Because the relative background was higher, diffuse emission detected with the IPC was often below threshold in HRI images. Thus only the brighter regions of 3C 58 and CTB 80 are visible in the HRI images. The HRI observ- ation of W 44 shows nothing when examined visually. A rate for W 44 was obtained by comparing the rate integrated over that part of the field covering the remnant with that from the part of the field outside the remnant. HRI rates given in table 2 have been corrected for vignetting, scattering in the telescope, and deadtime.
Uncertainties are both statistical and systematic.
Counting statistics are the dominant uncertainty for weaker sources and short observations. Possible errors in scattering correction and HRI sensitivity are dominant for the bright sources. The scattering correction is uncertain because it depends on the sometimes unknown energy spectrum. Such rates are footnoted in table 2. The dominant uncertainty in MPC counting rates is in background subtraction.