18358( 2) - 18-Aug-2026 18:21:30 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18358 Version: 2 Check-in Time: 18-Aug-2026 18:21:30 STScI Edit Number: 0 Title Ultra-rapid HST Target-of-Opportunity observations of a kilonova in LIGO-Virgo-KAGRA IR1 ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Charles Kilpatrick Northwestern University Y CoI: Dr. Armin Rest Space Telescope Science Institute N CoI: Dr. Louis-Gregory Strolger Space Telescope Science Institute N CoI: Dr. Tea Temim Columbia University in the City of New N York CoI: Dr. Ori Dosovitz Fox Space Telescope Science Institute N CoI: Dr. Ryan Ridden University of Canterbury N CoI: Dr. Qinan Wang Massachusetts Institute of Technology N CoI: Dr. Russell E. Ryan Jr. Space Telescope Science Institute N CoI: Dr. Annalisa Calamida Space Telescope Science Institute N CoI: Dr. Suvi Gezari University of Maryland N CoI: Dr. Justin Pierel The Johns Hopkins University N CoI: Dr. Yossef Zenati The Johns Hopkins University N CoI: Dr. Iair Arcavi Tel Aviv University - Wise Observatory N CoI: Prof. Gautham Narayan University of Illinois at Urbana - N Champaign CoI: Dr. Alexander Thomas Gagliano Massachusetts Institute of Technology N CoI: Mr. Patrick David Aleo University of Illinois at Urbana - N Champaign CoI: Dr. Konstantin Malanchev Carnegie Mellon University N CoI: Prof. Jeff Cooke Swinburne University of Technology N CoI: Ms. Jielai Zhang University of Toronto N CoI: Dr. Dougal Dobie Swinburne University of Technology N CoI: Prof. Robert P. Kirshner Harvard University N CoI: Maria Drout University of Toronto N CoI: Aaron Tohuvavohu California Institute of Technology N CoI: Marcelle Soares-Santos University of Michigan N CoI: Dr. Antonella Palmese Carnegie Mellon University N CoI: Dr. James Annis Fermi National Accelerator Laboratory N (FNAL) CoI: Dr. Andy Howell Las Cumbres Observatory Global Telescope N Network CoI: Dr. Curtis McCully Las Cumbres Observatory Global Telescope N Network CoI: Mr. Jamison Burke University of California - Santa Barbara N CoI: Prof. Daichi Hiramatsu University of Florida N CoI: Dr. Craig Pellegrino NASA Goddard Space Flight Center N CoI: Dr. Saurabh W. Jha Rutgers the State University of New Jersey N CoI: Prof. Ryan Foley University of California - Santa Cruz N CoI: David Oscar Jones University of Hawaii N CoI: Dr. Phillip Macias University of California - Santa Cruz N CoI: Mr. Cesar Rojas-Bravo University of California - Santa Cruz N CoI: Dr. Matthew Ryan Siebert Space Telescope Science Institute N 18358( 2) - 18-Aug-2026 18:21:30 - [ 2] CoI: Dr. Kirsty Taggart Universities Space Research Association N CoI: Dr. Samaporn Tinyanont National Astronomical Research Institute N of Thailand CoI: Prof. Stefano Valenti University of California - Davis N CoI: Prof. David J. Sand University of Arizona N CoI: Francisco Forster Universidad de Chile, CMM N CoI: Prof. Daryl Haggard McGill University N CoI: Dr. David Coulter The Johns Hopkins University N CoI: Dr. Peter M. Garnavich University of Notre Dame N CoI: Dr. Charlotte M. Wood North Carolina Agricultural and Technical N State University CoI: Yen-Chen Pan National Central University N CoI: Dr. Enrico Ramirez-Ruiz University of California - Santa Cruz N CoI: Dr. Daniel Kasen University of California - Berkeley N CoI: Dr. Alexander Pung Ji University of Chicago N CoI: Dr. Joshua D. Simon Carnegie Institution of Washington N CoI: Dr. Anthony Piro Carnegie Institution of Washington N CoI: Kyle Davis University of California - Santa Cruz N CoI: Dr. Peter McGill Lawrence Livermore National Laboratory N CoI: Dr. Maria Jose Bustamante Rosell Universities Space Research Association N CoI: Mike Engesser Space Telescope Science Institute N CoI: Tomas Cabrera Carnegie Mellon University