The Advanced X-ray Astrophysics Facility (AXAF) is a major NASA space observatory and is scheduled for launch in 1998. AXAF will perform high spatial and spectral resolution observations of celestial sources in the soft x-ray band 0.1 - 10 keV. The High Resolution Camera (HRC) is one of two focal plane instruments being developed for the AXAF. The HRC will be capable of observing point and extended sources with high sensitivity and high spatial resolution and will be used to record the high resolution spectra produced by an objective transmission grating. The HRC is based on microchannel plates (MCPs). We describe the design and development of the HRC, its expected performance, and some of its observational goals. The HRC consists of two separate detectors, HRC-I (Imaging) and HRC-S (Spectroscopy). HRC-I is used for imaging and has a field of view of 31arc min x 31arc min and a spatial resolution of < 25 um (equivalent to < 0.5 arc sec). HRC-S is optimized to readout the spectrum of AXAF's Low Energy Transmission Grating (LETG) and this combination will achieve resolving powers in excess of 1000 at low energies and cover a wavelength range of 4 to 140 Angstroms.
Keywords: X-rays, microchannel plates, x-ray observatory, x-ray astronomy, x-ray spectroscopy
1. INTRODUCTION
NASA will launch the Advanced X-ray Astrophysics Facility (AXAF) in late 1998. AXAF will perform high spatial and spectral resolution observations of celestial sources in the soft x-ray band 0.1 - 10 keV. The celestial objects that AXAF will observe include neutron stars, black hole candidates, normal stars, supernovae remnants, quasars, active galactic nuclei (AGN's), and hot gas in individual galaxies and clusters of galaxies. By these observations and others, AXAF will play a significant role in solving many of the problems of contemporary physics, astronomy, astrophysics, and cosmology. AXAF will represent a considerable advance over previous x-ray astronomy missions.
For background, we present a very brief overview of AXAF. Two
recent references (1,2) discuss the AXAF in more detail. AXAF is
comprised of three major components: the spacecraft, the x-ray
telescope, and the science instrument module containing the focal
plane
instruments. Fig. 1 is an exploded view of the AXAF. AXAF will be
launched into a highly elliptical orbit: a perigee altitude of
10,000
km, an apogee altitude of 140,000 km. The orbital period is 64.3
hours and the inclination is 28 degrees. The design life is five
years.
The high orbit means that the percentage of time above high
radiation regions is about 85% and uninterrupted observations of
almost as
long as 200 ks will be possible. The solar panels can be rotated
about one axis and this results in the ability to point towards any
region
of the sky (except for Sun, Earth, and Moon avoidance regions) at
any time of the year.
The spacecraft provides electric power, communications, command capability, data management, thermal control, propulsion, pointing control, aspect determination, radiation monitoring, and structures and mechanisms.
The X-ray telescope consists of the following major components: the High Resolution Mirror Assembly (HRMA) consisting of four pairs of grazing incidence mirrors, the optical bench assembly, and the Objective Transmission Gratings (OTG). The mirror assembly has a focal length of 10 m (plate scale: 49 micron per arc sec) , an outer diameter of 1.2 m, and a spatial resolution of < 1/2 arc sec (FWHM). The Objective Transmission Gratings (OTG) are mounted directly behind the HRMA and consist of the Low Energy Transmission Grating (LETG) and the High Energy Transmission Grating (HETG). Together with the HRMA and a focal plane instrument they comprise the Low Energy Transmission Grating Spectrometer (LETGS) and the High Energy Transmission Grating Spectrometer (HETGS) which will be capable of performing high resolution (resolving power > 1000) dispersive spectroscopy of point x-ray sources.
