WFC3/UVIS: New FLC External CTE Monitoring 2009-2020

With the recent release of a new pixel-based CTE-correction (v2.0) in the calwf3 (v3.6.0)calibration pipeline, we present updates to the Charge Transfer Efficiency (CTE) residual flux losses measured in the WFC3/UVIS External CTE programs (2009-2020). Using observations of 47 Tuc (NGC 104) and NGC 6791 we analyze the flux loss of stars due to diminishing CTE by comparing relative aperture photometry of a given source at varying distances from the readout amplifiers on a variety of image background levels. A photometric correction model that depends on source flux, observation date, and the source distance from the readout amplifiers was fit to the data for each background level. The coefficients derived from fitting the model to the data are provided for observers to (further) correct point-source photometry in new calwf3 (v3.6.0) flc.fits files. We show through the residual CTE flux losses after the application of either the v1.0 or v2.0 CTE-correction that neither model works perfectly. The v2.0 correction often under-corrects photometry, while the v1.0 correction often over-corrects. The v1.0 correction works best at a background of 12 e-, where the faintest sources measured (250-500 e-within a 3-pixel radius aperture furthest from the readout) are over-corrected by ∼5% in 2020. At higher backgrounds, the over-correction is larger. The new v2.0 model does its best job at a background of 20 e-, where the faintest sources measured (furthest from the readout amplifier) are under-corrected by ∼17% in 2020. The best pre-observation CTE mitigation strategy still remains to minimize losses in the first place, by placing the target near the readout amplifier (when possible) and post-flashing to ensure adequate total background (sky+dark+post-flash). As of April 2021, all flc and drc products retrieved from MAST will use the v2.0 correction, but users have the option of calibrating UVIS data with the 1.0 correction following the instructions in a recently published Jupyter-Notebook.