Slightly revised, following Peter McCullough's suggestion to nest dithers inside filter changes. ----- Attempting Submission 1 (Fri Sep 23 16:06:47 GMT 2011) ----- ----- Attempting Submission 2 (Wed Apr 18 04:10:04 GMT 2012) ----- Very faint X-ray transient accretion states (peak L X < 10^36 ergs/s) are hard to explain with standard theories of disk instabilities. The unusual globular cluster transient M15 X-3 has only been seen at Lx~6e33 and at 2-6e31 ergs/s. We aim to determine the nature of the companion star through near-simultaneous Chandra and HST imaging, measuring its X-ray spectrum and optical colors. We plan to use our one HST orbit to obtain images of the central region of M15 with the WFC3 camera in F438W (B), F606W (broad V) and F814W (I), within 1 week before or after our Chandra observation, to identify the nature of M15 X-3’s secondary. To ensure the highest spatial resolution, we will take 4 observations in each filter, using a box dither pattern. We aim for a uniform SNR of 23 for M15 X-3 (F555W=22.2) in all filters. In order to fit the 12 exposures into a single orbit it is necessary to use the 1k*1k subarray. We note that this still gives us well-dithered deep imaging of the full core of M15, enabling a variety of ancillary science. We request that the HST observations be carried out within 1 week before or after our Chandra observation. 12 false 0.0 0.0 false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false 1334721791905 /private/var/folders/Zy/ZykLH3gZG7mByzWdZSUcTk+++TI/-Tmp-/OPVisit592690591215380888.xml The scheduling data was generated by CASM 20.0. CASM was run in Phase II mode. This visit should be scheduled within one week before or after the Chandra observation for this joint Chandra/HST program. The Orient requirements ensure that the diffraction spikes of a nearby bright star do not fall on the key program star. Revised following suggestion by Peter McCullough, April 16, 2012. (Tic-Visit :proposal-id 12751 :visit-id "02" :proposal-version -1 :trans-version "20120125" :date "17-Apr-2012 22:03:10.0" :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 213904 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12751 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12751 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2907 :al-overhead-at-end 76 :internal-dur-at-end 270 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3245 :pcs-gap 0 ) true 1 0 Increased by 1 s