XMM-Newton SAS Home Page
XMM-Newton Science Analysis System

omfastflat (omfastflat-1.22) [xmmsas_20211130_0941-20.0.0]


CCD Single Photon Counting Mode used at X-ray wavelengths is not applicable at optical wavelengths. At optical wavelengths, each detected event corresponds to a 'fractional' photon count, dependent on gain variations over the CCD and modulo-8 fixed pattern noise (due to the OM centroiding algorithm).

In order to produce photometrically calibrated light-curves the OM FAST mode event-list must be corrected for pixel-to-pixel gain variations and modulo-8 fixed pattern noise. This task applies these corrections using the OM in-orbit flatfield (as produced by OMFLATGEN) and the OM OSW modulo-8 map (produced by OMMODMAP).

This task then calculates a photon-count correction factor for each event in the input event-list, (based on the event RAWX,RAWY pixel co-ordinates), using the corresponding flatfield and modulo-8 image values. A new column containing this correction factor is added to the eventlist and is used when constructing photometrically accurate light-curves or images from the OM FAST mode event list. An image is also computed from the corrected eventlist, and a QUALITY array is attached. This image is suitable for OMMODMAP.

Note: Modulo-8 spatial fixed-pattern noise correction of the event list is currently not available and may require a new task. The flatfielded image produced is currently corrected using the existing Image Mode task OMMODMAP.

XMM-Newton SOC -- 2021-11-30