National Aeronautics and Space Administration

Significant past projects include the project BBXRT where pioneering work on X-ray foil optics was done. BBXRT was flown in 1990 on broad space shuttle Columbia . It first demonstrated the potential for foil optics and opened a new direction for astronomical X-ray imaging. This is followed by ASCA, formerly named Astro-D. ASCA was launched in 1993 from Japan and re-enter the atmosphere in 2001. The project, Suzaku , involved 5 telescopes with extensive nesting of foils. These telescopes used the same principle of X-ray foil optics working at grazing incidence, but with a different fabrication technique than the previous missions. The new technique of epoxy-replication of metallic surface gives a much smoother surfaces, less scattering, and hence a better angular response than the telescopes in the aforementioned previous missions.

BBXRT, a US Space Shuttle mission flown in December of 1990, was the pioneer X-ray astronomy experiment, using thin foil mirrors, to observe the X-ray sky with telescopes effective over a broad energy band at moderate angular resolution. BBXRT consists of two aligned segment that focus X-rays by reflection at grazing angles of incidence. The X-rays are focused on solid state detectors that are cooled to a temperature of about 100 K.

For BBXRT, nested thin foil mirrors were constructed from thin aluminum foil reflectors made of high luster commercial aluminum. Foil segments were first formed into conical reflectors which were then coated with acrylic lacquer for increased smoothness. This was then followed by approximately 500 Angstroms of vacuum deposited gold for enhanced reflectivity in the X-ray band. Two BBXRT telescopes, each of which was 40 cm in diameter and with 377 cm focal length, had an effective area of 450 cm² at 1 keV and 180 cm² at 7 keV. The mirror weight of the two telescopes was 40 kg.

The ASCA satellite was launched in February 1993. It was known before launch as Astro-D. It is the fourth Japanese X-ray mission and was developed in collaboration between the USA and Japan. The four ASCA X-Ray Telescopes were improved versions of the BBXRT design with particular emphasis on the high energy efficiency and ultra light weight. The combined effective area exceeds 1200 cm² and 600 cm² at 1.5 keV and 7 keV, respectively. The total weight (including thermal sheilds) of four mirrors was less than 40 kg.

ASCA XRT Design Parameters

Mirror Parameters	Design values
Mirror Substrate	127 um aluminum foil
Mirror surface	Acrylic lacquer 10 um
	with 500 A Au
Mirror length	100 mm
Mirror number	120 foils
Outer diameter	345 mm
Inner diameter	120 mm
Focal length	3500 mm
Incident angles	0.24-0.70 degree
Number of telescopes	4
Mirror Weight (4 telescopes)	39.340 kg
Geometrical area	2232 cm2
Field of View	24' (1 keV)
	16' (7 keV)

Astro-E is an X-ray imaging mission with a similar concepts as ASCA. As part of the Japan-US collaboration, the X-Ray Mirror Group at Goddard was responsible for the research and production of all its X-ray telescopes. There were 5 telescopes for the Astro-E project. Four of them had X-ray charged couple devices as detectors at a focal distance of 4.75m; while one had an X-ray micro-calorimeter positioned at a focal distance of 4.5m. Each of these telescopes has 168-176 nestings of cylindrical foils, produced in quadrants. Nearly 10,000 foil-reflectors were made for this project. The processes involved the shaping and heat-forminig of foils, coating and replication of surfaces, optical and X-ray testing of individual foils, and assembly and testing of the telescopes. All these were done in-house.

In a two stage design of an X-ray telescope , X-rays are reflected off two surfaces consecutively at grazing angles. Instead of the Wolter Type parabolic-hyperbolic surfaces, the Astro-E mirrors used a much simpler approximation of conical surfaces. This simplication made a straightforward replication process but limits the resulting resolution of the telescope. The use of thin foil, however, allows an much more extensive nesting of reflecting surfaces.

The five mirrors were delivered to ISAS, Japan, were successfully integragted. However, the launch in February, 2000, did not successfully put the instrument in orbit. The project Astro-E2, hence renamed Suzaku is a replacement of the original mission. The design parameters are nearly the same, but with the addition of, for each telescope, a pre-collomator stage, which was designed and made at Japan's JAXA/ISAS. It was successfully placed in orbit in 2005.