N CoI: Dr. Melissa Shahbandeh Space Telescope Science Institute N CoI: Dr. Jacob Jencson California Institute of Technology N CoI: Dr. Sebastian Gomez University of Texas at Austin N CoI: Muryel Guolo The Johns Hopkins University N CoI: Rodrigo Angulo The Johns Hopkins University N CoI: Brad Tucker Australian National University N CoI: Dr. Mandeep S Gill Stanford University N CoI: Dr. Brendan O'Connor University of Maryland N CoI: Dr. Estefania Padilla Gonzalez The Johns Hopkins University N CoI: Dr. Thomas Moore Space Telescope Science Institute N CoI: Conor Larison Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract The upcoming LVK run (IR1) will detect new neutron star mergers in gravitational waves, with wide-field optical and infrared survey support for electromagnetic follow up, but interpretation depends on day-timescale kilonova evolution: the early UV and faint optical are where ground-based follow-up is weakest. Only GW170817/AT 2017gfo has been characterized in both gravitational waves and across the electromagnetic spectrum in enough detail to map ejecta composition, geometry, and heating. A comparable event would again require rapid space-based UV spectroscopy and imaging while the photosphere is hot. We request one ultra-disruptive, ultra-rapid HST Target-of-Opportunity trigger in Cycle 34 (overlapping IR1), totaling 10 orbits for coordinated WFC3/UVIS imaging and STIS spectroscopy matched to a ground-based discovery, prioritizing UV throughput and HST's rapid response. Our science goals are (1) to separate weak- and heavy-r-process signatures using early UV/blue photometry with JWST data, (2) to combine electromagnetic ejecta constraints with gravitational-wave chirp mass to inform neutron star radii, equation of state, and maximum mass, and (3) to test models for early UV and optical emission. Theory predicts blue-to-red kilonova diversity, and our HST window extends to ~12 days post-merger while redder optical/NIR monitoring continues from the 18358( 2) - 18-Aug-2026 18:21:30 - [ 3] ground and with JWST. Joint gravitational-wave and electromagnetic analysis will constrain merger dynamics and the remnant. We will coordinate openly, document triggering criteria, and waive proprietary periods so the multimessenger community can use these data immediately. ------------------------------------------------------------------------------------ Observations Description ------------------------ This Phase II program implements one ultra-disruptive Target-of-Opportunity activation (10 orbits) for early UV/optical observations of a kilonova counterpart to a LIGO-Virgo-KAGRA IR1 gravitational-wave event. We will not trigger unless the first visit can begin within 5 days of GW detection, aiming for first-epoch observations within <=3 days of merger for counterparts with r < 22 mag. Following the proposal Special Requirements, APT uses a simple five-epoch, two-orbit sequence (10 orbits total), with all visits On Hold until trigger, Sequential Observations in Time (After Observation links with variable cadence), and Non-Interruptible Sequential Observations within each visit. Exposure times and filter/instrument choices follow Scenario A of Table 1 (GW170817-like, m_F606W~19.5 at t~1 d), and will be optimized toward Scenario A or B at trigger. Epoch 1 (t~1.0 d): WFC3/UVIS G280 spectroscopy with F225W/F275W/F336W imaging and STIS/CCD G430L (52x2 slit). Epoch 2 (t~1.5 d): STIS G430L plus F225W/F275W/F336W. Epoch 3 (t~3.0 d): F275W/F336W/F438W. Epoch 4 (t~5.0 d): F336W/F438W. Epoch 5 (t~8-12 d): F438W. Early cadence is 0.5-2 days; later cadence widens to a few days, ending by ~12 days post-merger. Within each visit, STIS exposures are placed first (STIS/CCD ACQ with MIRROR+F28X50LP, then