PSPC

There are two (nominally identical) position sensitive proportional counters on ROSAT. PSPC C was in use up to Jan 25, 1991, after which PSPC B only was used.

The PSPC is a multiwire proportional counter which provides both spectral and spatial resolution. The counter gas is argon/methane/xenon, enclosed by a 1 $\mu$ polypropylene/carbon/lexan window supported by a grid with 72% transmission. A window support structure consists of 8 radial ribs meeting at a 20 arcminute diameter ring. The PSPC has a 2^o diameter. It has effective area between 0.1 and 2.2 keV, peaking at 330 cm² just below the carbon edge at 0.28 keV.

In general, both spectral and spatial resolution improve with increasing energy, but although the PSPC has 256 pha channels, it can make only $\sim$ 5 independent spectral measures. The spectral resolution varies as

FWHM/E = 0.43 (E/0.93)^-0.5

The PSPC detector used for observations was swapped (from detector `C' to detector `B') on 25th January 1991, and the detector gain changed on 11th October 1991, so different combinations of response matrix and effective area file must be used for observations taken between these dates.

The recommended combinations are:

before 25th January 1991 - pspcc_eff.sdf and drm_06.sdf
25th January 1991 to 11th October 1991 - pspcb_eff.sdf and drm_06.sdf
after 11th October 1991 - pspcb_eff.sdf and drm_36.sdf

For further information on using these files, see the Leicester Database and Archive Services (LEDAS) User Guide or the ASTERIX documentation.

http://ledas-www.star.le.ac.uk/info/user_guide/node24.html

The PSPC PSF encloses 50% of counts at 1 keV in a radius of 0.22 arcminutes. A model describing the the PSF as a function of energy and off-axis angle (within the 20 arcminute ring) is a good fit to the data. However, at large off-axis angles the PSF can no longer be treated as circularly symmetric, and at the lowest energies electronic 'ghosting' occurs, significantly broadening the PSF below 0.19 keV. Neither of these extreme cases is covered by the PSF model.

The PSPC was occasionally used with a boron filter in the X-ray beam. This filter has little transmission between the boron edge (0.188 keV) and the carbon edge (0.28 keV), and thus can provide extra spectral information in the energy range where the intrinsic spectral resolution is poorest. The boron filter only covered the central part of the PSPC field of view.

Since the end of 1994, the gas supply to the PSPC has been exhausted. The HRI is now permanently at the focus. A typical PSPC image can be seen in Figure 1.