We demonstrate that 80-140 keV hard X-rays from the X-ray star Cygnus X-1 could be used, in principle, to image the interior of an unknown target spacecraft. A simulated radiograph shows good signal-to-noise in a 1000-second exposure with ~2 cm spatial resolution. Because of the high collimation and short wavelength of the radiation, an image can be formed at almost any target-detector distance. Practical application of the technique would require the detector spacecraft to assume a parallel trajectory with the target and maintain station accurately enough to hold the radiograph shadow on its sensitive surface. Further research is needed on 1) detector background minimization in high-latitude and high-altitude orbits; 2) image formation for rotating targets, which is a problem similar to computerized tomography; and 3) optimization of navigation and station-keeping.
The mining of Virtual Observatories (VOs) is becoming a powerful new method for discovery in astronomy. Here we report on the development of SkyDOT (Sky Database for Objects in the Time domain), a new Virtual Observatory, which is dedicated to the study of sky variability. The site will confederate a number of massive variability surveys and enable exploration of the time domain in astronomy. We discuss the architecture of the database and the functionality of the user interface. An important aspect of SkyDOT is that it is continuously updated in near real time so that users can access new observations in a timely manner. The site will also utilize high level machine learning tools that will allow
sophisticated mining of the archive. Another key feature is the real time data stream provided by RAPTOR (RAPid Telescopes for Optical Response), a new sky monitoring experiment under construction at Los Alamos National Laboratory (LANL).
The Rapid Telescopes for Optical Response (RAPTOR) experiment is a spatially distributed system of autonomous robotic telescopes that is designed to monitor the sky for optical transients. The core of the ystem is composed of two telescope arrays, separated by 38 kilometers, that stereoscopically view the same 1500 square-degree field with a wide-field imaging array and a central 4 square-degree field with a more sensitive narrow-field ``fovea" imager. Coupled to each telescope array is a real-time data analysis pipeline that is designed to identify interesting transients on timescales of seconds and, when a celestial transient is identified, to command the rapidly slewing robotic mounts to point the narrow-field ``fovea'' imagers at the transient. The two narrow-field telescopes then image the transient with higher spatial resolution and at a faster cadence to gather light curve information. Each ``fovea" camera also images the transient through a different filter to provide color information. This stereoscopic monitoring array is supplemented by a rapidly slewing telescope with a low resolution spectrograph for follow-up observations of transients and a sky patrol telescope that nightly monitors about 10,000 square-degrees for variations, with timescales of a day or longer, to a depth about 100 times fainter. In addition to searching for fast transients, we will use the data stream from RAPTOR as a real-time sentinel for recognizing important variations in known sources. All of the data will be publically released through a virtual observatory called SkyDOT (Sky Database for Objects in the Time Domain) that we are developing for studying variability of the optical sky. Altogether, the RAPTOR project aims to construct a new type of system for discovery in optical astronomy---one that explores the time domain by "mining the sky in real time".
We describe the design of Lobster-ISS, an X-ray imaging all-sky monitor (ASM) to be flown as an attached payload on the International Space Station. Lobster-ISS is the subject of an ESA Phase-A study which will begin in December 2001. With an instantaneous field of view 162 x 22.5 degrees, Lobster-ISS will map almost the complete sky every 90 minute ISS orbit, generating a confusion-limited catalogue of ~250,000 sources every 2 months. Lobster-ISS will use focusing microchannel plate optics and imaging gas proportional micro-well detectors; work is currently underway to improve the MCP optics and to develop proportional counter windows with enhanced transmission and negligible rates of gas leakage, thus improving instrument throughput and reducing mass. Lobster-ISS provides an order of magnitude improvement in the sensitivity of X-ray ASMs, and will, for the first time, provide continuous monitoring of the sky in the soft X-ray region (0.1-3.5 keV). Lobster-ISS provides long term monitoring of all classes of variable X-ray source, and an essential alert facility, with rapid detection of transient X-ray sources such as Gamma-Ray Burst afterglows being relayed to contemporary pointed X-ray observatories. The mission, with a nominal lifetime of 3 years, is scheduled for launch on the Shuttle c.2009.
The prospect of making a lobster-eye telescope is drawing closer with recent developments in the manufacture of microchannel-plate optics. This would lead to an x-ray all-sky monitor with vastly improved sensitivity and resolution over existing and other planned instruments. We consider a new approach, using deep etch x-ray lithography, to making a lobster-eye lens that offers certain advantages even over microchannel-plate technology.
The effort and expense required to build and maintain an optical-quality telescope increases dramatically with the size of the telescope aperture, and this is especially so in space. But scenarios have been proposed for deploying telescopes with very large but considerably less than optical-quality apertures. Our interest is in ameliorating the effects of the low quality aperture in order to exploit the raw size of the aperture to obtain high resolution images. We describe an algorithm for generating an adaptive binary mask to correct the time-varying aberrations of very large apertures which are many wavelengths out of figure. The technique is limited to monochromatic imagery, though the wavelength at which observations are taken can be easily changed on the fly, and for earth-pointing applications, the limited light-gathering power imposed by the monochromatic filter is not a problem. The mask itself can be placed at the exit pupil of the telescope, which permits implementation on a large scale. A similar approach, in which the pixels of the mask are half-wave phase shifters instead of opaque optical elements, was described by Love et al.