G430L spectroscopy), followed by WFC3/UVIS imaging/spectroscopy, minimizing SI changes and placing target acquisition immediately before STIS science. Visit 01 uses an Ultra Disruptive response window of 1-2 days; visits 02-05 use Disruptive windows of 1-14 days to accommodate After Observation cadence. ------------------------------------------------------------------------------------ 18358( 2) - 18-Aug-2026 18:21:30 - [ 4] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 1 KILONOVA EXT-STAR, SUPERNOVA ELECTROMAGNETIC COUNTERPART V = 19.5 (KILONOVA) TO A LIGO-VIRGO-KAGRA IR1 GW EVENT WITH AT LEAST ONE NEUTRON STAR, AT A DISTANCE CONSISTENT WITH THE GW LOCALIZATION, AND WITH R < 22 MAG AT TRIGGER. ------------------------------------------------------------------------------------------------------------------------------------ 18358( 2) - 18-Aug-2026 18:21:30 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.149 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 14.04 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18358( 2) - 18-Aug-2026 18:21:30 - [ 6] Visit: 01 (KN-E1-1.0d) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 1.0D; TOO MAX RESPONSE TIME 2.0D On Hold Comments: On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. Additional Comments: Scenario A, t~1.0 d (2 orbits): WFC3/UVIS G280 slitless spectroscopy and F225W/F275W/F336W imaging, plus STIS/CCD G430L spectroscopy (52x2 slit), per Table 1. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 KILONOVA STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 40 S (40 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 KILONOVA STIS/CCD ACCUM 52X2 G230LB 2375 CR-SPLIT=2 1 1908 S (1908 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 KILONOVA WFC3/UVI ACCUM UVIS2-C51 F336W FLASH=21 1 50 S S 2C-SUB see below (49 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 KILONOVA WFC3/UVI ACCUM UVIS2-C51 F275W FLASH=21 1 150 S PATTERN 1 4 S 2C-SUB see below (298 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 KILONOVA WFC3/UVI ACCUM UVIS2-C51 F225W FLASH=20 1 250 S PATTERN 1 5 S 2C-SUB see below (498 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 KILONOVA STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 40 S (40 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 KILONOVA STIS/CCD ACCUM 52X2 G430L 4300 CR-SPLIT=2 1 500 S see below (498 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ KILONOVA 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A KILONOVA 2 STIS/CCD ACCUM 52X2 G230LB 2375 none none none 1 1 N/A KILONOVA 2 STIS/CCD ACCUM 52X2 G230LB 2375 none none none 2 1 N/A KILONOVA 3 WFC3/UVI ACCUM UVIS2-C5 F336W none none none none 2 49.0 S S 12C-SUB KILONOVA 4 WFC3/UVI ACCUM UVIS2-C5 F275W 1 none none none 2 149.0 S S 12C-SUB KILONOVA 4 WFC3/UVI ACCUM UVIS2-C5 F275W 2 none none none 2 149.0 S S 12C-SUB 18358( 2) - 18-Aug-2026 18:21:30 - [ 7] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ KILONOVA 5 WFC3/UVI ACCUM UVIS2-C5 F225W 1 none none none 2 249.0 S S 12C-SUB KILONOVA 5 WFC3/UVI ACCUM UVIS2-C5 F225W 2 none none none 2 249.0 S S 12C-SUB KILONOVA 6 STIS/CCD ACQ F28X50LP MIRROR none none none none 2 N/A KILONOVA 7 STIS/CCD ACCUM 52X2 G430L 4300 none none none 1 2 249.0 S KILONOVA 7 STIS/CCD ACCUM 52X2 G430L 4300 none none none 2 2 249.0 S 18358( 2) - 18-Aug-2026 18:21:30 - [ 8] Visit: 02 (KN-E2-1.5d) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 1.0D; TOO MAX RESPONSE TIME 14.0D On Hold Comments: On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. Additional Comments: Scenario A, t~1.5 d (2 orbits): STIS/CCD G430L spectroscopy and WFC3/UVIS F225W/F275W/F336W imaging, per Table 1. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 KILONOVA STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 40 S (40 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 KILONOVA STIS/CCD ACCUM 52X2 G430L 4300 CR-SPLIT=2 1 1814 S (1814 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 KILONOVA WFC3/UVI ACCUM UVIS2-C51 F336W FLASH=19 1 50 S (50 POS TARG 0, 0; SEQ 3-5 S 2C-SUB S) NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 4 KILONOVA WFC3/UVI ACCUM UVIS2-C51 F275W FLASH=18 1 300 S (300 SAME POS AS 3 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 5 KILONOVA WFC3/UVI ACCUM UVIS2-C51 F225W FLASH=15 1 737 S (737 SAME POS AS 3 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 6 KILONOVA WFC3/UVI ACCUM UVIS2-C51 F275W FLASH=18 1 300 S (300 POS TARG .099, .106; SEQ S 2C-SUB S) 6-8 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 7 KILONOVA WFC3/UVI ACCUM UVIS2-C51 F336W FLASH=19 1 50 S (50 SAME POS AS 6 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 8 KILONOVA WFC3/UVI ACCUM UVIS2-C51 F225W FLASH=15 1 737 S (737 SAME POS AS 6 S 2C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ KILONOVA 1 STIS/CCD ACQ F28X50LP MIRROR none none none none 1 N/A KILONOVA 2 STIS/CCD ACCUM 52X2 G430L 4300 none none none 1 1 N/A KILONOVA 2 STIS/CCD ACCUM 52X2 G430L 4300 none none none 2 1 N/A KILONOVA 3 WFC3/UVI ACCUM UVIS2-C5 F336W none none none none 2 N/A S 12C-SUB KILONOVA 4 WFC3/UVI ACCUM UVIS2-C5 F275W none none none none 2 N/A S 12C-SUB KILONOVA 5 WFC3/UVI ACCUM UVIS2-C5 F225W none none none none 2 N/A S 12C-SUB 18358( 2) - 18-Aug-2026 18:21:30 - [ 9] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ KILONOVA 6 WFC3/UVI ACCUM UVIS2-C5 F275W none none none none 2 N/A S 12C-SUB KILONOVA 7 WFC3/UVI ACCUM UVIS2-C5 F336W none none none none 2 N/A S 12C-SUB KILONOVA 8 WFC3/UVI ACCUM UVIS2-C5 F225W none none none none 2 N/A S 12C-SUB 18358( 2) - 18-Aug-2026 18:21:30 - [ 10] Visit: 03 (KN-E3-2.0d) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 1.0D; TOO MAX RESPONSE TIME 14.0D On Hold Comments: On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. Additional Comments: Scenario A, t~3.0 d (2 orbits): WFC3/UVIS F275W/F336W/F438W imaging, per Table 1. The Table 1 t~2 d epoch is absorbed into the cadence between epochs 2 and 3. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 KILONOVA WFC3/UVI ACCUM UVIS2 F336W FLASH=19 1 150 S (300 PATTERN 1 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 KILONOVA WFC3/UVI ACCUM UVIS2 F275W FLASH=19 1 963 S PATTERN 1 2 S (1926 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ KILONOVA 1 WFC3/UVI ACCUM UVIS2 F336W 1 none none none 1 N/A S KILONOVA 1 WFC3/UVI ACCUM UVIS2 F336W 2 none none none 1 N/A S KILONOVA 2 WFC3/UVI ACCUM UVIS2 F275W 1 none none none 1 N/A S KILONOVA 2 WFC3/UVI ACCUM UVIS2 F275W 2 none none none 1 N/A S 18358( 2) - 18-Aug-2026 18:21:30 - [ 11] Visit: 04 (KN-E5-5.0d) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 1.0D; TOO MAX RESPONSE TIME 14.0D On Hold Comments: On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. Additional Comments: Scenario A, t~5.0 d (2 orbits): WFC3/UVIS F336W/F438W imaging. Table 1 allocates 1 orbit here; the second orbit deepens F438W in this simplified 5x2 APT sequence. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 KILONOVA WFC3/UVI ACCUM UVIS2 F438W FLASH=15 1 250 S (500 PATTERN 1 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 KILONOVA WFC3/UVI ACCUM UVIS2 F336W FLASH=17 1 867 S PATTERN 1 2 S (1734 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ KILONOVA 1 WFC3/UVI ACCUM UVIS2 F438W 1 none none none 1 N/A S KILONOVA 1 WFC3/UVI ACCUM UVIS2 F438W 2 none none none 1 N/A S KILONOVA 2 WFC3/UVI ACCUM UVIS2 F336W 1 none none none 1 N/A S KILONOVA 2 WFC3/UVI ACCUM UVIS2 F336W 2 none none none 1 N/A S 18358( 2) - 18-Aug-2026 18:21:30 - [ 12] Visit: 05 (KN-E6-8) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 1.0D; TOO MAX RESPONSE TIME 14.0D On Hold Comments: On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. Additional Comments: Scenario A, t~8-12 d (2 orbits): WFC3/UVIS F438W imaging combining the Table 1 t~8 d and t~12 d epochs. