Engineering topics
Publications and source records attributed to Weisskopf, Martin C..
All of us are anticipating the launch of the Chandra X-Ray Observatory (CXO), formally known as the Advanced X-Ray Astrophysics Facility (AXAF). I will describe the highlights of the test and scientific calibration program that has preceded the launch activities and give an overview of the scientific investigations that are possible with NASA's next Great Observatory.
The Chandra X-Ray Observatory was launched early in the morning of 1999, July 23 by the Space Shuttle Columbia. The Shuttle launch was only the first step in placing NASA's latest great observatory into orbit. After release from the cargo bay, the Inertial Upper Stage performed two firings and separated from the observatory as planned. Finally, after five firings of Chandra's own Integral Propulsion System - the last of which took place 15 days after the initial launch - the observatory was placed in its highly elliptical orbit of 140,000 km apogee and 10,000 km perigee. After Observatory activation, the first x-rays focussed by the telescope were observed on 1999, August 12. Beginning with this initial observation one could conclude that the telescope had survived the launch environment and was operating as expected. The month following the opening of the sunshade door was spent adjusting the focus for each set of instrument configurations, determining the optical axis, calibrating the star camera, establishing the relative response functions, determining the energy scale(s), and performing a series of "publicity" images. Each observation proved to be far more revealing than was expected. Preliminary results will be presented and the status of the instrumentation on the observatory will be discussed.
The Chandra X-ray Observatory observed the Crab Nebula and Pulsar using the Low-Energy Transmission Grating (LETG) with the High-Resolution Camera (HRC). Time-resolved zeroth-order images reveal that the pulsar emits x rays at all pulse phases. Analysis of the flux at minimum -- most likely nonthermal in origin -- places an upper limit (T(sub infinity) < 2.1 MK) on the surface temperature of the underlying neutron star. In addition, analysis of the pulse profile appears to confirm the absolute timing of the Observatory to within about 0.2 ms.
Soon, NASA's Advanced X-ray Astrophysics Facility (AXAF) will begin its exploration of the x-ray universe, providing unprecedent angular and spectral resolution. Here we summarize the results of the extensive calibration of the AXAF observatory, performed at MSFC's X-Ray Calibration Facility (XRCF), through the effort of several AXAF science and engineering teams.
We present simulated AXAF spectra of accreting white dwarfs, using parameters appropriate for magnetic cataclysmic variables. The very high spectral resolution that can be obtained with the High-Energy Transmission Grating of AXAF can resolve the keV X-ray emission lines that characterize the temperature, density and velocity profiles of the shock-heated emission regions of these systems. These simulations demonstrate that actual spectra will allow us to place constraints on the white-dwarf mass and the accretion rate of the systems. The high-resolution spectra also allow the measurement of the velocity of the accretion flow in regions close to the white-dwarf surface.
The Flux Contamination Monitor (FCM) on the Advanced X-ray Astrophysics Facility (AXAF) serves the purposes of transfering the absolute flux calibration from the ground calibration at the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center to operation on orbit and of detecting any changes in molecular contamination of the High Resolution Mirror Assembly (HRMA) between ground calibration and the post-launch activation phase. we describe the design, construction, and characterization of the FCM radioactive sources, and their placement on the Forward Contamination Cover (FCC). We present results from FCM measurements with the AXAF focal plane instruments, particularly the AXAF CCD Imaging Spectrometer (ACIS), during the ground calibration phase at XRCF in 1997. Finally, we describe the plans for FCM on-orbit measurements during observatory activation and the subsequent analysis.
Results of a battery of X-ray tests on the AXAF Verification Engineering Test Article (VETA) X-ray optics which were performed shortly after their fabrication are presented. The optics, a paraboloid, and a hyperboloid in the classic Wolter-1 configuration are the outer mirrors of what is now the AXAF-I mission. The optical elements are each about 1.2 m in diameter and 0.8 m long. The principal proof that the optics met their specification was to demonstrate that the angular resolution was at least 0.5 arcsec. The final result was an angular resolution smaller than 0.25 arcsec.
