J/ApJS/280/29 Difference image analysis from DECam & Swope (Angulo+, 2025) ================================================================================ Optimizing convolution direction and template selection for difference image analysis. Angulo R., Rest A., Blair W.P., Jencson J., Coulter D.A., Wang Q., Foley R.J., Kilpatrick C.D., Li X., Piro A.L., Rojas-Bravo C. =2025ApJS..280...29A ================================================================================ ADC_Keywords: Photometry, ugriz; Supernovae Keywords: Astronomy image processing ; Astronomy data analysis ; Time domain astronomy Abstract: Difference image analysis (DIA) is a powerful tool for studying time-variable phenomena, and has been used by many time-domain surveys. Most DIA algorithms involve matching the spatially varying point-spread function shape between science and template images, and then convolving that shape in one image to match the other. The wrong choice of which image to convolve can introduce one of the largest sources of artifacts in the final difference image. We introduce a quantitative metric to determine the optimal convolution direction that depends not only on the sharpness of the images measured by their FWHM, but also on their exposure depths. With this metric, the optimal convolution direction can be determined a priori, depending only on the FWHM and depth of the images. This not only simplifies the process, but also makes it more robust and less prone to creating suboptimal difference images due to the wrong choice of the convolution direction. As an additional benefit, for a large set of images, we define a Figure-of-Merit based on this metric, which allows us to rank a list of images and determine the ones best suited to be used as templates, thus streamlining and automating the data reduction process. Description: We used images taken with DECam on the Blanco 4m telescope located at Cerro Tololo Inter-American Observatory (CTIO) in Chile. The field of view (FOV) of DECam is about 3deg2 with a pixel scale of 0.27". In order to probe fields with different stellar density, we selected extragalactic fields from the Young Supernova Experiment (YSE; Jones+ 2021ApJ...908..143J) DECam survey (fields 257A and 403C; PI: Rest, Program IDs: 2021A-0275 and 2023A-237157) and fields in the Galactic plane from the DECam Eta Car survey (fields ec0814 and ec0915; PI: Rest, Program IDs: 2021B-0325 and 2023A-643849). For this analysis, we use images taken in the griz filters. We also apply the same analysis for data from the Swope telescope, a 1m telescope located at Las Campanas Observatory in Chile, in Section 3.2. The FOV is 29.7x29.8arcmin2 with a pixel scale of 0.435". We chose two extragalactic fields with moderate stellar density from the Swope Supernova Survey (2022xkq and 2018jag; PI: Piro). The analysis of Swope data also uses images in the griz filters. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table3.dat 78 383 Blanco/DECam fields table4.dat 75 150 Swope fields -------------------------------------------------------------------------------- See also: J/ApJ/795/44 : PS1 SNe Ia (0.02