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Positional Accuracy


The uncertainties intrinsic in reconstructing the true sky position of an event detected by the WFC come from two main sources. Firstly, the resistive anode image readout system of the MCP detectors has a small but significant intrinsic distortion. This is determined largely by the physical parameters of the system (ie. anode resistance and capacitance) but asymmetries are introduced by variations in the resistivity of the anode as a result of the manufacturing process. Consequently, the distortion needs to be removed during the data processing. This task, termed `linearisation', is performed by means of a look-up table that gives the relationship between the detector coordinate system and that of the telescope.  Positional errors introduced by this process are approximately 12'. In-orbit experience demonstrates that the linearisation parameters have remained stable over the operational life of the detector.

A second source of uncertainty is the reconstruction of the WFC aspect from the star tracker data, which is subject to an error of around 12'.  Combined with the linearisation errors the limiting positional accuracy of the WFC is around 30', however this limit is achievable only for bright sources close to the optical axis.  For other cases the positional uncertainty is dominated by the PSF and by source statistics.   



Component Typical range Averaged
Auroral electrons 0-1000 1
Cosmic rays 4-25 10
He II 304Å S1: <1 <1
S2: 0-40 10
Other airglow emissions <1 <1
Spacecraft glow 0-300 20
Diffuse XUV background tex2html_wrap_inline17965 tex2html_wrap_inline17967
MCP intrinsic noise 2 2
Table 6.4: Field of view averaged contributions from the various background components in the WFC. Typical ranges of the background contribution from each component are shown (in units of counts per second per field of view), along with a typical orbit-averaged value



Filter Counts/s/FOV tex2html_wrap_inline17969 Counts/s/arcmin tex2html_wrap_inline17971
S1 25 3.5
S2 40 5.6
P1 110 22
P2 320 45
Table 6.5: Typical background rates in the various WFC filters

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