A proposal has been made to develop improved thin foil, replicated conical mirrors for use in x-ray astrophysics. In the NASA astrophysical programs in which such mirrors are used, they are required to be lightweight, and to offer moderate angular resolution and high throughput to afford required sensitivity in the photon-energy band from 0.1 to 20 keV. In the proposed improvement program, emphasis would be placed on increasing resolution and diameter:
- Angular resolution would be increased from the current level (half-power diameter of 1 arc minute, achieved for the Astro-E mission) to a higher level (half-power diameter of 15 arc seconds, required for the Constellation-X mission).
- Mirrors with diameters as large as 1.6 m would be developed.
Increased angular resolution of replicated conical foil mirrors depends mostly on the geometric accuracy and microscopic roughness of the surface being replicated, and on the accuracy with which the foil reflectors are held in position in mirror structures. According to the proposal, precise conical mandrels with root-mean-square surface roughness less than 5 Å would be either fabricated in house or bought to demonstrate the ability to make precise reflectors. Polished quartz or Zerodur (or equivalent low-thermal-expansion glass) mandrels as well as less expensive diamond-turned nickel mandrels would be tested. The following three approaches would be investigated as means for increasing the accuracy of holding fixtures: (1) highly precise electrical-discharge machining, (2) diamond turning, and (3) application, to silicon fixtures, of microlithographic and etching techniques used in fabricating microelectronic circuitry.
Simultaneously with the efforts to increase angular resolution, there would be efforts to develop a prototype mirror with a diameter of 1 m. This effort would entail procuring and replicating reflectors from 1-m-diameter mandrels, and fabricating housings of the appropriate dimensions. The housing for the 1-m mirror would likely be designed similarly to housings used for the current narrower conical foil x-ray mirrors, except for modifications needed to attain the required angular resolution.
This work was done by Robert Petre, William Zhang, and Peter Serlemitsos of Goddard Space Flight Center and Yang Soong of USRA. For more information, contact the Commercial Technology Office at Goddard Space Flight Center at (301) 286-5810.