Two additional orbits were approved by HSTMO and therefore added to the program. Proposal information and observing description have both been updated to reflect the additional approved orbits and changes due to RGM. -1277777392 ----- Attempting Submission 1 (Mon Oct 07 14:34:58 GMT 2024) ----- ----- Attempting Submission 2 (Thu Feb 20 17:28:06 GMT 2025) ----- WFC3 IR Time-Dependent Sensitivity: Spatial Scans We aim to measure the time dependent sensitivity loss for WFC3/IR using spatial scans of the open cluster M35. We use spatial scanning as the SNR achieved from scanning is higher than conventional staring observations. The spatial scans build on the F140W observations performed in HST programs 14020, 16031, 16439, 17261, and 17586 as well as the F098M observations from programs 16031, 16439, 17261, and 17586. Previous programs were designed to execute two identical visits ~3 months apart, however this is not currently feasible because RGM constrains the target visibility. Instead, we plan to execute a total of four identical visits: two during the window of visibility in Fall 2024, and two during the window of visibility in Fall 2025. This will extend the baseline to span approximately 6 years of WFC3/IR operations, enabling high precision analysis of time-dependent sensitivity. Additionally, for both pairs of visits, the short interval between each visit will provide additional sampling for the short-term repeatability. The four spatial scan visits are identical, and will be executed in pairs: the first two during the Fall 2024 visibility window and the second two during the Fall 2025 window. In RGM, we can no longer observe the target in the spring (as in prior cycles). Instead we observe it in the fall (flipped by 180 degrees). Because of this, we now dither in the opposite direction compared to previous progrmas. Therefore, observations in each visit are executed with the following strategy: - The first observation is a simple direct image in F140W. - The next 4 exposures are horizontal spatial scans (length ~13.2") in F140W with sequential dithers of 1'', perpendicular to the scan direction (the +Y direction), between exposures, as to match the experiment performed in 14020. - The following 10 exposures are performed in the same way, however with dithers of 1.4'' in the +Y direction to further mitigate persistence. - There are then 4 exposures taken with F098M, with the same 1.4" dithers in between each exposure. This puts one of the target stars at the top of the detector, thereby not allowing any further dithering in the previous direction. - Next, a large 17 arcsecond dither is performed in the +X direction (to avoid scanning on the same pixels where the star already landed), and 2 more F098M observations are taken, with subsequent dithers of 1.4" in the -Y direction. - We then switch back to F140W and take 5 more observations, with dithers of 3" in the -Y direction between exposures, as to ensure persistence is mitigated. - Lastly, we take a dark exposure with the same sample sequence (but using 11 reads) as to characterize persistence. 0 false false RESTRICTED STELLAR CLUSTER OPEN CLUSTER This object was generated by the targetselector and retrieved from the SIMBAD database. An advantage of an open cluster is that it is not too dense like a globular, and an open cluster will contain a lot of (photometrically stable) main sequence stars. 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