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 KILONOVA WFC3/UVI ACCUM UVIS2 F438W FLASH=10 1 1100 S PATTERN 1 1 S see below (2456 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ KILONOVA 1 WFC3/UVI ACCUM UVIS2 F438W 1 none none none 1 1228.0 S S KILONOVA 1 WFC3/UVI ACCUM UVIS2 F438W 2 none none none 1 1228.0 S S 18358( 2) - 18-Aug-2026 18:21:30 - [ 13] Visit: 06 (KN-E4-3.0d) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 1.0D; TOO MAX RESPONSE TIME 14.0D On Hold Comments: On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. Additional Comments: Scenario A, t~3.0 d (2 orbits): WFC3/UVIS F275W/F336W/F438W imaging, per Table 1. The Table 1 t~2 d epoch is absorbed into the cadence between epochs 2 and 3. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 KILONOVA WFC3/UVI ACCUM UVIS2 F438W FLASH=18 1 250 S (500 PATTERN 1 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 KILONOVA WFC3/UVI ACCUM UVIS2 F275W FLASH=19 1 865 S PATTERN 1 2 S (1730 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 KILONOVA WFC3/UVI ACCUM UVIS2 F336W FLASH=19 1 440 S PATTERN 1 3 S see below (878 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 KILONOVA WFC3/UVI ACCUM UVIS2 F275W FLASH=19 1 668 S PATTERN 1 4 S see below (1334 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ KILONOVA 1 WFC3/UVI ACCUM UVIS2 F438W 1 none none none 1 N/A S KILONOVA 1 WFC3/UVI ACCUM UVIS2 F438W 2 none none none 1 N/A S KILONOVA 2 WFC3/UVI ACCUM UVIS2 F275W 1 none none none 1 N/A S KILONOVA 2 WFC3/UVI ACCUM UVIS2 F275W 2 none none none 1 N/A S KILONOVA 3 WFC3/UVI ACCUM UVIS2 F336W 1 none none none 2 439.0 S S KILONOVA 3 WFC3/UVI ACCUM UVIS2 F336W 2 none none none 2 439.0 S S KILONOVA 4 WFC3/UVI ACCUM UVIS2 F275W 1 none none none 2 667.0 S S KILONOVA 4 WFC3/UVI ACCUM UVIS2 F275W 2 none none none 2 667.0 S S 18358( 2) - 18-Aug-2026 18:21:30 - [ 14] Visit: 07 (KN-E7-12) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 1.0D; TOO MAX RESPONSE TIME 14.0D On Hold Comments: On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. Additional Comments: Scenario A, t~8-12 d (2 orbits): WFC3/UVIS F438W imaging combining the Table 1 t~8 d and t~12 d epochs. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 KILONOVA WFC3/UVI ACCUM UVIS2 F438W FLASH=10 1 1100 S PATTERN 1 1 S see below (2456 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ KILONOVA 1 WFC3/UVI ACCUM UVIS2 F438W 1 none none none 1 1228.0 S S KILONOVA 1 WFC3/UVI ACCUM UVIS2 F438W 2 none none none 1 1228.0 S S 18358( 2) - 18-Aug-2026 18:21:30 - [ 15] Summary Form for Proposal 18358 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X2, F28X50LP, UVIS2, UVIS2-C512C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM, ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT, CR-SPLIT=2, FLASH=10, FLASH=15, FLASH=17, FLASH=18, FLASH=19, FLASH=20, FLASH=21 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ON HOLD , PATTERN 1 1, PATTERN 1 2, PATTERN 1 3, PATTERN 1 4, PATTERN 1 5, POS TARG .099, .106, POS TARG 0, 0, SAME POS AS 3, SAME POS AS 6, SEQ 3-5 NON-INT, SEQ 6-8 NON-INT, TOO MAX RESPONSE TIME 14.0D, TOO MAX RESPONSE TIME 2.0D, TOO MIN RESPONSE TIME 1.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F225W, F275W, F336W, F438W, G230LB, G430L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names KILONOVA ------------------------------------------------------------------------------------------------------------------------------------