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Arxiv: The James Webb Space Telescope Absolute Flux Calibration. II. Mid-Infrared Instrument Imaging and Coronagraphy Published: 9/16/2024 7:24:36 PM Updated: 9/16/2024 7:24:36 PM

Paper abstract: The absolute flux calibration of the Mid-Infrared Instrument Imaging andCoronagraphy is based on observations of multiple stars taken during the first2.5 years of JWST operations. The observations were designed to ensure that theflux calibration is valid for a range of flux densities, different subarrays,and different types of stars. The flux calibration was measured by combiningobserved aperture photometry corrected to infinite aperture with predictionsbased on previous observations and models of stellar atmospheres. A subset ofthese observations were combined with model point-spread-functions to measurethe corrections to infinite aperture. Variations in the calibration factor withtime, flux density, background level, type of star, subarray, integration time,rate, and well depth were investigated, and the only significant variationswere with time and subarray. Observations of the same star taken approximatelyevery month revealed a modest time-dependent response loss seen mainly at thelongest wavelengths. This loss is well characterized by a decaying exponentialwith a time constant of ~200 days. After correcting for the response loss, theband-dependent scatter around the corrected average (aka repeatability) wasfound to range from 0.1 to 1.2%. Signals in observations taken with differentsubarrays can be lower by up to 3.4% compared to FULL frame. After correctingfor the time and subarray dependencies, the scatter in the calibration factorsmeasured for individual stars ranges from 1 to 4% depending on the band. Theformal uncertainties on the flux calibration averaged for all observations are0.3 to 1.0%, with longer-wavelength bands generally having largeruncertainties.