|WFC3 Data Handbook v.4|
10.3 For UVIS data, the calibrated flt/flc files can be used directly, just as for staring-mode observations.There are several existing papers in the literature that illustrate the use of UVIS scan-mode data for astrometric measurements (e.g., Riess et al 2014, Casertano et al. 2016). In these investigations, the authors have been able to achieve an accuracy of about 30 micro" (more than 10× that of pointed observations) in the measurements of trigonometric parallaxes. The interested reader is directed to such references for more details about the data analysis process.In the case of photometry of bright, isolated sources, spatial scans have two key advantages over staring mode observations. First, the ability to collect of millions of source photons per exposure without saturating is enabled by spreading the light over hundreds of pixels. Second, illuminating hundreds of pixels averages out spatially-dependent sources of noise such as flat-field errors and enables sampling of different pixel phases along the direction of the scan.HST program 14878 is a Cycle 24 calibration program (ongoing at the time of this publication) intended to study the photometric repeatability of spatial scans of bright, isolated stars with WFC3/UVIS. Analysis of the first two identical visits of this program finds the photometric repeatability to be approximately 0.1% r.m.s. ( WFC3 ISR 2017-21), an improvement of more than a factor of 5 over the traditional staring mode results.An outline of the analysis procedure performed on program 14878 is summarized below (the aforementioned report contains additional details).
• Images are sky subtracted. The sky region is defined as all pixels excluding a 10-pixel border around the perimeter of the subarray, and a conservatively large 400 × 75 pixel rectangular aperture around the source center. The pixel values in the sky region are sigma clipped (iteratively) and the mean of the remaining sky pixels is subtracted from the science array to remove the sky.
• A rectangular aperture is centered on the scan using Photutils.RectangularAperture is used. For higher precision, the overlap of the apertures with the science array is set to the ‘subpixel’ mode to allow for subpixel centering of the scans. Finally, photutils.aperture_photometry is used to sum up source counts.