The optical/UV monitor (OM) on the ESA x-ray cornerstone mission XMM is designed to provide simultaneous optical and UV coverage of x-ray targets viewed by the observatory. The instrument consists of a 30 cm modified Ritchey-Chretien telescope. This feeds a compact photon counting detector operating in the blue part of the optical spectrum and the UV (1600 - 6000 angstrom). The OM has a square field of view of approximately 24 arcmin along the diagonal, and will cover the central region of the field of view of the EPIC x- ray cameras where the x-ray image quality is best. Because of the low sky background in space, the sensitivity of the OM for detecting stars will be comparable to that of a 4-m telescope at the Earth's surface; it should detect a B equals 24th magnitude star in a 1000 s observation using unfiltered light. The pixel size of the detector corresponds to 0.5 arc seconds on the sky in normal operation. In front of each of two redundant detectors are filter wheels containing broad band filters. The filter wheels also contain Grisms for low resolution spectroscopy of brighter sources (lambda/Delta lambda 200) and a 4x field expander which will allow high spatial resolution images of the field center to be taken in optical light.
Preliminary scoping exercises indicate that remote-sensing lidar can play a useful role in missions that involve determining regional weather patterns and atmospheric transport conditions. Both meteorological modeling and local atmospheric sensing should be employed. Satellite-based remote sensing systems, using an incoherent Doppler wind-sensor, seem feasible.
We propose a new technique for remote sensing: photon-counting laser mapping. Micro- channel plate detectors with crossed delay-line (MCP/CDL) readout combine high position accuracy and sub-nanosecond photon timing, at event rates of 106 detected photons per second and more. A mapping system would combine an MCP/CDL detector with a fast pulse, high repetition rate laser illuminator. The system would map solid targets with exceptional range and cross-range resolution. The resulting images would be intrinsically three- dimensional, without resorting to multiple viewing angles, so that objects of identical albedo could be discriminated. For a detector time resolution and pulse width of order 1010 seconds, the in-range resolution would be a few centimeters, allowing the discrimination of surfaces by their textures. Images could be taken at night, at illumination levels up to full moonlight, from ground, airborne, or space platforms. We discuss signal-to-noise as a function of laser flux and background level.
We set forth a conceptual design for an x-ray all-sky monitor based on lobster-eye wide-field telescopes. This instrument, suitable for a small satellite, would monitor the flux of objects as faint as 2 multiplied by 10-12 erg cm-2 s-1 (0.5 - 2.4 keV) on a daily basis with a signal-to-noise of 5. Sources would be located to 1 - 2 arc- minutes. Detailed simulations show that crosstalk from the cruciform lobster images would not significantly compromise performance. At this sensitivity limit, we could monitor not just x- ray binaries but fainter classes of x-ray sources. Hundreds of active galactic nuclei, coronal sources, and cataclysmic variables could be tracked on a daily basis. Large numbers of fast transients should be visible, including gamma-ray bursts and the soft x-ray breakout of nearby type II supernovae.
The MOnitoring X-ray Experiment (MOXE) is an X-ray all-sky monitor to be launched on the Russian Spectrum-X-Gamma satellite. It will monitor several hundred X-ray sources on a daily basis, and will be the first instrument to monitor most of the X-ray sky most of the time. MOXE will alert users of more sensitive instruments on Russia's giant high energy astrophysics observatory and of other instruments to transient activity. MOXE consists of an array of 6 X-ray pinhole cameras, sensitive from 2 to 25 keV, which views 4(pi) steradians (except for a 20 degree(s) X 80 degree(s) patch which includes the Sun). The pinhole apertures of 0.625 X 2.556 cm2 imply an angular resolution of 2 degree(s).4 X 9 degree(s).7 (FWHM on-axis). The flight instrument will mass approximately 118 kg and draw 38 Watts. For a non-focussing all-sky instrument that is limited by sky background, the limiting sensitivity is a function only of detector area. MOXE will, for a 24 hrs exposure, have a sensitivity of approximately 2 mCrab. MOXE distinguishes itself with respect to other all-sky monitors in its high duty cycle, thus having unprecedented sensitivity to transient phenomena with time scales between minutes and hours.
The Array of Low Energy X-ray Imaging Sensors (ALEXIS) satellite is Los Alamos' first attempt at building and flying a small, 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 VHF ionospheric experiment called BLACKBEARD. The spacecraft is controlled exclusively from a ground station located at Los Alamos. The 113-kg ALEXIS satellite was launched by a Pegasus booster into a 750 X 850 km, 70 degree inclination orbit on April 25, 1993. Due to damage sustained at the time of launch, ground controllers did not make contact with the satellite until late June. By late July, full satellite operations had been restored through the implementation of new procedures for attitude control. Science operations with the two onboard experiments began at that time. This paper will discuss our experience gained in launching and managing this small scientific and technology demonstration satellite.
