Solar Object Tracking For The Hubble Space Telescope

We consider multirate digital control systems which consist of an interconnection of a continuous-time nonlinear plant (described by ordinary differential equations) and a digital lifted controller (described by ordinary difference equations). The input to the digital controller consists of the multirate sampled output of the plant and the input to the continuous-time plant consists of the multirate hold output of the digital controller. We show that when quantizer nonlinearities are neglected, then under reasonable conditions (which exclude the critical cases), the stability properties (in the Lyapunov sense) of the trivial solution of the nonlinear multirate digital control system can be deduced from the stability properties of the trivial solution of its linearization. When quantizer nonlinearities are not neglected, corresponding results can be established for Lagrange stability rather than Lyapunov stability of an equilibrium.

Space telescopes for astrophysics and astronomy have provided some of the most significant contributions to science throughout the history of the international space program. The Hubble Space Telescope (HST) continues to amaze and astound the astronomical community with a seemingly endless stream of new discoveries and measurements while further captivating the public eye with its observations and imagery. The last HST servicing mission, which repaired existing instruments and installed two completely new instruments, has helped to ensure a continuation of the images and discoveries that have long captured the global public attention. In addition, the observations from HST are complemented by others, notably from the “warm” Spitzer Space Telescope mission, the Chandra X-Ray Observatory, the recently completed Herschel Space Observatory, and the Fermi Gamma-Ray Space Telescope (celebrating its fifth year on orbit). With these space-based observations complemented by observations from ground and airborne systems, the world is poised for continued discovery for many years to come. Though the budgetary environment remains challenging for all of these programs, in particular for new programs, significant future investment is in place for continued progress. As a major example, we are actively preparing for the results from the James Webb Space Telescope (JWST), on track for launch in 2018.

The NASA Hubble Space Telescope (HST) was designed to be deployed and later serviced for maintenance and upgrades, as required, by the space shuttle fleet, with a 5-year mission life for the batteries. HST was deployed 380 miles above the Earth, from Space Shuttle Discovery, on April 25, 1990. Four servicing missions, (SM1, SM2, SM3A, AND SM3B) have been performed. Astronauts have replaced or modified optics, solar arrays, a power control unit, and various science packages. A fifth servicing mission, SM4 scheduled for early 2004, is planned to replace the batteries for the first time. The HST is powered by solar array wings and nickel hydrogen (NiH/sub 2/) 22-cell batteries, which are grouped into two parallel battery modules of three parallel batteries each. With a design life of 7 years at launch, these batteries have surpassed 12 years in orbit, which gives HST the highest number of charge/discharge cycles of any NiH/sub 2/ battery currently in low earth orbit (LEO) application. Being in a LEO orbit, HST has a 36-minute umbra period, during which spacecraft power requirements normally force the batteries into discharge, and a 60-minute sun period, which is available for battery recharge. The intent of this paper is to address the issue of NiH/sub 2/ battery reliability and how battery capacity degradation can impact scheduling of a servicing mission to bring replacement batteries to HST, and extend mission life until deployment of Next Generation Space Telescope (NGST), planned for 2010 at the earliest.

When NASA Administrator Sean O’Keefe announced the cancellation of Servicing Mission 4 (SM4) of the Hubble Space Telescope (HST), scientists and politicians were outraged. A decision that could shorten the lifetime of this enormously successful observatory initially seems difficult to justify. However, a close examination of the history of the HST and its linkage to the Space Shuttle reveals that the decision was the right one for the future of both space-based astronomy and human spaceflight. By avoiding the pitfalls of connecting these two separate enterprises, NASA will be better able to implement the Vision for Space Exploration. Using Hubble as a case study, this paper demonstrates from a programmatic and a scientific perspective the weaknesses inherent in linking space-based astronomy to human spaceflight. Although the four servicing missions conducted so far have greatly augmented the scientific value of the HST, the telescope has encountered numerous setbacks because of its dependence on the Shuttle. Furthermore, with or without the SM4 upgrades made, Hubble will not survive long enough to serve as an adequate complement to the James Webb Space Telescope (JWST). A Next-generation Hubble Space Telescope (NHST) will be needed when the JWST’s planned five-year mission ends in 2016. The cancellation of SM4 could spur the development of such a telescope. Even if efforts to service Hubble robotically are not fully successful before the telescope goes dark, robotic servicing capabilities will have been developed for the NHST and other future space telescopes. The cancellation of Hubble Servicing Mission 4 signifies the end of an unnecessary linkage between human spaceflight and space-based astronomy. By removing this dependency as soon as possible, NASA will be able to reserve the costs and risks associated with human spaceflight for those activities for which humans are best suited – the exploration and development of the inner solar system.

