APEX is a proposed mission for a Small Explorer (SMEX) satellite. APEX will investigate the density, temperature, composition, magnetic field, structure, and dynamics of hot astrophysical plasmas (log T = ~5-7), which emit the bulk of their radiation at EUV wavelengths and produce critical spectral diagnostics not found at other wavelengths. APEX addresses basic questions of stellar evolution and galactic structure through high-resolution spectroscopy of white dwarf stars, cataclysmic variables, the local interstellar medium, and stellar coronae. Thus APEX complements the Chandra, Newton-XMM, FUSE, and CHIPS missions. The instrument is a suite of 8 near-normal incidence spectrometers (~90-275 Angstroms, resolving power ~10,000, effective area 30-50 cm2) each of which employs a multilayer-coated ion-etched blazed diffraction grating and a microchannel plate detector of high quantum efficiency and high spatial resolution. The instrument is mounted on a 3-axis stabilized commercial spacecraft bus with a precision pointing system. The spacecraft is launched by a Taurus vehicle, and payload size and weight fit comfortably within limits for the 2210 fairing. Of order 100 targets will be observed over the baseline mission of 2 years. These are selected carefully to maximize scientific return, and all were detected in the EUVE and the ROSAT WFC surveys.
The Cosmic Origins Spectrograph (COS) is a new instrument for the Hubble Space Telescope that will be installed during servicing mission 4, currently scheduled for May, 2005. The primary science objectives of the mission are the study of the origins of large scale structure in the universe, the formation, and evolution of galaxies, the origin of stellar and planetary systems and the cold interstellar medium. As such, COS has been designed for the highest possible sensitivity on point sources, while maintaining moderate (λ/Δλ = 20,000) spectral resolution. COS has recently (summer 2003) completed an initial calibration. Performance is essentially as predicted. Detailed results from that calibration are presented in Wilkinson, et al, this volume.
We present a status report on CHIPS, the Cosmic Hot Interstellar Plasma Spectrometer. CHIPS is the first NASA University-Class Explorer (UNEX) project, and was launched on January 13, 2003.
The grazing incidence CHIPS spectrograph is surveying selected regions of the sky for diffuse emission in the comparatively unexplored wavelength band between 90 and 260 Å. These data are providing important new constraints on the temperature, ionization state, and emission measure of hot plasma in the "local bubble" of the interstellar medium.
The Cosmic Hot Interstellar Plasma Spectrometer satellite (CHIPSat) was launched on January 12, 2003 and is successfully accomplishing its mission. CHIPS is NASA’s first-ever University-Class Explorer (UNEX) project, and is performed through a grant to the University of California at Berkeley (UCB) Space Sciences Laboratory (SSL). As a small start-up aerospace company, SpaceDev was awarded responsibility for a low-cost spacecraft and mission design, build, integration and test, and mission operations. The company leveraged past small satellite mission experiences to help design a robust small spacecraft system architecture. In addition, they utilized common industry hardware and software standards to facilitate design implementation, integration, and test of the bus, including the use of TCP/IP protocols and the Internet for end-to-end satellite communications. The approach called for a single-string design except in critical areas, the use of COTS parts to incorporate the latest proven technologies in commercial electronics, and the establishment of a working system as quickly as possible in order to maximize test hours prior to launch. Furthermore, automated ground systems were combined with table-configured onboard software to allow for "hands-off" mission operations. During nominal operations, the CHIPSat spacecraft uses a 3-axis stabilized zero-momentum bias "Nominal" mode. The secondary mode is a "Safehold" mode where fixed "keep-alive" arrays maintain enough power to operate the essential spacecraft bus in any attitude and spin condition, and no a-priori attitude knowledge is required to recover. Due to the omnidirectional antenna design, communications are robust in “Safehold” mode, including the transmission of basic housekeeping data at a duty cycle that is adjusted based on available solar power. This design enables the entire mission to be spent in "Observation Mode" with timed pointing files mapping the sky as desired unless an anomalous event upsets the health of the bus such that the spacecraft system toggles back to "Safehold". In all conditions, spacecraft operations do not require any time-critical operator involvement. This paper will examine the results of the first six months of CHIPSat on-orbit operations and measure them against the expectations of the aforementioned design architecture. The end result will be a "lessons learned" account of a 3-axis sun-pointing small spacecraft design architecture that will be useful for future science missions.