Two photon counting imaging detectors are mounted on a movable platform called the Science Instrument Module (SIM). On command the SIM's translation mechanism can alternately place one of the two detectors at the focus of the telescope. The SIM also has the capability of moving in the direction of the optical axis of the telescope so that on-orbit focussing can be performed. This is the first time such a capability has been provided for an x-ray observatory and it is present to compensate for any displacements along the optical axis due to the launch or to shrinkage of the optical bench with time. The two focal plane instruments are the AXAF CCD Imaging Spectrometer (ACIS), based on charge coupled devices (CCDs) and the High Resolution Camera (HRC), based on microchannel plates (MCPs).
The ACIS (3) is comprised of two arrays of CCDs, one optimized for imaging wide fields, the other optimized for grating spectroscopy and for imaging narrow fields. Each array is shaped to conform to the relevant focal surface. In concert with the HRMA, the ACIS imaging array will provide simultaneous time-resolved imaging and spectroscopy (resolving power = 50 @ 6 keV) in the energy range 0.5 - 10.0 keV. The ACIS spectroscopic array will acquire high resolution (resolving power = 1000 @ 1 keV) spectra of point sources when used with the HETG.
The HRC has the highest spatial resolution imaging capabilty on AXAF (a Gaussian PSF with a FWHM < 20 microns or < 0.4 arc sec). It is comprised of two detectors, HRC-I for imaging and HRC-S for spectroscopy (as an imaging readout for LETG). HRC-I has a large field of view (31 arc min x 31 arc min) and HRC-S has a large format (300 mm readout length). The HRC is capable of detecting photons with energies lower than 0.1 keV and as high as the mirror assembly cut-off of 10 keV. The HRC will be discussed in greater detail in the sections below.
The eight mirror elements for the HRMA have been successfully fabricated and are undergoing coating and assembly. The objective gratings and focal plane detectors have undergone critical design reviews and are now under construction. Major components of the instruments are now starting to be calibrated. Delivery of instruments for integration and end-to-end calibration at the Marshall Space Flight Center (MSFC) X-ray Calibration Facility (XRCF) are planned for the summer of 1996 and winter of 1997, respectively.
2. THE HRC
The HRC is a single photon counter that provides position, time of arrival, and energy (low resolution; resolving power = 1 @ 1 keV) of each detected x-ray photon. The HRC uses microchannel plates (MCPs) as it primary x-ray sensor and an electronic readout to provide signals to the spacecraft telemetry. It owes its heritage to the highly successful High Resolution Imagers (HRIs) that have flown on the Einstein (4,5) (for 2 1/2 years) and ROSAT (6,7) (for five years and still operating) Observatories. Several objectives of the scientific program to be carried out with the HRC are:
o The nature and origin of the cosmic X-ray background and the relative contributions of discrete sources to the cosmic x-ray background
o The nature and origin of nuclear activity in galaxies and quasars
o The structure and evolution of galaxy clusters
o The mass and nature of halos of galaxies
o The origin of stellar activity as manifested in x-ray emission from winds and coronae
Fig. 2 is a schematic of the HRC focal plane geometry. The sensor
of the imaging detector, HRC-I, consists of a two stage chevron of
100 mm x 100 mm Galileo Electro-Optics Corp. low noise
(radioisotope free (8)) lead oxide glass MCPs. The sensor of the
spectroscopy
readout detector, HRC-S, 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 arranged in a rectangle. The individual
segments were cut from 100 mm x 100 mm MCPs. UV/Ion shields
consisting of aluminized Lexan provided by Luxel Corporation
prevent low energy ions, low energy electrons, and EUV and UV
radiation from reaching the detectors.
Figs. 3 - 5 illustrate the operating principles of the HRC-I. The
front MCP is coated with a material (photocathode) of high
photoelectric yield (CsI). An incident x-ray photon interacts with
the photocathode producing 1 or more electrons in an MCP channel
(or pore). The initial electron or electrons are accelerated by an
applied electric field and generate secondary electrons; this
process
ultimately produces a cloud of about 10^7 electrons which emerges
from the rear of the second MCP. The electron cloud is collected
on
two parallel planes of resistively coupled wires. The charge cloud
centroid, which yields the location of the detected x-ray , the
cloud
arrival time, and the magnitude of the charge cloud can is
determined by measuring the signal on just three amplifiers (spaced
every 8
wires on 1.6 mm centers). The HRC-S electronic readout is similar
except that only one plane consists of wires; the other consists of
thin conductive strips deposited on a ceramic substrate (9). The
HRC-S electronic readout also lacks a reflector plane.
