HRC is the High Resolution Camera, a microchannel plate (MCP) detector. The HRC has the highest spatial resolution imaging on AXAF, matching the HRMA point spread function most closely. The HRC emphasizes lower energies where the mirror area is largest. It has a large field of view and is useful for imaging large objects (e.g., galaxies, supernova remnants, clusters of galaxies), or a large region of the sky. The HRC may have some limited energy resolution and good time resolution (which is valuable for the analysis of bursts, pulsars, etc.). The HRC has two sets of detectors: a 10 cm square detector optimized for imaging; and a 20 x 300 mm rectangular device optimized as a detector for grating spectra - especially for the LETG - for which its large format with many pixels gives high spectral resolution and wide spectral coverage. Further instrument characteristics are given below.
The HRC is a photon counting detector similar to the highly successful HEAO-2 (Einstein Observatory) High Resolution Imaging Detector (HRI) developed by SAO, which operated flawlessly for 2-1/2 years. An HRI was also built at SAO as one of the focal plane instruments of the ROSAT Observatory's X-ray telescope. The ROSAT Observatory is a joint mission of West Germany (DARA,MPE), the United Kingdom (SERC), and the United States (NASA) and was launched in early 1990. The ROSAT HRI is still operating flawlessly after five years.
The Instrument Principal Investigator is Dr. Stephen Murray of the Smithsonian Astrophysical Observatory.
The main HRC characteristics, summarized in the table below, include high spatial resolution and high time resolution over the entire field of view, low internal background, low sensitivity to cosmic ray induced background, high x-ray quantum efficiency from 0.1 to 10 keV, and modest energy resolution over this spectral band. The AXAF HRC will have substantially increased capability compared with the Einstein HRI in the areas of quantum efficiency, detector size, background rate, and intrinsic energy resolution.
These properties of the HRC, combined with the large area of the AXAF x-ray optics lead to an increase in point source sensitivity of greater than 50 compared to the Einstein HRI. The low background of the HRC also provides a very sensitive detector for studies of diffuse sources. Finally, the good response at low energies allows the HRC to be used as an efficient readout for the LETGS.
MCP bias angle HRC-I, HRC-S 6 degrees
MCP L/D HRC-I, HRC-S 120:1
MCP pore diameter HRC-I 10 microns
HRC-S 12.5 microns
MCP channel pitch HRC-I 12 microns
HRC-S 15 microns
MCP open area fraction HRC-I 68%
HRC-S 63%
Photocathode HRC-I, HRC-S CsI
Energy range HRC-I 0.1 - 10 keV
HRC-S 0.8 - 6 keV
Effective Area HRC-I/HRMA, @ 0.1 keV 10 cm^2
HRC-I/HRMA, @ 1 keV 225 cm^2
HRC-S/HRMA/LETG, @ 0.1 keV 8 cm^2
HRC-S/HRMA/LETG, @ 1 keV 30 cm^2
Focal plane geometry HRC-I 90 mm x 90 mm
HRC-S 3 x 19 mm x 100 mm
FOV HRC-I 30 arcmin x 30 arcmin
HRC-S 7 arcmin x 97 arcmin
Spectral range HRC-I 0 - 60 Angstroms
HRC-S 0 - 160 Angstroms
Spatial resolution (FWHM) HRC-I, HRC-S < 25 micron
< 0.5 arc sec
Plate scale HRC-I, HRC-S 20 arc sec/mm
Dispersion HRC-I/LETG, HRC-I/LETG 1.15 Angstrom/mm
Spectral resolution HRC-I (non-dispersive) 1 @ 1 keV
HRC-S 0.03 Angstrom
Quantum efficiency HRC-I, HRC-S @0.1 - 3.0 keV: 20%-50%
@3.0 - 8.0 keV: 10%-20%
Time resolution HRC-I, HRC-S 16 microseconds
Max. count rate (TM) HRC-I, HRC-S 184 ct s-1
Background (est.) HRC-I, HRC-S internal: 1 x 10^-6
imaged gal. X-rays: 1 x 10^-6
imaged extragal. X-rays: 3 x 10^-7
stray visible and UV: neglig.
