0 Assigned ID: 6545 ----- Attempting Submission 1 (Thu Apr 10 17:22:17 GMT 2025) ----- HST Phase I Proposal 6545 successfully submitted. Receipt: # 6545-1 ----- Attempting Submission 2 (Thu Apr 10 17:28:23 GMT 2025) ----- HST Phase I Proposal 6545 successfully submitted. Receipt: # 6545-2 ----- Attempting Submission 3 (Thu Apr 10 17:42:12 GMT 2025) ----- HST Phase I Proposal 6545 successfully submitted. Receipt: # 6545-3 ----- Attempting Submission 4 (Thu Apr 10 17:54:04 GMT 2025) ----- HST Phase I Proposal 6545 successfully submitted. Receipt: # 6545-4 ----- Attempting Submission 5 (Thu Apr 10 18:09:17 GMT 2025) ----- HST Phase I Proposal 6545 successfully submitted. Receipt: # 6545-5 ----- Attempting Submission 6 (Thu Apr 10 18:45:15 GMT 2025) ----- HST Phase I Proposal 6545 successfully submitted. Receipt: # 6545-6 ----- Attempting Submission 7 (Thu Apr 10 23:53:07 GMT 2025) ----- I spread the four F625W imaging exposures over the six-orbit visit. I introduced a nested WFC3-UVIS-DITHER-LINE-3PT (primary) × WFC3-UVIS-DITHER-BOX (secondary) pattern for the F645N, and WFC3-UVIS-DITHER-BOX for the F625W exposures. -1522463584 ----- Attempting Submission 1 (Thu Aug 14 23:13:30 GMT 2025) ----- ----- Attempting Submission 2 (Mon Oct 20 22:05:06 GMT 2025) ----- ----- Attempting Submission 3 (Thu Jun 11 22:08:31 GMT 2026) ----- The ~30 arcsecond Dragon arc at z=0.72 created by the Abell 370 cluster is the most prolific known source of microlensing events of individual stars at a cosmological distance. A total of ~50 stars have been detected across only two ultra-deep UV-optical HST and two shallow JWST visits. Recent work finds that the observed microlensing events may exhibit a broad bias towards the negative-parity side of the critical curve, which holds the promise to distinguish among forms of substructure produced by different dark-matter candidates. Improved constraints on the location of the cluster's critical curve, however, are needed. Furthermore, the microlensing events, if they can be combined with a spatially resolved recent star-formation history, will yield constraints on the upper-end of the initial-mass function (IMF) across the giant arc. Here we propose to take advantage of a pair of coincidences between the WFC3 UVIS F645N filter and [OII] nebular emission from the Dragon arc at z=0.72, and between the JWST NIRCam F323N filter and Paschen alpha emission. Since nebular emission is not, as a rule, found at the same locations as star-forming clumps visible in broadband images, the proposed data yield a new and independent set of constraints on the path of the critical curve. Pairs of counterimages of HII regions identified by their [OII] / Paschen alpha ratio will precisely bracket the location of the critical curve. Moreover, the fluxes of HII regions should not be affected by stellar microlensing, so we can, for the first time, unambiguously detect the existence of millilensing due to dark-matter subhalos in a cluster. The program will acquire a total of five orbits of WFC3 UVIS F645N narrow-band and a single orbit of WFC3 UVIS F625W observations of the Abell 370 galaxy-cluster field. The WFC3 UVIS F645N narrowband filter will have sensitivity to [OII] 3726, 3729 nebular emission from the Dragon arc at z=0.72. Coordinated JWST NIRCam F323N imaging corresponds to Paschen alpha emission from the arc. After subtracting continuum emission estimated from F625W for the WFC3 UVIS imaging, these will allow which will enable identification of counterimages of HII regions which constrain the location of the critical curve. For scheduling flexiblity, the total of six orbits are broken into a set of three two-orbit visits. Each visit include four dithered exposures. POS TARGS are used to accomplish dithering across the full set of three visits for F645N. Visit 2 differs from Visit 1 and Visit 3. The first orbit consists of the four F625W exposures, and the second of four F645N exposures. For each filter, we use the POS TARG values for the WFC3-UVIS-DITHER-BOX from https://hst-docs.stsci.edu/wfc3ihb/appendix-c-dithering-and-mosaicking/c-2-wfc3-patterns The FLASH level is set to 17 for the F645N exposures, and FLASH is set to 3 for the F625W exposures. We use the UVIS1-FIX aperture to target the center of the Dragon Arc so that the arc falls away from the chip gap at all roll angles. Preferred orient is 85.2, 175.2, 265.2, or 355.2 to match diffraction spike orientation of archival F625W observations for improved subtraction. Orients are set to be the same for three visits. PI, Patrick Kelly, is an assistant professor at the University of Minnesota. He is an expert in interpreting Hubble observations of microlensing events, and has led the Flashlights HST program in search for the microlensing events in the Hubble Frontier Fields (HFF) clusters. Co-I, Wenlei Chen, is a postdoctoral researcher at the University of Minnesota and is working on modeling magnification maps for the HFF clusters based on Flashlights data and recently-discovered microlening events. Co-I Masamune Oguri, is an assistant professor at the university of Tokyo and has developed Glafic, a software for analyzing gravitational lensing in the HFF clusters. Co-I, Adi Zitrin, is an assistant faculty at Ben Gurion University of the Negev who has developed a model for the gravitational lensing of the HFF clusters. Co-I, Jose Diego, is a Permanent Staff CSIC Researcher and has also developed a gravitational lensing called WSLAP+ for the HFF clusters. Co-I Fengwu Sun is an expert in lensing and galaxy populations. Stellar Physics and Stellar Types Galaxies 6 false false false false false false false true PRIMARY 3.4 true SUPPORTED CLUSTER OF GALAXIES GRAVITATIONAL LENS J2000 J2000 WFC3/UVIS Imaging No No F645N 1781215589789 The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. preferred orient is 85.2, 175.2, 265.2, or 355.2 to match diffraction spike orientation of archival F625W observations for improved subtraction (Tic-Visit :proposal-id 18106 :visit-id "01" :proposal-version -1 :trans-version "20260319" :date "11-Jun-2026 17:06:29.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 4619936 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18106 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18106 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18106 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2594 :al-overhead-at-end 126 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18106 :visit-id "01" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2592 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18106 :visit-id "01" :orbit-number 4 :pcs-time 380 :si-vis-utilization 2580 :al-overhead-at-end 120 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18106 :visit-id "01" :orbit-number 5 :pcs-time 380 :si-vis-utilization 2594 :al-overhead-at-end 126 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18106 :visit-id "01" :orbit-number 6 :pcs-time 380 :si-vis-utilization 2578 :al-overhead-at-end 120 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 6 0