Constellation-X Mission Update

The 2004 calendar year has been an eventful one for the Constellation-X team, highlighted by the formal assignment of management responsibility to GSFC for NASAÕs Beyond Einstein program with the signing of the FAD (Formulation Authorization Document). The flagship missions within Beyond Einstein are LISA and Constellation-X, with LISA first in the queue having recently entered into Phase A and Constellation-X (Con-X) scheduled to move into Phase A in FY 07. The schedule for the Beyond Einstein missions and for Con-X in particular is still being re-worked in response to budget changes necessitated in part by the introduction of the Exploration Initiative in NASAÕs FY 05 budget. A planned RFP (request for proposal) to industry to study the design and assembly methods of the large Con-X Spectroscopy X-ray Telescope (SXT) was therefore put on hold. The team has taken advantage of this slip to push harder on the mirror and detector technologies needed to meet the most stringent Con-X mission design goals.

Due in part to the re-direction of NASA priorities in the last year, a series of National Academy-level reviews (ie, the Decadal Report mid-course review), and NASA Road-mapping efforts have been begun. To date these reviews have validated the conclusions of the most recent Decadal survey, which gave Con-X very high priority (second only to JWST among large space-based projects).

On the optics front, the SXT team has made significant progress toward making reflectors that meet the Con-X angular resolution requirement of 15" final HEW (half energy width) for the full 4-telescope system. In order to meet this requirement, the contribution to the HEW from axial figure errors on each reflector pair must be less than 10". The best reflectors produced in this past year have an axial figure that approach this 10" HEW requirement. Metrology indicates that the X-ray scattering from the reflectors will fall within the allowed range, but X-ray reflectivity measurements have not yet been performed on the latest reflectors. Measurement of the X-ray reflectivity of individual reflectors, and possibly an imaging demonstration using an aligned pair of reflectors, will be performed by the end of this calendar year.

Figure 23: A prototype Con-X mirror segment being removed from a replication mandrel.

The baseline SXT reflector manufacturing plan is to replicate the final surface of each reflector using a thin layer of epoxy between the thermally formed glass substrate and a gold film deposited onto a precision replication mandrel. Significantly, this 10" axial figure was achieved on the glass reflector substrate alone, suggesting that it may be possible to simplify the manufacturing process, or alternatively, that continued efforts to improve the figure and scattering may yield reflectors whose axial figures approach the 5" HEW angular resolution goal.

The Hard X-ray Telescope team continues work on both the Nickel and Glass substrate designs for this telescope which will extend the energy range of Con-X to at least 40 keV. Both designs are expected to have <30" HEW, which meets the design requirements by better than a factor of 2. Con-X technology development in this area has been significantly helped by the involvement of many Con-X team members in a robust NASA high energy balloon program, which includes InFocus, HEFT, and HERO. The HERO balloon program is based at MSFC, where several dozen nickel shells have been fabricated (for the HERO payload) with diameters ranging from 4 to 10 cm.; full illumination X-ray tests on individual shells over an energy range of 20 - 60 keV showed a range of 11" - 13" resolution HPD (half power diameter). Larger diameter thinner nickel shells which meet the mass requirements of Con-X have been fabricated at Brera Observatory. Measurements of the first shell (130 micron thick 30 cm diameter) at MPE Panter facility with full illumination X-ray tests at 1.5 keV showed a 25" resolution HPD.

The Reflection Grating Spectrograph (RGS) team has made significant progress both in the lab and in the modeling of the RGS over this year. Two grating geometries have been extensively studied - the baseline "in-plane" grating design, and a new "off-plane" design. At the start of the year there were significant differences in the modeled performance of the candidate off-plane gratings. These have now been understood, and in some configurations the off-plane design may offer higher throughput and resolution. Several design studies of the full Con-X telescope system which were carried out this year used this newer off-plane grating configuration. An off-plane grating was tested at the Brookhaven synchrotron source, and absolute efficiencies and polarization sensitivities were measured. The grating efficiency was found to be dependent upon the polarization of the incoming beam, but the magnitude of this effect is highly dependent upon the detailed structure of the grating grooves.

On the detector front, the calorimeter team has also made significant progress. Both the Ge and Si based devices have seen improvements, with resolutions of less than 5 eV being reported. Single Si based devices using Transition Edge Sensors (TES) have reached as low at 2.5 eV at 6 keV, but the Con-X devices must be large arrays. Design and testing work on the SQUID multiplexers needed to read out an array of these devices is progressing.

On the science side, one of the most significant efforts of the Facility Science Team (FST) and the FST Science Panels over the past year has been an in-depth update of the basic Con-X science case. The original science case (detailed in glossy booklets, web pages such as http://constellation.gsfc.nasa.gov/, etc.) was formulated prior to the launch of Chandra and XMM. In support of this update a series of mini workshops were held in the late fall and winter of 2004 to discuss recent scientific advances from Chandra, XMM and other facilities within the context of the technical capabilities of Con-X. Panels were also asked for inputs to a scientific trade study of the mission parameters, and to indicate which were the most important. These mini-workshops and some work by various science panels at their home institutions, covered topics ranging from cosmology to planets. A series of white papers resulting from these meetings are nearing completion and should soon appear on the main Constellation-X science web page: http://constellation.gsfc.nasa.gov/science/.

Another significant effort has been a series of engineering design studies lead by the Con-X Project Office. These have been motivated in part by the new availability of the Delta 4-Heavy launch vehicle, which has significantly more lifting power to L2 than the baseline Atlas 5. Several different configurations have been studied, including one using a single telescope consisting of a pair of formation flying spacecraft (one holding the detectors, the other the mirrors) separated by up to 50m.

NASA re-direction away from International Space Station (ISS) has impacted the European/Japanese XEUS X-ray mission, which no longer plans to utilize the ISS for assembly and then operate nearby in order to allow servicing. Instead, XEUS will operate at L2, as Con-X will. While the primary XEUS science is imaging of very faint objects, and that of Con-X is high resolution spectroscopy of brighter objects, both are large area, high throughput missions. This has opened up the possibility of a collaboration between ESA, JAXA and NASA. Discussions and joint engineering studies are underway to see what common ground there is in the science objectives and what degree of sharing of technologies might be beneficial to both agencies.

In closing we would like to welcome Ann Hornschemeier to the Con-X team. Ann is our new acting Deputy Project Scientist, filling in for Kim Weaver who is on a 1-year detail to NASA-HQ.