Exposure Correction

The program xrtcorr performs exposure correction on binned datasets. The effects listed in the following table are corrected for:

Table 1: Correction factor dependencies

Correction	Energy	Time	Position
Vignetting	YES	NO	YES
PSF scattering	YES	NO	YES
Filter absorption	YES	NO	NO
Dead time	NO	YES	NO
Wires	NO	NO	NO

All these effects are applied directly to the data when exposure correcting a time series or an image, however, only the dead time correction is applied to a spectrum; in this case the energy dependent corrections and the effect of the wires are applied to the detector matrix. For a time series or an image, the energy dependent corrections, vignetting, PSF scattering and filter absorption, are applied assuming a mean energy. The default energy for this correction is 0.2 keV but it may be overridden on the command line, by :

        > xrtcorr energy=1.0

The vignetting and filter corrections are read from the effective area and detector matrix files held in the $XRTCAL directory. The PSF correction is calculated from an analytical function and is a strong function of off-axis angle as well as energy. The wire correction is a single value (1.27) and dead time is calculated from count rate values stored in the eventrate file and is usually $\sim 2\%$.

The vignetting, PSF and filter corrections are ENERGY dependent and NOT pulse height dependent. This means that they cannot be directly applied to the data in a spectrum but rather must be applied to the detector matrix used in spectral fitting. The data in a spectrum are simply multiplied by the dead time factor and divided by the exposure time during exposure correction. The wire factor is not applied because this is also contained within the detector matrix.

Images and spectral_images are corrected for dead time and vignetting. The vignetting correction applied to an image assumes a mean energy and so is not accurate. The correction applied to a spectral_image, uses the approximate energy to pulse height bin mapping of E=(Ch.No.) / 100 keV. This again is not accurate and under no circumstances should a spectral image be corrected with xrtcorr and then used for spectral work by projecting the file into a spectrum.