HRI Update: September 1993

The HRI was turned off and removed from the telescope focus at the end of AO3 (June 1993). The instrument will remain inactive either until the AO4 PSPC observations have been completed or until the PSPC gas supply is exhausted. Once the AO4 PSPC observations are concluded, the ROSAT mission will be devoted primarily to high-spatial-resolution X-ray astronomy with the HRI.

When the HRI is moved back into the telescope focus, additional count rate and gain measurements of the X-ray sources HZ43, N132D, and A2256 will be made to determine whether an increase in the high voltage setting will be needed before AO4 HRI observations commence. (These X-ray sources have been monitored throughout the mission to track the long term stability of the HRI's quantum efficiency.) In spite of a slight decrease in the gain of the HRI detector (David et al.) during the first three years, the count rates of these calibration sources have not varied measurably.

For purposes of scientific analysis and the preparation of future proposals, an updated version of the HRI calibration report "The ROSAT High Resolution Imager (HRI)" (David, Harnden, Kearns, and Zombeck,1992) can be obtained via anonymous ftp either from GSFC (ftp heasarc.gsfc.nasa.gov, 128.183.8.233) or from SAO (ftp sao-ftp.harvard.edu, 128.103.42.3). The same directory structure is in place at each site. The report can be accessed via the following path: rosat/doc/hri/hri_report. (For a description of the available files, use the command get README.)

The calibration report contains an updated on-axis HRI point-spread function, which is well parameterized by the sum of two gaussians and an exponential. In the IRAF/PROS analysis system, the task rosprf can be used to generate a simulated image of an on-axis point source, which then can be used with deconvolution algorithms. (The on-axis point spread function fits imaged point sources very well within five arcmin of the telescope axis.)

There are also a number of off-axis observations of point sources available in the IRAF/PROS directory xrcal$ for use in the analysis of observations farther off-axis. Smoothing these images slightly and then using them with deconvolution algorithms will produce better results than will assuming some analytic two-dimensional form for the off-axis point-spread function.

The report also contains a discussion of HRI background characteristics that are very important for analyses of low-surface-brightness extended sources, such as clusters or galaxies.