Development of a flat calibration unit for accurate flat fielding in the mid-infrared region

Abstract. Time-domain astronomy is important in the field of modern astronomy, and monitoring observations in the mid-infrared region with 1% photometric accuracy to study the variables and transients is becoming essential. The non-uniformity of the sensitivity caused by the optical characteristics of instruments and differences in the response curves of individual detector pixels degrade photometric accuracy. Therefore, to achieve 1% photometric accuracy, a flat-field correction for the non-uniformity with an accuracy of better than 1% is required. We developed a flat calibration unit (FCU) consisting of a silicon lens, a blackbody source, and two flat folding mirrors. We conducted proof-of-concept tests of the FCU by measuring the accuracy and stability of flat frames obtained using the FCU. The accuracies of the flat frames were 0.23% at 7.7  μm, 0.43% at 9.6  μm, 0.34% at 11.5  μm, and 0.84% at 20.9  μm, which are sufficient to achieve 1% photometric accuracy. The flat frames obtained using the FCU were stable over a period of 29 h within the accuracies of 0.13% at 7.7  μm, 0.12% at 9.6  μm, 0.22% at 11.5  μm, and 0.52% at 20.9  μm, indicating that it is sufficient to obtain flat frames once per night.