out-of-band X-rays: 3 x 10^-7
pi 0 decay gammas: 5 x 10^-7
nuclear activation: 5 x 10^-7
TOTAL: 4 x 10^-6
(units are counts/arcsec^2/s)
Sensitivity HRC-I, HRC-S 2.5 x 10^-15 erg/cm^2/s
(5 sigma; point source;
300,000 second observation;
1.4 power law index;
NH = 3 x 10^20 cm-2)
HRC - Top View in Color
HRC-I - Color
HRC-I - B&W
Schematic view of HRC-I, the imaging detector. HRC-I's x-ray sensor is a two stage chevron of
100 mm x 100 mm Galileo Electro-Optics low noise (radioisotope free) lead oxide
glass MCPs (microchannel plates). The bias angle of both plates is 6 degrees.
The MCP channels are 10 microns in diameter on 12 micron centers, with an open area
fraction of 68%. The channel l/d (length to diameter ratio) is 120:1. The front
plare is coated with CsI to increase the quantum efficiency over that of bare glass.
The low noise glass, a major development by Galileo Electro-Optics, has
reduced the internal background of the detector by about a factor of 10 below
conventional MCPs. The measured background is about 0.04 ct cm-2 s-1.
The electronic readout system is a crossed grid charge detector (CGCD). 65 hybrid preamplifiers per axis divide the image plane into 64 x 64 coarse position elements. The "fine" position (digitized to 6.4 microns) is determined from a "three tap" centroid calculation of the charge collected on the grid wires (100 micron diameter on 200 micron centers). The demonstrated spatial resolution of the combined MCPs and readout is less than 20 microns (FWHM). This corresponds to less than 0.4 arc seconds at the focal plane of AXAF.
A shield (or blocking filter) mounted approximately 1 cm forward of the front MCP
blocks UV,FUV, and EUV and low energy charged particles. This UV/Ion shield is
held at a positive potential with respect to the front MCP in order to prevent
photoelectrons from the shield or from the MCP interchannel web from degrading
the image.
HRC-S - Color
HRC-I - B&W
Schematic view of HRC-S, the spectroscopy detector.
HRC-S has been designed to record the specta produced by the Low Energy
Transmission Grating (LETG). The HRC-S' x-ray sensor consists of three sets of
a two stage chevron of
100 mm x 27 mm Philips Photonics low noise (radioisotope free) lead oxide
glass MCPs. The bias angle of both plates is 6 degrees.
The MCP channels are 12.5 microns in diameter on 15 micron centers, with an
open area
fraction of 68%. The channel l/d (length to diameter ratio) is 120:1. The front
plate is coated with CsI to increase the quantum efficiency over that of bare
glass.
The low noise glass, a major development by Philips Photonics, has
reduced the internal background of the detector by about a factor of 10 below
conventional MCPs. The measured background is about 0.04 ct cm-2 s-1.
The electronic readout system is essentially the same as that used for HRC-I except that the CGCD is a hybrid consisting of one plane of wires wound in the cross-dispersion direction and the other plane consisting of photo-etched conductors in the dispersion direction. This readout design allows tilting of the outer two segments of the HRC-S towards the grating assembly in order to approximately match the Rowland circle, where best focusing of the spectra occurs. The demonstrated spatial resolution of the combined MCPs and readout is less than 20 microns (FWHM). This corresponds to a spectral resolution of less than 0.03 Angstrom.
Shields (or blocking filters)
block UV,FUV, and EUV and low energy charged particles. These UV/Ion shields are
held at a positive potential with respect to the front MCPs in order to prevent
photoelectrons from the shields or from the MCPs' interchannel web from degrading
the images of the spectral lines.
Anti-Co Shield
Schematic view of the anti-coincidence shield.
The detectors (HRC-I and HRC-S) are surrounded by a five-sided plastic scintillator
anticoincidence shield in order to reject high energy charged particle
induced events within the MCPs. This shield will also serve as a radiation
monitor for the AXAF spacecraft to warn of the presence of high intensity
charged particle fluxes that, for example, may occur as the result of a solar
flare.
HRC-I and HRC-S
Schematic view of HRC-I and HRC-S mounted to the vacuum housing bottom plate.
HRC-I, HRC-S, and anti-co shield
Schematic view of HRC-I and HRC-S mounted to the vacuum housing bottom plate. The anti-coincidence shield is also shown.
