0 Assigned ID: 2667 ----- Attempting Submission 1 (Sun Jan 30 03:00:54 GMT 2022) ----- HST Phase I Proposal 2667 successfully submitted. Receipt: # 2667-1 ----- Attempting Submission 2 (Sun Jan 30 05:34:42 GMT 2022) ----- HST Phase I Proposal 2667 successfully submitted. Receipt: # 2667-2 0 ----- Attempting Submission 1 (Thu Mar 17 18:58:15 GMT 2022) ----- HST Phase II Proposal 16922 successfully submitted. Receipt: # 16922-1 ----- Attempting Submission 2 (Thu Mar 17 21:22:30 GMT 2022) ----- HST Phase II Proposal 16922 successfully submitted. Receipt: # 16922-2 ----- Attempting Submission 3 (Thu Mar 17 21:47:45 GMT 2022) ----- 10s overun in IR orbit Late-time Observations of Calcium-Rich Transient SN 2021gno Supernova (SN) 2021gno in NGC 4165 (D=30.5 Mpc) is the second closest known Calcium-rich (Ca-rich) transient and only the second object in this class with an X-ray detection. Prompt, high-cadence follow-up of this transient across the EM spectrum has indicated that the progenitor star was likely low mass (e.g., white dwarf or <10 Msun) and surrounded by dense circumstellar material (CSM) whose geometry/density was capable of producing luminous X-ray emission as well as a double-peaked light curve. The close proximity of SN 2021gno provides only the second opportunity to track the photometric evolution of a Ca-rich transient at late phases (>300 days) when the SN luminosity is governed by radioactive decay and/or additional power sources e.g., CSM, and is too faint for ground-based observatories. These objects typically decrease in magnitude rapidly and thus their late-time decline rate and power source remains ambiguous. Here we propose multi-color imaging of SN 2021gno in order to understand its late-time bolometric behavior and to constrain the total masses of radioactive decay isotopes Ni-56 and Ni-57 synthesized in the explosion. This will allow us to test whether SN 2021gno is powered solely by radioactive decay or by additional CSM at large distances from the progenitor system. We propose to observe nearby supernova 2021gno with WFC3/IR in F110W and F160W filters in the first of our two allocated orbits. In the second orbit, we will observe SN 2021gno with WFC3/UVIS in filters F555W and F814W. Our optimal observing window is between 28-April-2022 and 01-June-2022. We have requested that IR imaging be taken within 3 days of the UVIS imaging orbit. Below is our observing plan for each filter in each orbit: Orbit 1: F110W/F160W: Four ~300s individual exposures for each filter using the WFC3-IR-DILTER-BOX-MIN pattern. Orbit 2: F555W: Total Exposure time = 1500s; Split into two 750s exposures using WFC3-UVIS-DITHER-LINE pattern F814W: Total Exposure time = 900s; Split into two 450s exposures using WFC3-UVIS-DITHER-LINE pattern Mr. Wynn Jacobson-Galan has extensive experience analyzing and reducing WFC3/UVIS data, the instrument proposed for this program. Mr. Jacobson-Galan is a graduate student with burgeoning expertise on the late-time light curves of thermonuclear supernovae observed with HST (2 published papers) and calcium-rich transients (3 published papers, 1 paper in prep). Mr. Jacobson-Galan has completed analysis on the early-time, multi-wavelength observations of the proposed target, SN 2021gno, and all data on this object will be included in a journal article planned to be submitted within the month. Prof. Raffaella Margutti is a leading expert on x-ray and radio observations of exotic transients such as SN 2021gno. Dr. Charles Kilpatrick has significant experience in reduction and analysis of HST/WFC3, ACS, and WFPC2 imaging data, including the prime instruments used in these proposed observations. Prof. Ryan Foley is an expert in transient surveys and follow up of all types of explosive transients. Dr. Armin Rest is one of the leading experts on HST observations and difference photomety. Dr. Yossef Zenati has extensive experience with theoretical modeling and interpretation of Ca-rich and other thermonuclear transient varieties. Huei Sears has extensive experience with HST reductions and will assist in the data analysis. 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