We describe the design and development of the CHIPS microchannel plate detector. The Cosmic Hot Interstellar Plasma Spectrometer will study the diffuse radiation of the interstellar medium in the extreme ultraviolet band pass of 90Å to 260Å. Astronomical fluxes are expected to be low, so high efficiency in the band pass, good out-of-band rejection, low intrinsic background, and minimal image non-linearities are crucial detector properties. The detector utilizes three 75mm diameter microchannel plates (MCPs) in an abutted Z stack configuration. A NaBr photocathode material deposited on the MCP top surface enhances the quantum detection efficiency. The charge pulses from the MCPs are centroided in two dimensions by a crossed-delayline (XDL) anode. A four panel thin-film filter array is affixed above the MCPs to reduce sensitivity to airglow and scattered radiation, composed of aluminum, polyimide/boron, and zirconium filter panes. The detector is housed in a flight vacuum chamber to preserve the hygroscopic photocathode, the pressure sensitive thin-film filters, and to permit application of high voltage during ground test.
The Cosmic Hot interstellar Plasma Spectrometer (CHIPS), successfully launched on 2003 January 12, provides astronomers with an observatory dedicated to observation of the hot interstellar medium in the extreme ultraviolet. We describe here the otpical and photometric performance of the spectrograph based on calibrations of the individual components, end-to-end vacuum tests, and in-orbit observations of the Moon.
The CHIPS observatory was launched on 12 January 2003, and is the first UNEX (NASA Goddard Spaceflight Center University Explorer class) mission. It is currently on-orbit and performing diffuse spectroscopy in the 90-260Å wavelength band. The instrument is integrated with a custom 3-axis stabilized mini-satellite, designed for roughly one year of operation. The purpose of the observatory is examination of details of the local bubble thermal pressure, spatial distribution and ionization history. The spectrometer consists of six spectrograph channels which deliver >lambda/100 resolution spectra to a single detector. Cost constraints of UNEX led to a design based on a traditional aluminum structure, and an instrument with a large field of view (5° x 26°) for the dual purpose of increasing sensitivity in the photon-starved 90-260Å band, and to reduce requirements on spacecraft pointing. All optomechanical systems on the spectrometer, including coalignment, thermal, front cover and vacuum door release are performing well on orbit. We discuss design, test and operational performance of these systems, as well as launch loads and thermal system considerations.
The proposed SuperNova/Acceleration Probe (SNAP) mission will have a two-meter class telescope delivering diffraction-limited images to an instrumented 0.7 square degree field in the visible and near-infrared wavelength regime. The requirements for the instrument suite and the present configuration of the focal plane concept are presented. A two year R&D phase, largely supported by the Department of Energy, is just beginning. We describe the development activities that are taking place to advance our preparedness for mission proposal in the areas of detectors and electronics.
The SNAP (Supernova/Acceleration Probe) mission's primary science goal is the determination of the properties of the dark energy. Specifically, observations of distant Type Ia supernovae will be used to measure the dark energy equation of state constant parameter, w0, and time varying parameter, w1, to a fractional uncertainty of 0.05 and 0.3 respectively. This places stringent requirements on the control of systematics and on the absolute color calibration of these supernovae. The overall calibration for the SNAP CCD and NIR imagers and spectrograph will be conducted through several routes. We envision employing a variety of well-studied stars, certainly including the HST spectrophotometric standard stars (and possibly the Sun) and performing indirect transfer calibrations that permit comparison with NIST irradiance standards to close the loop with fundamental MKS quantities. We discuss the basic issues and possible strategies in order to achieve approximately 2 - 3% color errors over the wavelength range of from 350 to 1700 nm.
We present the preliminary calibration results for the Cosmic Origins Spectrograph, a fourth generation replacement instrument for the Hubble Space Telescope due to be installed in mid-2005. The Cosmic Origins Spectrograph consists of two spectroscopic channels: a far ultraviolet channel that observes wavelengths between 1150 and 2000 Åand a near ultraviolet channel that observes between 1700 and 3200 Å. Each channel supports moderate (R≈20,000) and low (R≈2000) spectral resolution. We discuss the calibration methodology, test configurations, and preliminary end-to-end calibration results. This includes spectral resolution, system efficiency, flat fields, and wavelength scales for each channel. We also present the measured transmission of the Bright Object Aperture (BOA) and the measured spatial resolution.
