Hi Hal, How's this? I re-did some orbit numbers, and I auto-adjusted the pattern. ----- Submission 5 (Tue Oct 26 15:53:21 GMT 2004) ----- not running VP. A Deep Search for Companions in the Pluto-Charon System We propose to perform a deep search for new satellites in the Pluto-Charon system using the ACS in WFC mode. In a single orbit, ACS/WFC imaging will reach a sensitivity that is ~5.5 mag fainter than previous surveys, and we will be able to detect objects that are only ~5 km in diameter, if they have the same albedo as Pluto (50%), and ~17 km if the albedo is similar to that of cometary nuclei (4%). The discovery of a satellite in the Pluto-Charon system would be an exciting development in the burgeoning field of satellite formation theory and would also provide critical mission support for the New Horizons mission, which will be launched in January 2006 and will make a close flyby of Pluto-Charon as early as July 2015. Pluto is approaching the Milky Way, and observations near the time of this year's opposition will mitigate the background confusion problem, which generally becomes worse during the coming decade. The high spatial resolution of HST is ideal for this type of search in crowded stellar fields, and the field-of-view of the ACS/WFC is almost perfectly matched to the orbital stability radius of companions in the Pluto-Charon system. One additional image, taken ~5 days after the first, is needed to confirm that any detected objects are truly members of the Pluto-Charon system. Thus, the entire program requires a total of only 2 orbits.
The 90 arcsec radius of the stability region for Pluto satellites fits nicely in the FOV of the ACS WFC (202'' x 202'', 0.049 arcsec/pixel), so the search can be done using a single HST pointing. The ACS HRC is much less suitable, having a lower sensitivity and a smaller field. We plan to track on Pluto and, thus, will also be tracking on any satellites, which will be essentially fixed relative to Pluto in a single HST orbit. We plan to take three long exposures (~600s) per orbit, dithered so that bad pixels can be removed, and a single short exposure, with Pluto and Charon not overexposed, in order to provide an astrometric reference for satellite locations. Pluto has a geocentric motion of ~4 arcsec/hr, which means that stars will be trailed by ~14 WFC pixels in a single 600s exposure. Under these conditions, stars trails are easily removed using standard image reduction methods to produce clean backgrounds for making a sensitive search for companions. Using the ACS exposure time calculator (ETC), we find that S/N = 5 can be obtained in the F606W filter on an object having an R-band magnitude of ~27.5 in one orbit of HST time (2000 seconds of exposure time, with additional time for overhead). The CLEAR filter has greater throughput, but has significanlty poorer image quality so that confusion with background sources is more of a problem, so F606W is preferred. We need two orbits, separated by 3-8 days, to unambiguously separate Pluto satellites from background KBOs and to obtain initial orbit constraints. Although the presence of Pluto and Charon in the field will degrade the sensitivity somewhat relative to the above values, especially right next to Pluto and Charon, we should still be able to detect any companions larger than ~5-20 km in diameter, depending on their albedo. We have already looked carefully at the path of Pluto across the sky to search for the best times to perform our observations. The important constraints to be satisfied are: 1) Maximize Pluto's apparent rate of motion to make it easier to remove the effects of background stars and non-Pluto region objects. 2) Minimize background light by observing in Earth's shadow (or primarily so). 3) Do not observe when there are nearby, bright field stars. These constraints are generally met if the observations are scheduled between 5/15/2005 and 6/30/2005. However, there are specific dates (e.g., don't observe during 5/16-17/2005 because a cluster of stars is nearby and don't observe on 5/26/2005 because a bright star is nearby). Please let us know when the specific observing dates are selected, and we'll double-check to ensure that there are no nearby field objects that could spoil our observations. SOLAR SYSTEM 12 false 0.0 false false false false false SUPPORTED SOLAR SYSTEM PLANET PLUTO STD=PLUTO ACS/WFC Imaging No No F606W POS-TARG DITHER 3.011 LINE false false 2 85.28 ACS-WFC-DITHER-LINE 1 false /var/tmp/OPVisit30042.xml (Tic-Visit :proposal-id 10427 :visit-id "01" :proposal-version -1 :trans-version "20041107" :date "14-Mar-2005 11:21:33.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1421120 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10427 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10427 :visit-id "01" :orbit-number 1 :pcs-time 365 :si-vis-utilization 2711 :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 3236 :pcs-gap 0 ) true 1 0 /data/hedwig1/sherwin/10427/OP-cache/10427-v01.xml /var/tmp/OPVisit30043.xml (Tic-Visit :proposal-id 10427 :visit-id "02" :proposal-version -1 :trans-version "20041107" :date "14-Mar-2005 11:21:50.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 1421120 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 10427 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 10427 :visit-id "02" :orbit-number 1 :pcs-time 365 :si-vis-utilization 2711 :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 3236 :pcs-gap 0 ) true 1 0 /data/hedwig1/sherwin/10427/OP-cache/10427-v02.xml