On future astronomical instruments for the soft x-ray to FUV, stray light may be a significant cause of background events. Currently, we are engaged in an ongoing program to identify materials that are suitable for use as low- reflectance surfaces in space based instruments. As a result, we have measured the scattering performance in this spectral region, of wide a selection of low-reflectivity materials, produced with a range of processes. We present preliminary measurements of the absolute bidirectional reflectance distribution function (BRDF) for a selection of seven of these materials. Measurements were obtained at a five spectral lines, including strong geocoronal lines, over the wavelength range 44 to 1216 angstrom at near grazing incidence. We find that in most cases for constant incident and scatter angles, the total variation of BRDF with wavelength over this range is only a factor of order ten. We also find that although we have identified materials which in many instances have lower reflectances than bead blasted aluminum, it is still a good choice for most applications given its low cost and convenience.
The Array of Low Energy X-ray Imaging Sensors (ALEXIS) satellite is Los Alamos' first attempt at building and flying a low cost, rapid development, technology demonstration and scientific space mission. The ALEXIS satellite contains the two experiments: the ALEXIS telescope array, (which consists of six EUV/ultrasoft x- ray telescopes utilizing multilayer mirrors, each with a 33 degree field-of-view), and a VHF ionospheric experiment called Blackbeard. A ground station located at Los Alamos exclusively controls the spacecraft. The 248 pound ALEXIS satellite was launched by a Pegasus booster into a 400 x 450 nautical mile, 70 degree inclination orbit on April 25, 1993. Images from a video system on the rocket indicated that ALEXIS had been severely damaged during launch with one of the 4 solar panels breaking away from its mounting. (It later turned out that the solar paddle was still attached to the spacecraft but only through cable bundles.) Attempts at communicating with the satellite were unsuccessful until a surprised ground crew received a short transmission on June 2. By mid July, ground station operators had regained full control of the satellite and began to initiate scientific operations with both the telescope array and the VHF experiment. In this paper we will discuss a preliminary analysis of the on-orbit performance of EUV telescopes on ALEXIS.
Several spherically curved microchannel plate (MCP) stack configurations were studied as part of an ongoing astrophysical detector development program, and as part of the development of the ALEXIS satellite payload. MCP pairs with surface radii of curvature as small as 7 cm, and diameters up to 46 mm have been evaluated. The experiments show that the gain (greater than 1.5 x 10 exp 7) and background characteristics (about 0.5 events/sq cm per sec) of highly curved MCP stacks are in general equivalent to the performance achieved with flat MCP stacks of similar configuration. However, gain variations across the curved MCP's due to variations in the channel length to diameter ratio are observed. The overall pulse height distribution of a highly curved surface MCP stack (greater than 50 percent FWHM) is thus broader than its flat counterpart (less than 30 percent). Preconditioning of curved MCP stacks gives comparable results to flat MCP stacks, but it also decreases the overall gain variations. Flat fields of curved MCP stacks have the same general characteristics as flat MCP stacks.
The Array of Low Energy X-ray Imaging Sensors (ALEXIS) experiment consists of six wide angle EUV/ultrasoft Xray
telescopes utilizing normal incidence multilayer mirrors, flown on a miniature satellite to map out the sky in three narrow
bandpasses around 66, 7 1, and 95eV.The 66 and 7 1 eV bandpasses are centered on intense Fe emission lines which are
characteristic of million degree plasmas such as the one thought to produce the soft X-ray background. The 95eVbandpass
has a higher throughput and is more sensitive to continuum sources. The mission will be launched into orbit on the Pegasus
Air Launched Vehicle in mid-1991.
We will present the details of the ALEXIS telescope optical design, initial characterizations of the first flight mirrors
and detectors, and the current schemes for characterizing and calibrating the completed telescope assemblies. We will also
discuss the details of a novel "wavetrap" feature incorporated into the multilayer mirror structure to greatly reduce the mirror's
reflectivity at 304A, a major background contamination flux of He II emission from the geocorona.