The ground calibration of Chandra included an intensive and extensive program for calibrating the full Observatory and its individual subsystems
The calibration results to date clearly demonstrate that Chandra instrumentation provides the capabilities - high-resolution (sub-arcsec) imaging, spectrometric imaging, and high-resolution dispersive spectroscopy - we desired. The goal is a highly accurate calibration (often misquoted to be at the one per-cent level - there were numerous parameters to be calibrated with required accuracies that range from parts per million e.g. for relative flux in the wings of the point spread function to many percent e.g. for the absolute efficiency of the HRC/HRMA combination). The calibration program to reach this goal is a tremendous effort which requires continued analysis and interpretation.
Calibration of the Observatory used, of course, the flight High-Resolution Mirror Assembly (HRMA) and flight objective transmission gratings (OTGs - the Low-Energy Transmission Grating (LETG) and the High-Energy Transmission Grating (HETG)).
Besides the flight focal-plane instruments - the High-Resolution Camera (HRC) and AXAF CCD Imaging Spectrometer (ACIS) - the calibration employed 4 non-flight detectors. The HRMA X-ray Detector Assembly (HXDA), designed and fabricated by SAO, comprises a Solid-State Detector (SSD) with aperture wheel, a Flow Proportional Counter (FPC) with aperture plate, and a microchannel-plate High-Speed Imager (HSI). In addition, the ACIS Two-Chip (ACIS-2C), custom built by MIT and SAO, served as an ACIS surrogate during rehearsal and calibration prior to arrival of the flight ACIS.
In addition to the joint system-level calibration of the Chandra Observatory at the XRCF, individual teams conducted extensive sub-system calibrations. The SAO Mission Support Team (MST) and the Physikalisch-Technische Bundesanstalt (PTB) is calibrating SSDs and FPCs from the HRMA X-ray Detector System, at the Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung (BESSY). Using the precisely calculable synchrotron radiation from a countable number of electrons of known energy, absolute calibrations are, in principle, possible.
Furthermore, the SAO Mission Support Team (MST), with the Los Alamos National Laboratory (LANL) and Lawrence Berkeley National Laboratory (LBNL), is using the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) and the Advanced Light Source (ALS) at LBNL to determine the optical constants of iridium coatings deposited on witness flats during qualification runs and coating of the Chandra optics. The objective is to combine these measurements of reflectance with the SAO-developed finite-element and ray-trace models to simulate the performance of the HRMA.
Each of the OTG teams has used its own laboratories, BESSY, and other facilities for sub-system calibrations. The LETG team calibrated the LETG grating period and period variations at the Max-Planck-Institut für extraterrestrische Physik (MPE) and the transmission efficiency of individual gratings at BESSY. The HETG team calibrated the HETG grating periods and period variations at MIT and the transmission efficiency of individual gratings at MIT, NSLS, and BESSY.
Likewise, each of the focal-plane-instrument teams has performed sub-system calibrations at various locations. The SAO HRC team and the University of Leicester (UK) characterized and calibrated microchannel plates MCPs in their laboratories and measured atomic-edge structure of flight-like CsI-coated MCPs at the Daresbury Synchrotron Radiation Source (UK). With the Istituto e Osservatorio Astronomico G. S. Vaiana (Italy), the HRC team also characterized the UV/ion shields. The ACIS team calibrated CCD absolute efficiency using atomic-fluorescence sources at MIT and synchrotron measurements at BESSY; they calibrated the energy scale and spectral resolution using a reflection- grating spectrometer at MIT. In addition, they characterized the optical blocking filters at PSU and, with SAO, at the NSLS.
Detailed results of the ground calibration are provided in the Calibration Reports, accessible through the Project Science calibration web page at:
http://wwwastro.msfc.nasa.gov/xray/xraycal/
Release 1 of the Calibration Report occurred in 1997 October; release 2 is nearing completion and most contributions are already available.
The calibration results confirm Chandra's unprecedented capabilities for high-resolution imaging and spectrometric imaging. The Observatory's on-axis full-width at half maximum (FWHM) and half-power diameter (HPD) are each less than an arcsecond. Consistent with, and based on, these results, the current performance prediction for the on-orbit encircled energy gives at least 50% of the flux included within a 1-arcsec circle for an on-axis source, over the entire Chandra energy band (0.08-10 keV).
Furthermore, the ACIS spectral resolution is excellent and fairly consistent with models for the CCD performance, at least for the front-illuminated CCDs. As anticipated, the 2 back-illuminated CCDs in the ACIS-S array exhibit poorer energy resolution and are not yet modeled as accurately; however, their energy resolution is more than adequate for separation of OTG spectral orders and well within specification.
The efforts to model the measured effective area of the HRMA (as opposed to HRMA and focal plane instrument where the tolerance was larger) have proved problematic. While efforts continue with some success to identify and remove systematic differences among the efficiencies of the detectors, the measured effective area remains about 10% below model predictions, for energies above 2 keV.
Currently, we believe that this discrepancy results, in part, because of different approaches used in modeling the effects of surface roughness. Basically, the synchrotron program to determine the optical constants of iridium-coated flats uses one technique, while the analysis of the Observatory calibration uses another. Specifically, the synchrotron program employs the Nevot-Croce approximation to calculate extinction of the specular radiation due to surface roughness, without regard to the angular distribution of the scattered radiation. In contrast, the HRMA ray-trace modeling explicitly calculates the angular distribution of the scattered radiation using scalar scattering theory. Although these two approaches are, in principle, mutually consistent, their application may not be totally consistent and complete.
In addition, other factors may affect the reflectance of the Chandra HRMA mirrors. For example, either gradients in the density of the iridium coating near its surface or the presence of molecular contamination on the coating's surface can play a role. In most reasonable cases, the former effect would decrease the reflectance, while the latter would increase it.
Modeling of the HETG and LETG efficiencies, sub-system measurements, and the Observatory calibration data now agree to within about 15%. While this is approximately the accuracy goal for the OTG calibrations, the HETG and LETG teams continue to refine models for the efficiency of individual grating facets - including, bar shape, as well as size and spacing - and then synthesizing the efficiency of an ensemble of hundreds of individual facets. Note that, because the efficiencies are obtained by comparing OTG-in to OTG-out measurements, the determined values are independent of the HRMA effective area and any uncertainties associated with it.
We are looking forward to the successful launch.
- Martin C. Weisskopf & Stephen L. O'Dell