Spatially extended quasi-monoenergetic x-ray beams will allow improved performance for many applications in diagnostic radiology. X-rays with well-defined energies between 15 keV and 20 keV can be used to enhance the contrast of mammography images while reducing dose to the patient. Diagnostic radiology using iodine, xenon, or barium as a contrast agent can be made more sensitive by using nearly monoenergetic x-rays with energies just above and below their K edges near 33 keV, 34 keV, and 37 keV. We describe the design and preliminary performance measurements of a prototype thin film multilayer x-ray narrow-band filter or monochromator designed to produce fan-shaped beams of x-rays at 33 keV. A set of closely spaced thin foil substrates coated with graded Pt/C multilayers provides energy selectivity when illuminated by a diverging broad-band x-ray beam incident on the foils at near-grazing angles from 0.2 degrees to 0.3 degrees. The individual thin foil mirrors are mounted into top and bottom precision alignment structures formed by deep reactive ion etching 1-mm thick silicon wafers.
We describe recent progress toward producing a segmented mirror that meets the mass and angular resolution requirements for the Constellation-X Spectroscopy X-ray Telescope (SXT). While the segmented approach has its heritage in conical thin foil X-ray mirrors pioneered at GSFC, the Constellation-X implementation introduces innovations in nearly all components. The baseline configuration uses thermally formed glass for reflector substrates; thermally formed Be is being investigated as an option. Alignment is performed using etched Si microstructures that locate reflectors to submicron accuracy. The only aspect preserved from previous mirrors is epoxy replication of the X-ray reflecting surface. Thus far, all developments have been at the component level. Nonetheless, we have made substantial progress toward meeting the Constellation-X SXT angular resolution goal.
Segmented mirrors are one of the two approaches being investigated for both the Spectroscopy X-ray Telescope (SXT) and the Hard X-ray Telescope (HXT) on Constellation-X. Mirrors based on the grazing incidence foil optics pioneered by GSFC will meet the stringent Constellation-X SXT weight requirement, but the currently achieved resolution falls short of the 15 inch half-power diameter (HPD) required for Constellation-X. Significant contributions to the blur arise from the figure of individual reflectors and from inaccurate mounting. Only a small contribution to the HPD of the existing mirrors arises from the conical approximation. In this paper, we describe our program for improving the spatial resolution of segmented mirrors to meet the COnstellation-X requirement. Our effort incorporates accurately figured replication mandrels, mechanically more robust reflector substrates, high accuracy alignment, and ultimately a transition from conical to curved reflecting surfaces.
We describe plans for several science programs in crowded fields using the faint object spectrograph (FOS) which rely critically on the enhanced angular resolution provided by the COSTAR corrective optics. Based on ground-based calibration of the COSTAR and on-orbit performance of the FOS, the anticipated performance of the COSTAR+/FOS should allow many important scientific studies to be completed which have had to be postponed due to the spherical aberration in the HST primary mirror. Particularly impacted by spherical aberration, and thus able to benefit most dramatically from the installation of COSTAR, are spectroscopic observations with FOS in crowded fields. Many of the most important science goals for the FOS instrument involve observations of crowded fields, for which the restored high- angular resolution of HST is essential - not only to isolate features of small angular extent, but also to eliminate optical contamination from the surrounding luminous regions. Spectroscopy of the nuclei of galaxies to obtain rotation curves and velocity dispersions which might reveal the presence of central black holes benefits dramatically from the enhanced angular resolution of HST as restored by COSTAR. We present models based on our current understanding of the dynamics of galaxy nuclei to illustrate the dramatic improvement in sensitivity in searching for black holes made possible by restoring HST's image quality. Two other categories of scientific investigations in crowded fields which will benefit greatly from restoring HST's image quality are spectroscopy of the luminous material (presumably distant galaxies) surrounding quasistellar objects, and spectroscopy of individual stars in globular clusters. The promise of finally being able to carry out these exciting scientific programs with the FOS on the restored HST explains why our team of scientists, along with many other astronomers, look forward to using the restored HST to carry out the scientific investigations for which it was originally intended.
We describe recent orbital test data acquired with the COSTAR/Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST). These data exhibit possible internal thruput deficit and scattering features in the FOS. We then describe some data from laboratory measurements of COSTAR optical components carried out at Ball Aerospace Corporation and NASA/Goddard Space Flight Center, which we initially thought might bear on some of the orbital test data. We use next, a theoretical model to simulate some of the HST/COSTAR/FOS imaging and spectral performance. We apply the model to the current data to assist in evaluating COSTAR iamging performance, to attempt to isolate the origin of the thruput deficit and scattering features. The results of these studies will permit a better understanding of the limits of HST performance and permit development of optimal strategies for performing spatially resolved spectroscopy in the FOS scientific observing program.
Observations of a standard star (BD+75D325) have been used to measure the Hubble Space Telescope (HST)/Faint Object Spectrograph (FOS) scattering characteristics in the wavelength range 115 to 250 nm. Spectra of the standard star were obtained as the star was progressively offset from the optical line of sight axis of the telescope, both in the in- dispersion and cross-dispersion directions. These data have been reduced and analyzed to determine the scattering function of the telescope-spectrograph combination. Two primary results have been obtained. (1) The resulting scattering function exhibits three characteristics: (a) the inner core ((theta) < 4') is dominated by the large Point Spread Function (PSF) of the HST; (b) the outer wings of the scattering function (4' < (theta) < 32') show a (theta) -3 dependence consistent with predictions for the HST Airy disc; and (c) the wavelength dependence of this scattering function follows (lambda) -1, suggesting that the ultraviolet (UV) micro roughness contribution to the scatter is quite small, and hence the HST primary mirror is very smooth at ultraviolet wavelengths. (2) The FOS scattering contribution is limited only by grating scatter, and is consistent with pre-launch grating calibration measurements.
The on-orbit performance of the HST + FOS instrument is described and illustrated with examples of initial scientific results. The effects of the spherical aberration from the misfiguring of the HST primary mirror upon isolated point sources and in complex fields such as the nuclei of galaxies are analyzed. Possible means for eliminating the effects of spherical aberration are studied. Concepts include using image enhancement software to extract maximum spatial and spectral information from the existing data as well as several options to repair or compensate for the HST's optical performance. In particular, it may be possible to install corrective optics into the HST which will eliminate the spherical aberration for the FOS and some of the other instruments. The more promising ideas and calculations of the expected improvements in performance are briefly described.