|ROSAT Home Page||ROSAT
This section describes the expected performance of the complete WFC telescope ( for the full aperture). Values for the point response function, vignetting and effective area were obtained directly from in-flight calibration and full aperture test at MPE's Panter X-ray beam line facility.
Figure 5.2 shows the on-axis effective area of the complete WFC for each filter design. The effective area decreases with increasing off-axis angle, dropping linearly to 67% of its on-axis value at 2.5 degrees off-axis. The vignetting function (Figure 5.3 ), which describes the relative efficiency as a function of off-axis angle, is dominated by geometric baffling. Rejection of scattered flux from bright sources outside the field of view is good. Less than 0.5% and 0.01% of the source flux is detected for sources 3 degrees off-axis and 5 degrees off-axis respectively. Note that some of the available WFC filters have a secondary transmission band in the far-ultraviolet. Although the far-UV transmission is much weaker than that in the primary XUV bands, problems may potentially arise when observing objects that have a particularly strong far-UV component in their spectrum. Details of the far-UV sensitivity, the ``UV leak'', are given in § 12.6 .
The point response function of the WFC (Figure 5.4 ) is dominated by the performance of the mirrors. Figuring errors determine the on-axis response but aberrations inherent in the optical design dominate at the edge of the field of view. As the energy of the incident photons increases the scattering wings becomes more pronounced. This is illustrated in Figure 5.5 . At 277 eV and 40.8 eV the on-axis half energy widths (HEW) are 2.9' and 1.7' respectively. However, at the edge of the field of view the HEW is 4.4' and independent of photon energy. The average HEW over the field of view is 3.5'.
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 (i.e., anode resistance and capacitance) but assymmetries 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 ``linearization'', is performed by means of a look-up table that gives the relationship between the detector coordinate system and that of the telescope. Figure 5.6 a shows the image from an early calibration observation displayed in raw detector coordinates. Figure 5.6 b shows the corresponding image after correction for the anode distortion, i.e. ``linearization'', whilst Figure 5.6 c shows the image in celestial coordinates after correction for the spacecraft attitude changes during the observation (the main attitude changes here are the intentional aspect ``wobble'' - see § 3.4 ). The ``linearization'' correction introduces positional errors which are negligible compared with other sources of uncertainty (see § 12.8 ).