The Extreme Universe Space Observatory-EUSO-is devoted to the exploration from space of the highest energy processes present and accessible in the Universe. The results will extend the knowledge of the extremes of the physical world and address unresolved issued in a number of fields such as fundamental physics, cosmology and astrophysics. Several kind of detectors have been so far proposed for EUSO, all of them requiring some sort of ancillary optics to collect the light from the image produced by the main optics on the focal surface, for an efficient coupling to the detectors. Optical adapters must be selected taking in account several inputs: feasibility, cost, mass budget. Two main options are here investigated: imaging optics (by means of small lenses) and non imaging optics (by means of compound parabolic concentrators). The first kind of focal plane optics is easy and feasible, but it does not guarantee a high concentration ratio. Non imaging optics present much higher efficiency with a concentration close to the theoretical limit, but it also pose new technological diffculties and challenges. This work aims to clarify how this focal plane optics can be made, their limits in terms of concentration of radiation according to the laws of geometrical and physical optics and finally to identify the possible solution to this problem, including available technologies to be used for the construction.
Radiation induced phosphorescence of UV window materials has been identified as a source of background signal in UV detectors for as long as these detectors have flown in space, but there is little detailed knowledge of the spectrum, decay time constants or thermal dependence of the phosphorescence. We present initial results of a study undertaken to characterize this source of background signal, including spectra, decay timescale analysis, and preliminary assessment of depopulation/deexcitation techniques. The ultimate goals of this study are to identify and evaluate phosphorescence mitigation techniques and to identify the source of the phosphorescence in optical materials.
The nitride-III semiconductors, in particular GaN (band gap energy 3.5 eV), AlN (band gap 6.2 eV) and their alloys AlxGa1-xN are attractive as UV photo-convertors with applications as photocathodes for position sensitive detector systems. These can “fill the gap” in the 150-400nm wavelength regime between alkali halide photocathodes (<2000Å), and the various optical photocathodes (>4000Å, mutlialkali & GaAs). Currently CsTe photocathodes have fairly low efficiency (Fig. 1) in the 100nm to 300nm regime are sensitive to contamination and have no tolerance to gas exposure. We have prepared and measured a number of GaN photocathodes in opaque and semitransparent modes, achieving >50% quantum efficiency in opaque mode and ~35% in semitransparent mode (Fig. 2). The GaN photocathodes are stable over periods of >1 year and are robust enough to be re-activated many times. The cutoff wavelength is sharp, with a rapid decline in quantum efficiency at ~380-400nm. Application of GaN photocathodes in imaging devices should be feasible in the near future. Further performance improvements are also expected as GaN fabrication and processing techniques are refined.
Our work with GaN based photocathodes shows a strong dependence on the photo-emission response versus the carrier concentration and conductivity of the films. Films with quantum efficiency (QE) as high as 56% in opaque mode and as high as 30% in transmission mode have been made. Although surface activation plays a key
role, the characteristics of the films, e.g. the thickness, film structure, minority carrier diffusion length, and doping, all play a role in affecting the photo-emission QE and especially its spectral dependence. The QE of films with the various properties is discussed and the utility of using measurements of the film properties to predict the optimal performance of the resulting photocathode is demonstrated.
Significant advances in readout elements of microchannel plate based sensors have led to the development of detectors with less than 10 μm spatial resolution. We have shown that cross strip (XS) anodes have spatial resolution as small as 5 μm FWHM when a simple and fast center of gravity centroiding technique is used. In this paper we investigate the variation of XS anode spatial resolution for several types of centroiding algorithms (N-finger center of gravity, Gaussian, Lorentzian, parabolic and hyperbolic cosine centroiding) and determine the optimum algorithm in terms of spatial resolution
and image linearity. We found that for existing 32x32 mm2 cross strip anodes and associated electronics, the best resolution and linearity is achieved with center of gravity centroiding with properly chosen thresholds on the data set. The images of USAF resolution test target obtained with MCP's with 6 μm pores on 7.5 μm centers resolve 71.8 line pairs per mm (Group 6 element 2, with line width of only ~7 μm). Thus the ~7 μm spatial resolution of the detector with cross strip anode is indeed limited now by the size of the MCP pores, while the resolution of XS readout is on the order of
only few micrometers FWHM.
We describe the development of high quantum efficiency UV photocathodes for use in large area two dimensional microchannel plated based, detector arrays to enable new UV space astronomy missions. Future UV missions will require improvements in detector sensitivity, which has the most leverage for cost-effective improvements in overall telescope/instrument sensitivity. We use new materials such as p-doped GaN, AlGaN, ZnMgO, SiC and diamond. We have currently obtained QE values > 40% at 185 nm with Cesiated GaN, and hope to demonstrate higher values in the future. By using controlled internal fields and nano-structuring of the surfaces, we plan to provide field emission assistance for photoelectrons while maintaining their energy distinction from dark noise electrons. We will transfer these methods from GaN to ZnMgO a new family of wide band-gap materials more compatible with microchannel plates. We also are exploring technical parameters such as doping profiles, internal and external field strengths, angle of incidence, field emission assistance, surface preparation, etc.