The launch of the Far Ultraviolet Spectroscopic Explorer (FUSE) has been followed by an extensive period of calibration and characterization as part of the preparation for normal satellite operations. Major tasks carried out during this period include the initial coalignment, focusing, and characterization of the four instrument channels and a preliminary measurement of the resolution and throughput performance of the instrument. We describe the results from this test program and present preliminary estimates of the on-orbit performance of the FUSE satellite based on a combination of these data and prelaunch laboratory measurements.

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The Far Ultraviolet Spectroscopic Explorer satellite was launched in 1999 and began a three-year prime mission to collect high-resolution spectra in the far ultraviolet bandpass. Two and a half years after launch, mechanical failure of two out of four reaction wheels reduced the satellite to two-axis control, halting science observations. This failure prompted modification of the FUSE attitude control system software to restore three-axis control using a hybrid of existing magnetic and reaction wheel actuators. Pointing accuracy and stability are once again at the sub-arcsecond level, close to the pre-wheel failure performance. The range of stable attitudes is limited, but ground-based modeling software now directs the planning process so that observations and maneuvers stay within the limits of the actuators. Despite these constraints, efficient science operations are ongoing, and over the course of a year, the entire sky is available for observation.

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission is the first focusing X-ray telescope in the hard X-ray (3-79 keV) band. Among the phenomena that can be studied in this energy band, some require high time resolution and stability: rotation-powered and accreting millisecond pulsars, fast variability from black holes and neutron stars, X-ray bursts, and more. Moreover, a good alignment of the timestamps of X-ray photons to UTC is key for multi-instrument studies of fast astrophysical processes. In this paper, we describe the timing calibration of the NuSTAR mission. In particular, we present a method to correct the temperature-dependent frequency response of the on-board temperature-compensated crystal oscillator. Together with measurements of the spacecraft clock offsets obtained during downlinks passes, this allows a precise characterization of the behavior of the oscillator. The calibrated NuSTAR event timestamps for a typical observation are shown to be accurate to a precision of ∼65 μs.

THEMIS-a five-spacecraft constellation to study magnetospheric events leading to auroral outbursts-launched on February 17, 2007. All aspects of operations are conducted at the Mission Operations Center at the University of California at Berkeley. Activities of the multi-mission operations team include mission and science operations, flight dynamics and ground station operations. Communications with the constellation are primarily established via the Berkeley Ground Station, while NASA's Ground Network provides secondary pass coverage. In addition, NASA's Space Network supports maneuver operations near perigee. Following a successful launch campaign, the operations team performed on-orbit probe bus and instrument check-out and commissioning tasks, and placed the constellation initially into a coast phase orbit configuration to control orbit dispersion and conduct initial science operations during the summer of 2007. Mission orbit placement was completed in the fall of 2007, in time for the first winter observing season in the Earth's magnetospheric tail. Over the course of the first 18 months of on-orbit constellation operations, procedures for instrument configuration, science data acquisition and navigation were refined, and software systems were enhanced. Overall, the implemented ground systems at the Mission Operations Center proved to be very successful and completely adequate to support reliable and efficient constellation operations. A high degree of systems automation is employed to support lights-out operations during off-hours.

We present new optical identiÐcations of previously unidentiÐed faint extreme ultraviolet sources. Our total sample of 30 identiÐed sources, of which 22 are new identiÐcations, includes 24 late-type stars, three white dwarfs, two cataclysmic variables (CVs), and one active galactic nucleus. These sources are joint detections of the faint sources from the all-sky surveys of the Extreme Ultraviolet Explorer (EUV E) in the 58È174 (0.071È0.214 keV) EUV band and of the ROSAT Position Sensitive Proportional Counter A in the 5È120 (0.1È2.4 keV) X-ray band. We obtained medium-resolution spectra of the possible optical A counterparts with the Shane 3 m telescope at Lick Observatory using the Kast double spectrograph covering a bandpass of 3600È7500 Our sample of active late-type stars is dominated by K and M A. stars showing strong Balmer and Ca II emission lines. The white dwarfs are fairly typical for those detected in the EUV E survey with and log g ranging from 35 to 53 kK and 7.6 to 8.7, respectively. T eff We found strong H and He emission lines typical of cataclysmic variables (CVs) for EUVE J0854]390 and EUVE J1802]180. EUVE J0854]390 is a newly identiÐed cataclysmic variable showing radial velocity shifts to the red as large as B400 km s~1. We associate EUVE J1802]180 with the previously identiÐed CV, V884 Her (RX J1802.1]1804). Including the present work (22 new identiÐcations), EUV E optical identiÐcation campaigns have identiÐed B28% of the presently cataloged NOID sources.

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We present optical identifications of 30 previously unidentified extreme ultraviolet (EUV) and soft x-ray sources coinciding with the position of faint sources detected during the all-sky surveys of the Extreme Ultraviolet Explorer (EUVE) in the 58-174 Â (0.071-0.214 keV) band and of the ROSAT Position Sensitive Proportional Counter (PSPC) in the 5-120 Â (0.1-2.4 keV x-ray) band. We present low-resolution spectra of the possible optical counterparts of the x-ray and EUVE sources obtained with 1.5 m telescope at CTIO in 1995 May and December and 1996 February. The optical counterparts for 23 sources are identified as late-type (dKe and dMe) stars, and 4 are identified as new DA white dwarfs. All spectra of sources identified as late-type stars show the Balmer series and Ca H and K fines in emission, indicating the signature of chromospheric activity. For the white dwarfs, the joint detection by the ROSAT PSPC and the EUVE Lexan band indicates that they have a low-abundance of metals and that their EUV emission is mostly attenuated by the local interstellar medium (ISM).