0 Assigned ID: 396 ----- Attempting Submission 1 (Fri Apr 07 04:37:16 GMT 2017) ----- Changed aperture from WFC to WFC1 based on recommendation from instrument team 1340601551 ----- Attempting Submission 1 (Fri Jul 14 20:36:00 GMT 2017) ----- ----- Attempting Submission 2 (Tue Apr 10 11:07:34 GMT 2018) ----- Double source plane (DSP) gravitational lenses are extremely rare alignments of a massive lens galaxy with two background sources at distinct redshifts. The presence of two source planes provides important constraints on cosmology and galaxy structure beyond that of typical lens systems by breaking degeneracies between parameters that vary with source redshift. While these systems are extremely valuable, only a handful are known. We have discovered the first DSP lens, the "Eye of Horus", in the Hyper Suprime-Cam survey and have confirmed both source redshifts with follow-up spectroscopy, making this the only known DSP lens with both source redshifts measured. Furthermore, the brightest image of the most distant source (S2) is split into a pair of images by a mass component that is undetected in our ground-based data, suggesting the presence of a satellite or line-of-sight galaxy causing this splitting. In order to better understand this system and use it for cosmology and galaxy studies, we must construct an accurate lens model, accounting for the lensing effects of both the main lens galaxy and the intermediate source. Only with deep, high-resolution imaging from HST/ACS can we accurately model this system. Our proposed multiband imaging will clearly separate out the two sources by their distinct colors, allowing us to use their extended surface brightness distributions as constraints on our lens model. These data may also reveal the satellite galaxy responsible for the splitting of the brightest image of S2. With these observations, we will be able to take full advantage of the wealth of information provided by this system. We aim to image the double source plane lens, the "Eye of Horus", using the ACS/WFC. Our goal is to obtain a S/N = 3 per pixel for both S1 (z=1.30) in the F606W filter, as well as S2 (z=1.99) in the F850LP filter. To attain this, we require 2 orbits in F606W and 1 orbit in F850LP. We use the ACS-WFC-DITHER-BOX 4-point dither pattern for our observations to eliminate defects and improve the PSF sampling. The entire target is ~10 arcsec across, which easily fits within the ACS field of view. Our proposed observations consist of a single visit of 3 orbits, using the WFC aperture. Orbit 1: Dither 1 (2 exposures in F606W, 1 exposure in F850LP), Dither 2 (1 exposure in F606W) Orbit 2: Dither 2 (1 exposure in F606W, 1 exposure in F850LP), Dither 3 (2 exposures in F606W) Orbit 3: Dither 3 (1 exposure in F850LP), Dither 4 (2 exposures in F606W, 1 exposure in F850LP) Cosmology Galaxies and the IGM 6 false false false false false false SUPPORTED GALAXY EINSTEIN RING GRAVITATIONAL LENS J2000 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F850LP POS-TARG DITHER 0.262 LINE 0.192 false false 68.14 4 18.39 ACS-WFC-DITHER-BOX 1 false 1523358403869 /private/var/folders/yy/cvpg_zyx6qs5cpyzhphj5n940000gn/T/OPVisit8230325515907791009.xml The scheduling data was generated by CASM 25.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 15325 :visit-id "01" :proposal-version -1 :trans-version "20171009" :date "10-Apr-2018 20:06:42.0" :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 3410688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 15325 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15325 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2869 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3202 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15325 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2979 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3202 :pcs-gap 0 ) (Tic-Orbit :proposal-id 15325 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2978 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3202 :pcs-gap 0 ) true 3 0