Herein we report on a preliminary study to assess the use of the Hubble Space Telescope (HST) for the direct detection and spectroscopic characterization of exoplanets and debris disks - an application for which HST was not originally designed. Coronagraphic advances may enable the design of a science instrument that could achieve limiting contrasts approx.10deg beyond 275 milli-arcseconds (4 lambda/D at 800 nm) inner working angle, thereby enabling detection and characterization of several known jovian planets and imaging of debris disks. Advantages of using HST are that it already exists in orbit, it's primary mirror is thermally stable and it is the most characterized space telescope yet flown. However there is drift of the HST telescope, likely due to thermal effects crossing the terminator. The drift, however, is well characterized and consists of a larger deterministic components and a smaller stochastic component. It is the effect of this drift versus the sensing and control bandwidth of the instrument that would likely limit HST coronagraphic performance. Herein we discuss the science case, quantifY the limiting factors and assess the feasibility of using HST for exoplanet discovery using a hypothetical new instrument. Keywords: Hubble Space Telescope, coronagraphy, exoplanets, telescopes

Skylab's Apollo Telescope Mount (ATM) was this country's first full scale manned astronomical observatory in space. It was a predecessor to the currently proposed Spacelab experiment observational systems. The ATM experiment canister, which housed the eight principal solar telescopes, was three m long, over two m in diameter, and weighed approximately 11,200 kilograms. The system was designed to operate within a pointing accuracy of ± 2.5 arc-sec and within a stability accuracy of ± 2.5 arc-sec over a 15-minute observation period. Review of photographic data indicated that the Experiment Pointing Control System (EPCS), which controlled the ATM experiment canister, performed with a stability of better than 1.0 arc seconds, exceeding its design requirement by a factor of 2.5. The EPCS was a part of the Skylab Attitude and Pointing Control System (APCS) which utilized momentum exchange devices and cold gas thrusters for overall vehicle control. The EPCS operated in a nested configuration within the vehicle control system. It was designed to isolate the ATM experiments from vehicle disturbances in two axes, up/down and left/right. Roll positioning control was also available. A fine sun sensor (FSS) provided position information while rate information was obtained from canister mounted rate gyros. Torque motors on each of the dynamic axes responded to controller signals to position the experiment canister.

One of the many components of the software systems used to operate the Hubble Space Telescope (HST) are the applications packages for calibration and scientific data analysis. Although of great importance to the observer, data analysis is at the end of the line in this extensive system. Calibration is a more integral part of telescope operations, in that the STScI will provide all HST observers with nominally calibrated data. However, it is recognized that many observers will wish to verify or modify the calibration themselves, so that the calibration software is also an integral part of the data analysis system. STSDAS — the Space Telescope Science Data Analysis System — is the software tool kit that allows HST observers to calibrate, reduce, and analyze HST observations.