The detectors 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 spacecraft to warn of the presence of a high intensity charged particle flux.
Fig. 6 shows a schematic view of the HRC assembly. Both the HRC-I
and HRC-S are mounted within a single vacuum housing with a
protective door. This housing, along with the anti-coincidence high
energy charged particle shield, shutter assembly (for focus
determination), radioactive calibration sources, power supplies,
and processing electronics are mounted to the Science Instrument
Module (SIM). The total weight of the HRC is slightly under 250
lbs. and the power consumption is about 40 Watts.
The HRC's parameters are summarized in Table 1.
Table 1. HRC Parameters.
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 300 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 93 mm x 93 mm
HRC-S 3 x 20 mm x 93 mm
FOV: HRC-I 31 arcmin x 31 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%-30%
Time resolution HRC-I, HRC-S 16 microseconds
Max. count rate (TM) HRC-I, HRC-S 188 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 10^-15 erg/cm^2/s
(Point source, 300,000
second observation; 1.4
power law photon index)
2.1 HRC-I
The HRC-I's x-ray sensor is a two stage chevron of 100 mm x 100 mm
square Galileo Electro-Optics low noise (radioisotope free) lead
oxide glass MCPs. 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 length to diameter ratio is
120:1. The front plate will be coated with CsI to increase the
quantum efficiency over that of the bare glass. The low noise
glass, a major development by Galileo Electro-Optics (10), has
reduced the
internal background of the detector by about a factor of 10 below
conventional MCPs. The laboratory (11) measured background is 0.04
ct
cm-2 s-1. Fig. 7 illustrates the construction of the HRC-I MCP
assembly.
The electronic readout system is a crossed grid charge detector
(CGCD) briefly described above. 65 hybrid preamplifiers per axis
divide the image plane into 64 x 64 coarse position elements. The
fine position (readout to a digital resolution of 6.4 microns) is
determined from a "three tap" centroid calculation of the charge
collected on the grid wires (illustrated in Fig. 5). The
demonstrated
spatial resolution of the engineering model HRC-I is < 20 microns
FWHM (Fig. 8).
A filter consisting of 700 Angstroms of aluminum and 6000 Angstroms of Lexan (a polycarbonate) mounted approx. 1 cm in forward of the front MCP blocks scattered UV, FUV, and EUV from the Sun and low energy charged particles. The filter also blocks UV, FUV, and EUV from imaged hot stars. This filter is held at a positive potential with respect to the front MCP in order to prevent a "halo" (7) in the PSF of the detector. The transmission of this filter is shown in Fig. 9. Figure 9. Transmissions of the HRC-I and HRC-S UV/Ion shields as a function of energy (note: this represents a design change as of May 1996.)
Fig. 10 shows the predicted effective area as a function of
incident energy for the combined HRMA and HRC-I. This curve is
based
upon an estimate of the CsI quantum efficiency based upon previous
laboratory measurements. We have not yet coated the flight plates
so the definitive curve will have to wait for our calibration
measurements.
CsI coated MCPs show very modest energy resolution and this
capability will be exploited on AXAF. Fig. 11 shows the response of
a
similar MCP detector (the ROSAT HRI) to "soft" and "hard" celestial
x-ray sources (7).
