The transformations from the OM instrumental system to the standard Johnson's system were established based on in-flight observations. Several fields have been observed from the ground with the standard Johnson UBV filters and with the XMM-Newton OM. The fitting was limited to stars with less than 20% coincidence loss, and 5% statistical errors. 363 cross-identified stars have been used to make the color transformation.
The Color-transformations for UV filters are based on the simulations because we have not got enough calibration observations for UV filters. A calibration field with many blue stars (B-V 0.4 ) will be observed, then we will update the UV color transformation based on in-flight observations.
We have derived the flux conversion factors from OM observations of seven spectrophotometric standard stars (white dwarfs: Hz4, GD50, GD153, LBB227, BPM16274, Hz2 and G93-48). For each filter, if you multiply the countrates (cts/s) from SAS by these conversion factors, you will get the flux (erg//s/A).
The AB system for OM has been derived from simulations based in the results obtained from the same white dwarfs.
These flux conversion factors have been added as keywords in the COLORMAG extension of the CCF as defined above.
A series of calibration observations have been requested to refine the conversion factors used in the count rate to flux calculations according to spectral type. These observations will provide data for a longer term strategy to introduce colour correction terms that can be applied to fluxes calculated from count rates received by two or more filters.