Rockwell Scientific Company is developing a new type of HgCdTe 1K 1K detector, called WFC3-1R, with cutoff wavelength at 1.7 m and 150K operating temperature. The detector will be installed on the Wide Field Camera 3, the fourth generation panchromatic instrument for the Hubble Space Telescope (HST) to be installed during HST Servicing Mission 4, currently scheduled for 2004. The detector uses HgCdTe MBE grown on a CdZnTe substrate and a new type of multiplexer, the Hawaii-1R MUX. Six lots of detectors have been produced so far, and have demonstrated the capability to meet or exceed the project requirements. In particular, detectors show quantum efficiency as high as ~90% at =1.4-1.6 m and greater than 50% at >1.0 m, readout noise of 30 e- rms with double correlated sampling, and dark current <0.2 e/s/pix at 150K. We illustrate the behavior of the reference pixels, showing that they allow the compensation of drifts in the dc output level. A number of detectors show a peculiar instability related to the variations of diode polarization, still under investigation. We also report on the environmental testing needed to qualify the WFC3- 1R detectors as suitable for flight on the HST. We finally provide an update of the project status.

Quantification of the Effects of Respiration and Parallax on Inferior Vena Caval Filter Position

The SOFIE pointing control system (PCS) locates and tracks the top edge of the sun and periodically scans the solar disk for calibration. Primary hardware components are a steering mirror assembly (SMA), sun sensor, vibration isolation system (VIS), and associated electronics. The SMA has a 100-Hz control bandwidth and is capable of ±1.6 mechanical degree deflection in azimuth and elevation axes. The sun sensor uses a 1024x1024 pixel, radiation-hardened focal plane array and coarse and fine tracking algorithms to report the solar centroid and edge positions to the PCS. The PCS control law uses this information to command the SMA. A change in launch loads necessitated the development of the VIS, which features passive viscoelastic damping to protect the SMA. A rapid prototyping methodology was used to develop the control laws for the inner SMA feedback loop and outer PCS feedback loop. The methodology features integrated end-to-end modeling of structural dynamics, controls, and optics; automatic C-code synthesis from block diagrams; real-time hardware-in-the-loop (HIL) testing; and the ability to change control parameters "on the fly." Extensive testing of the PCS shows stable pointing performance of about 2 arcsec in the presence of 60-arcsec disturbances, compared to the requirement of 15 arcsec.

We analyze CO and H2 absorption lines of the foreground molecular cloud in the Carina nebula. We use HST-STIS (Hubble Space Telescope - Space Telescope Imaging Spectrograph) &amp; IUE (International Ultraviolet Explorer) INES data to analyze the A-X (v=0→2) absorption band of CO for several hot stars toward the Carina nebula, while 9 stars of them have FUSE (Far Ultraviolet Spectroscopic Explorer) spectra to analyze the (v=0→4) vibrational band in the Lyman series of H2. The column densities of CO and H2 varies in the vicinity of N(CO) ~ 1013cm−2 and N(H2) ~ 1019cm−2, respectively. The resultant CO-to-H2 abundance ratio is about 10−6. We investigate the variation of the abundance ratio according to the relative position of the target stars to morphology the molecular cloud in the Carina nebula.

ABSTRACTWe present deep Hubble Space Telescope optical imaging observations of two Galactic globular clusters, 47 Tucanae and M15, using the Space Telescope Imaging Spectrograph (STIS) longpass (LP) filter and the Wide Field and Planetary Camera 2 (WFPC2) F606W and F814W filters. These globular clusters have very different metallicities ([Fe/H ]∼ - 0.7 dex for 47 Tuc, and [Fe/H ]∼ - 2.2 dex for M15), essentially spanning the metallicity distribution of the entire Milky Way globular cluster system, and were chosen to investigate the relationship between magnitudes in the nonstandard STIS LP system and the better characterized WFPC2 magnitudes. We determine this relationship by combining our new STIS data with archival WFPC2 data, taking care to provide a robust and reliable transformation. Examining the 47 Tuc and M15 data separately, there is no evidence for a metallicity dependence in the transformation relations. This allows us to combine the data for both clusters, and also include the stars in the Small Magellanic Cloud that lie within the field of view of the 47 Tuc images, to determine final transformations that are valid for main‐sequence stars with colors in the range 0.3≲V606− I814≲1.8. These final relations predict STIS magnitudes within 0.05 mag (1 σ) of those measured for stars in both clusters. In an , we discuss the differences between our empirical transformation relations and those predicted by synthetic photometry.

