MASK is probably a slightly misleading term which is used by the PSF system for a sub-menu or sub-option.
ONAXIS_3 is a recommended MASK if the source is on axis.
The ONAXIS_3 mask returns a single on-axis PSF. It combines three functions which account for three of the factors which make up the composite on-axis PSF;
- primary electron generation is represented by a Gaussian function
- penetration of the counter gases which is an exponential function
- mirror scattering which is a Lorentzian function
At greater than 20 arc minutes off axis the off-axis blur becomes the
dominant factor in the PSF and causes a noticeable asymmetry in the
distribution of source counts. Because of the blur effect a PSF which
varies with off-angle is a widely preferred option.
VARPROFILE is the recommended MASK if the source is off axis.
The VARPROFILE mask creates a variable PSF which contains both off-axis angle and energy dependence.
The VARPROFILE mask uses a `data cube' - a file in which calibration data has been pre-processed and stored. The values of radius contained in US Rod Calibration Tables have been transformed into the `data cube' with ENERGY, OFF AXIS and RADIAL BIN components. The `data cube' applies to the whole ROSAT field of view and can be used to calculate an individual PSF for each data set. The comment `Loaded XRT PSF cube with 10 energy channels' indicates that the data cube has been loaded.
THE CUBE'S DIMENSIONS
DATA CUBE BINS BIN SIZE
ENERGY 1-10 0.24keV
OFF AXIS 1-30 2 arcmin
RADIAL BIN 1-40 Variable @0.018 arcmin
With the VARPROFILE mask the user will also be prompted for a value of AUX.
The AUX value provides the program with a value of mean photon energy in keV. For typical source spectra 0.5keV is a recommended value for AUX. For very hard or soft sources other values may be required - look at a spectrum of the source to decide.
If the VARPROFILE option is being used with a MODEL the smallest recommended grid size is 0.02 (where axis values are in degrees).