Studies of cosmic X-ray sources have suffered from lack of focussing optics in the hard X-ray spectral region (E above 10 keV); in the absence of imaging optics, celestial X-rays are masked by the cosmic ray background, which severely degrades the detector sensitivity. There are several possible ways to develop grazing incidence imaging optics for this spectral region; we describe here one approach which utilizes numerous large diameter silicon wafers to form a flat-plate imaging telescope. A prototype imager of this type has been constructed, and we present measurements of surface quality, coalignment accuracy, and imaging ability.
The scientific goals of X-ray astronomy are considered to evaluate the relative advantages of using classical Wolter-1 optics or using a different approach. The portion of the X-ray band over 10 keV is unexploited in the present X-ray optics technology, and focussing in this portion of the band is crucial because nonfocussed experiments are background limited. The basic design of 'hard' X-ray optics is described theoretically emphasizing the very small angles of incidence in the grazing-incidence optics. Optimization of the signal-to-noise ratio is found to occur at a finite angular resolution. In real applications, the effective area reduced by the efficiency of the two reflections is 80 percent at energies up to 40 keV, and the quality of the reflecting surface can be monitored to minimize scattering. Focussing optics are found to offer improvements in signal-to-noise as well as more effective scientific return because microelectronic focal-plane technology is employed.
A proportional counter is introduced in which the levels of energy and spatial resolutions and background rejection permit the application of the device to X-ray astronomy. A multistep approach is employed in which photons cause a signal that triggers the system and measures the energy of the incident photon. The multistep approach permits good energy resolution from parallel geometry and from the imaging stage due to coupling of the imaging and amplification stages. The design also employs fluorescence gating to reduce background, a method that is compatible with the multistep technique. Use of the proportional counter is reported for NASA's supernova campaign, and the pair background is below 0.0001 counts/sq cm sec keV at the xenon k-edge. Potential improvements and applications are listed including the CASES, POF, and EXOSS mission programs.
The optics and instruments of the Advanced X-ray Astrophysics Facility (AXAF) are described. The instrument capabilities are reviewed so that potential users of AXAF may plan supporting research in the years prior to launch. The AXAF is to be built around a large-area high-resolution grazing-incidence X-ray telescope, with a complement of imaging and spectroscopic instruments which can be maintained and/or replaced in orbit. An important feature of the AXAF is the aspect system. It utilizes solid state star cameras and fiducial lights to permit both image reconstruction (on the ground) with minimal blurring due to spacecraft and internal motions, and placement of the X-ray image on the sky to an accuracy of 1 arcsec.
This paper presents an overview of NASA's Advanced X-Ray Astrophysics Facility (AXAF), which is planned for launch in the 1990s and is expected to operate in space for at least 15 years. The design of the X-ray telescope is discussed and the instruments that may fly at the start of the AXAF mission, the principal investigator of each instrument, and the instrument performance parameters are listed. The AXAF instruments will include low-energy and high-energy transimission gratings, a Bragg crystal spectrometer, CCD imagers, an X-ray calorimeter, and a microchannel plate imager. The long lifetime of the AXAF will make it possible to follow up on the HEAO-2/Einstein results, the more recent X-ray observations performed by European and Japanese satellites, and the future ROSAT all-sky survey. The long lifetime of the AXAF, together with the servicing capability that will be offered by the Space Station, will also provide the opportunity of incorporating other instruments into the observatory.
NASA's Advanced X-ray Astrophysics Facility (AXAF), scheduled for launch in the early 1990s, will be an X-ray observatory built around a large-area, high-resolution grazing incidence X-ray telescope. The heart of the AXAF will be an X-ray telescope consisting of six nested Wolter type I paraboloid-hyperboloid pairs ranging in diameter from 0.6 to 1.2 m. The rsulting geometric collecting area will be 1700 sq cm and the focal length will be 10 m. Potential AXAF instruments are presented.