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.
A multi-national consortium of research groups are developing the XMM (x-ray multi-mirror mission) optical monitor to provide a capability for optical identification and photometry of x-ray sources observed by the XMM observatory. This will be the first multi-wavelength facility dedicated to monitoring the variability of diverse sources from the optical through to x-ray wavelengths. Here we describe the system design and discuss progress in the breadboard phase of the development program.
A crossed delay line (CDL) readout for use in conjunction with a z-stack of 40 mm diameter microchannel plates (MCPs) has been designed using standard transmission line theory. The CDL readout was subsequently wound and tested in various configurations to determine if the electrical characteristics followed predicted trends. The agreement was good. The optimized CDL and MCPs were then designed into a sealed tube format which could be manufactured with either a visible or ultraviolet photocathode. The components were assembled into a vacuum system version and tested. The prototype detector displayed 15 micrometers resolution and a maximum deviation from linearity of 180 micrometers over the 40 mm format.
We report the launch and rescue of the ALEXIS small satellite. ALEXIS is a 113-kg satellite that carries an ultrasoft x-ray telescope array and a high-speed VHF receiver/digitizer (BLACKBEARD), supported by a miniature spacecraft bus. It was launched by a Pegasus booster on 1993 April 25, but a solar paddle was damaged during powered flight. Initial attempts to contact ALEXIS were unsuccessful. The satellite finally responded in June, and was soon brought under control. Because the magnetometer had failed, the rescue required the development of new attitude control techniques. The telemetry system has performed nominally. The BLACKBEARD experiment was turned on shortly after contact, and has returned its first data. We discuss preliminary lessons learned from ALEXIS.
We describe a sensitive technique for detecting small space debris that exploits a fast photon- counting imager. Microchannel plate detectors using crossed delay-line readout can achieve a resolution of 2048 X 2048 spatial pixels and a maximum count rate of about 106 photons per second. A baseline debris-tracking system might couple this detector to a 16-cm aperture telescope. The detector yields x, y, and time information for each detected photon. When visualized in (x,y,t) space, photons from a fast-moving orbital object appear on a straight line. They can be distinguished from diffuse background photons, randomly scattered in the space, and star photons, which fall on a line with sidereal velocity. By searching for this unique signature, we can detect and track small debris objects. At dawn and dusk, a spherical object of 1.3 cm diameter at 400 km will reflect sunlight for an apparent magnitude of V approximately equals 16. The baseline system would detect about 16 photons from this object as it crosses a 1 degree field of view in about 1 second. The line in (x,y,t) space will be significant in a diffuse background of approximately 106 photons. We discuss the data processing scheme and line detection algorithm. The advantages of this technique are that one can (1) detect cm-size debris objects with a small telescope, and (2) detect debris moving with any direction and velocity.
The Array of Low Energy X-ray Imaging Sensors (ALEXIS) experiment consists of a mini-satellite containing six wide angle EUV/ultrasoft X-ray telescopes. Its purpose is to mp out the sky in three narrow (5%) baridpasses around 66, 71, arid 93 eV. The mission will be launched on the Pegasus Air Launched Vehicle in 1992 into a 400 nautical mile, high inclination orbit. The project is a collaborative effort between Los Alamos National Laboratory, Sandia National Laboratory, and the University of California-Berkeley Space Sciences Laboratory. The six telescopes are arranged in three pairs in such a manner that as the satellite spins twice a minute they scan the entire antisolar hemisphere. Each f/i telescope consists of a spherical multilayer coated mirror with a spherical microchannel plate detector located at the prime focus and a thin aluminum or lexan/boron filter in front of the detector. The multilayer coatings determine the bandpasses of the telescopes. Each telescope has a field of view of 33 degrees. Unlike grazing incidence x-ray telescopes, the point spread function is uniform over the entire field of view with a FWHM of O.5degrees determined by spherical aberration. In this paper we present the status of the project as of July i992 as well as summary results from the pre-flight telescope calibration procedures.
We have investigated a slit aperture as an alternative to the square pinhole aperture for the MOXE detectors, which are to be put on the Soviet satellite Spectrum X-Gamma. A slit offers advantages for better discrimination of sources in crowded regions, eliminates the need for support structures for the aperture window, and does not compromise the signal-to-noise ratio (S/N) of a point source. We find that in a single 24 hr pointing of the satellite, MOXE can determine the position of a 10 mCrab source to better than 0.5 degrees with the slit. The structure of a titanium grate which supports the detector''s beryllium window constrains the slit to be 0.5 cm x 2.56 cm, oriented at an angle of 26.6 degrees to either side of the center lines of the detector. We illustrate an arrangement of the slits on each of the six detectors which optimizes source localization for a number of pointings.
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.