2.2 HRC-S
The Objective Transmission Gratings (OTG) are mounted directly
behind AXAF's HRMA (Fig. 1) and consist of the Low Energy
Transmission Grating (LETG) and the High Energy Transmission
Grating (HETG). Together with the HRMA and the focal plane
detectors they will be capable of performing high resolution
dispersive spectroscopy of point x-ray sources. Fig. 12 and 13 show
the
operating principles of the OTG. The HRC-S is designed to readout
the spectra produced by the LETG.
The HRC-S' x-ray sensor consists of three sets of a two stage
chevron of 100 mm x 27 mm rectangular 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 63%. The channel
length to diameter ratio is 120:1. The front plates will be coated
with
CsI to increase the quantum efficiency over that of the 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 laboratory (11) measured
background is 0.04 ct cm-2 s-1. Fig. 14 illustrates the
construction of the HRC-S assembly.
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 hybrid readout permits the tilting (Fig. 15) of the outer two
segments of the HRC-S towards the OTG in order to approximately
match the curvature of the Rowland circle where best focus occurs.
The demonstrated (9) spatial resolution of the laboratory readout
combined with an MCP pair is < 20 micron.
Figs. 16 and 17 show the predicted resolving power and effective
area of the combined HRC-S/LETG/HRMA, respectively.
Kenter, et al. (12) discuss in greater detail the laboratory performance of the HRC MCPs: gain, gain uniformity, gain vs bias voltage, background, background uniiformity, image uniformity, and fixed pattern noise.
3. INSTRUMENT BACKGROUND
We have taken a number of successful measures to reduce the background in both HRC-I and HRC-S. The major sources of background for MCPs operating as x-ray detectors in space are
o internal background produced by the beta decay decay of naturally occurring radioisotopes in the MCP glass
o imaged galactic and extragalactic background x-rays
o scattered UV, FUV, and EUV from the Sun or bright Earth (both imaged and non-imaged)
o charged particle background
o non-imaged hard x-rays (diffuse background and point sources)
o galactic cosmic ray induced prompt gamma and x-rays
o galactic cosmic ray induced activation (spallation products)
We have reduced the MCP internal background by about a factor of ten by obtaining plates virtually free of naturally occurring radioisotopes. The UV, FUV, and EUV background have been reduced to negligible amounts by means of the UV/Ion shields and stray-light baffles. Background from non-imaged hard x-rays has been reduced by means of tantalum shielding around the detectors. The charged particle background has been reduced to a negligible amount by means of a five-sided anticoincidence shield.
The predicted on-orbit background for either the HRC-I or HRC-S is
given in Table 1. The background from all sources except for the
imaged galactic and extragalactic x-ray background (which may be
entirely resolved into point sources) is 2 x 10-6 ct arcsec-2 s-1
(1 ct
arcsec-2 week-1 !).
4. CALIBRATION
The HRC will undergo extensive characterization and calibration prior to integration (12). Subsystem level calibrations will include quantum efficiency, energy resolution, background, spatial resolution, geometric non-linearity, etc.. These calibrations will occur at discrete energies. Witness samples of coated MCPs and UV/Ion shields will be taken to synchrotron facilities13 in order to make measurements over a continuum of energies and with high spectral resolution.
After integration of the HRC, system level calibration with the AXAF mirror assembly and transmission gratings will take place at MSFC's X-ray Calibration Facility (XRCF). On-orbit calibrations will verify the ground calibration and indicate any temporal variations.
ACKNOWLEDGEMENTS
We thank Gary Meehan, Jack Gomes, and Gerry Austin for their technical assistance and advice in preparing this article. This work was supported by NASA Contract NAS8-38248. Previous reviews (14,15) of the HRC program have been helpful in preparing this article. The development of the HRC is being led at the Smithsonian Astrophysical Observatory by a team under the direction of Dr. S. Murray, the HRC Principal Investigator. The University of Leicester, U.K. and the Istituto e Osservatorio Astronomico G.S. Vaiana, Palermo, Italy are playing major roles in the development of HRC detector components, calibration, and mission planning.
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