The Einstein Observatory HRI Source List

SPIE, 106, 196, 1977) and by Giacconi, et al (1979ApJ...230..540G). The following summary is meant only as a convenient reference for features of the HRI with which users should be familiar. The Instrument: --------------- The performance characteristics of the telescope-HRI combination included: spatial resolution (FWHM): 3 arcsec at the telescope focus field of view: 24 arcmin on a side temporal resolution: 7.8125 microsec background in orbit: 0.46 cts/sec/arcmin**2 (away from the South Atlantic Anomaly) quantum efficiency: ~10% at 1 keV maximum event rate: 100 counts/sec.

(telemetry limited) dead time correction: median of 1.02 for mission energy range: 0.15-3.0 keV (HRI-2) 0.15-3.5 keV (HRI-3) The mirror effective area decreased as a function of off-axis angle to 0.9 at 15 arcminutes. This "vignetting" must be taken into account when calculating count rates. The quantum efficiency of the microchannel plates varied by +/-5% across the field of view of the HRI, which must also be allowed for in determining source strengths.

The two-dimensional HRI used a pair of cascaded microchannel plates (MCPs) as an X-ray image converter and photoelectron multiplier, and a crossed-wire grid as a two-dimensional position-sensitive charge detector. Position determination was accomplished by electronic interpolation between the coarse grid wires. The detector provided the arrival time and position of each X-ray event which occurred within the field of view.

The MCP photocathode surface was shielded from ultraviolet light and ions by a thin, metallized self-supporting plastic filter. This UV/ion shield was the only component to differ among the three HRIs on-board the Einstein Observatory. HRI-2 showed a splotchy artifact early in the mission; from December 15, 1978, therefore, HRI-3 was used instead.

An Objective Grating Spectrometer (OGS) could be used in conjunction with the HRI. 53 sequences were observed with this configuration. These sequences have not been included in the current HRI source list. Point Response Function: ------------------------ An analytical approximation to the Point Response Function (PRF) for the HRI at 1.5 keV has been given by J.P. Henry: PSF(r) = 0.02885 x [ exp (-r/1.96") + 0.01 exp (-r/12.94") ] /arcsec**2 This was derived from long exposures of point sources on axis, and appears to be a reasonably accurate description within 5 arcminutes of the field center for sources such as Cyg X-2 and 3C273 (i.e., for sources with non-pathological spectra).

The formula is also a good fit to the 1.5 keV ground calibration data. It is valid for r < 1'.