We present a nearly complete rapid neutron-capture process (r-process) chemical inventory of the metal-poor ([Fe/H] = −1.46 ± 0.10) r-process-enhanced ([Eu/Fe] = +1.32 ± 0.08) halo star HD 222925. This abundance set is the most complete for any object beyond the solar system, with a total of 63 metals detected and seven with upper limits. It comprises 42 elements from 31 ≤ Z ≤ 90, including elements rarely detected in r-process-enhanced stars, such as Ga, Ge, As, Se, Cd, In, Sn, Sb, Te, W, Re, Os, Ir, Pt, and Au. We derive these abundances from an analysis of 404 absorption lines in ultraviolet spectra collected using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and previously analyzed optical spectra. A series of appendices discusses the atomic data and quality of fits for these lines. The r-process elements from Ba to Pb, including all elements at the third r-process peak, exhibit remarkable agreement with the solar r-process residuals, with a standard deviation of the differences of only 0.08 dex (17%). In contrast, deviations among the lighter elements from Ga to Te span nearly 1.4 dex, and they show distinct trends from Ga to Se, Nb through Cd, and In through Te. The r-process contribution to Ga, Ge, and As is small, and Se is the lightest element whose production is dominated by the r-process. The lanthanide fraction, log X La = −1.39 ± 0.09, is typical for r-process-enhanced stars and higher than that of the kilonova from the GW170817 neutron-star merger event. We advocate adopting this pattern as an alternative to the solar r-process-element residuals when confronting future theoretical models of heavy-element nucleosynthesis with observations.

The coronagraphic instrument (CGI) currently proposed for the Wide-Field Infrared Survey Telescope–Astrophysics Focused Telescope Assets (WFIRST-AFTA) mission will be the first example of a space-based coronagraph optimized for extremely high contrasts that are required for the direct imaging of exoplanets reflecting the light of their host star. While the design of this instrument is still in progress, this early stage of development is a particularly beneficial time to consider the operation of such an instrument. We review current or planned operations on the Hubble Space Telescope and the James Webb Space Telescope with a focus on which operational aspects will have relevance to the planned WFIRST-AFTA CGI. We identify five key aspects of operations that will require attention: (1) detector health and evolution, (2) wavefront control, (3) observing strategies/postprocessing, (4) astrometric precision/target acquisition, and (5) polarimetry. We make suggestions on a path forward for each of these items.

The NASA Engineering and Safety Center (NESC) was established in 2003 (after the Columbia accident) to provide an independent technical resource for the resolution of challenging technical problems (through the use of studies, analysis, tests, etc.) for NASA programs and projects. Since its inception, NESC has completed nearly 1000 technical assessments for NASA’s Human Exploration and Operation Mission Directorate (HEOMD), Science Mission Directorate (SMD), Space Technology Mission Directorate (STMD), and Aeronautics Research Mission Directorate (ARMD). Of the SMD related assessments, several were for the resolution of technical problems, analysis, or studies related to NASA’s astrophysics missions in various phases of the project from design to operation. Some of the recent examples of NESC technical support for NASA astrophysics missions have been for: Hubble Space Telescope (HST), Chandra X-ray Observatory (CXO), Fermi Gamma-ray Space Telescope, Kepler Space Telescope, Transiting Exoplanet Survey Satellite (TESS), James Webb Space Telescope (JWST), and Laser Interferometer Space Antenna (LISA). In this paper, we outline some of the technical challenges faced by these astrophysics missions and describe how NESC contributed to their resolution. The case studies cover a wide range of disciplines involving space telescopes, detectors, lasers, and attitude control systems. These efforts include innovative solutions for extending the life of the missions, technical resolution of challenging problems, strategies for risk mitigation, and failure investigations combined with lessons learned reports to advance discipline knowledge, enhance NASA capabilities, and avoid future problems.