbits to image the comet with identical sequences. For each orbit, we will primarily use the F606W filter to image the comet, and also take images through the F438W filter to determine the color of the nucleus and coma. We will include two exposure levels for the F606W filter, 50 sec and 274 sec. For the F438W filter, we will use one exposure level of 300 sec. To avoid bad pixels and cosmic rays, we will both take multiple long exposures at the same pointing and use two-point dither. We plan to use a 1k-by-1k subarray (UVIS2-C1K1C-SUB) to reduce the instrument overhead and eliminate any issues associated with dumping the data buffer. In addition, in order to mitigate the possible CTE loss for the exposures through F438W filter due to the faint sky and possibly faint comet at this wavelength, we will use post flash value of 6 to add background to about 12 e-. The imaging sequence of each single orbit is: 50s F606W 272s F606W x2 copies 50s F606W 300s F438W Dither to a 2nd location 300s F438W 50s F606W 272s F606W x2 copies 502 F606W The purpose of imaging the comet three times in three separate orbits is to bracket the photometric range of the nucleus in a full rotation to avoid potential bias in the size determination. Since the rotational period is completely unknown, we need to separate the three orbits based on the statistical characteristics of the rotational periods of all comets previously measured. The best strategy is to execute the second orbit 3-6 hrs (2-4 orbits) after the first orbit, and execute the third orbit 12-24 hrs (8-16 orbits) after the second orbit. The exact timing is not critical here, as long as the two separations are not nearly commensurable. SOLAR SYSTEM Default false false false false false SUPPORTED SOLAR SYSTEM COMET Comet C/2012 S1 (ISON) TYPE=COMET,Q=0.01250258834862941,E=1.000004161371302,I=61.85935281653595,O=295.7375188569599,W=345.5090969848318,T=28-NOV-2013:18:42:46,TTimeScale=UTC,EQUINOX=J2000,EPOCH=07-JAN-2013:00:00:00,EpochTimeScale=TDB EARTH WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F438W (Tic-Visit :proposal-id 13198 :visit-id "01" :proposal-version -1 :trans-version "20121031" :date "25-Mar-2013 19:40:50.0" :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 178256 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13198 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13198 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2896 :al-overhead-at-end 0 :internal-dur-at-end 231 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) true 1 0 (Tic-Visit :proposal-id 13198 :visit-id "02" :proposal-version -1 :trans-version "20121031" :date "25-Mar-2013 19:41:18.0" :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 178256 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13198 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13198 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2896 :al-overhead-at-end 0 :internal-dur-at-end 231 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) true 1 0 (Tic-Visit :proposal-id 13198 :visit-id "03" :proposal-version -1 :trans-version "20121031" :date "25-Mar-2013 19:41:19.0" :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 178256 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13198 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13198 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2896 :al-overhead-at-end 0 :internal-dur-at-end 231 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3229 :pcs-gap 0 ) false 1 0