HRI

The ROSAT HRI is an enhanced version of the HRI flown on the Einstein satellite. It consists of two cascaded microchannel plates with a crossed grid readout system. The quantum efficiency of the HRI is maximised by use of a CsI coating on the front plate. The HRI field of view is the intersection of a circle and a square, having an effective radius of $\sim$ 18 arcminutes. Unlike the PSPC, it is able to make full use of the very small mirror PSF, and has an on-axis 50% power radius of 3 arcseconds. However, point images also show a large diameter, low surface brightness halo

The effective area of the HRI is less than that of the PSPC at all energies, particularly so at the lowest energies. Like the PSPC, it has a strong carbon edge at 0.28 keV.

The HRI has 16 pha channels, and is easily able to distinguish UV photons from X-rays. Some X-ray spectral resolution may also be possible, however this would be very limited. Spatial gain variations have not been corrected in the HRI datasets. A typical HRI image can be seen in Figure 2.