Tammy Jones/Jim Sahli March 4, 1997
Goddard Space Flight Center
Greenbelt, Md.
(301) 286-5566/8955

HUBBLE SPACE TELESCOPE STATUS REPORT

During the approximately two weeks since the Hubble Space Telescope (HST) was deployed by the astronauts onboard Space Shuttle Discovery, ground controllers have methodically brought it back to normal operations as a spacecraft and have made good progress in checking out the new instruments and other spacecraft components. After a two-day period in which HST was controlled by direct real-time commands from the ground, it successfully resumed normal, unconstrained operations, commanded by its on-board computer and guided by its on-board pointing control system. The Servicing Mission Orbital Verification (SMOV) process, which will require a total of about 18 weeks to complete, is on schedule.

All of the new spacecraft hardware, inserted by the astronauts during EVA's, has been brought up to operational status. The new Reaction Wheel (RWA-1), Solar Array Drive Electronics (SADE-2), Data Interface Unit (DIU-2), Solid State Recorder (SSR) and Engineering & Science Tape Recorder (ESTR-2) are working properly and in fact have now been integrated into routine operations.

Two days ago, the new Space Telescope Imaging Spectrograph (STIS) obtained its first calibration images of isolated stars with its CCD detector. The Team reported good initial image quality, so that only minor "tweaking" of the instrument's optical focus and alignment settings will be necessary. The CCD detector itself is working well, with very low measured background noise levels. The STIS Team has been mapping the positions of the instrument's apertures relative to the telescope's Fine Guidance Sensors (FGS's) and relative to its own detectors, so that astronomical targets can be accurately positioned in STIS's field of view.

The new Near Infrared Camera And Multi-Object Spectrometer (NICMOS) has acquired observations of its internal calibration light sources which indicate good performance of its detectors. It's initial optical focus and alignment condition will be checked with observations of isolated calibration stars obtained later today. Engineering tests have been run to assess the simultaneous operation of NICMOS's three cameras, the gain settings of the detectors and the operation of the instrument's mechanisms, such as the filter wheel motors. These tests have demonstrated nominal operations of NICMOS.

The new Fine Guidance Sensor (FGS-1) has returned its first observations of guide stars and the initial optical data quality is good. This FGS is the first to incorporate an adjustable mirror to allow optimization of its optical alignment. It is clear from these first trial observations that the alignment process, scheduled during the next several weeks, should produce excellent FGS data quality.

The Wide Field and Planetary Camera (WFPC-2) has been used to acquire observations of "standard stars" to monitor the cleanliness of HST's and its own optics. These sensitivity monitoring observations show that HST's optical sensitivity was unaffected by the servicing process - the response to starlight was the same as when last measured in November, 1996, before the Second Servicing Mission.

The only significant technical issue that has arisen during the first two weeks of post-servicing operation is a higher than expected susceptibility of some of the electronic circuits in both STIS and NICMOS to upsets induced by trapped particle radiation in the South Atlantic Anomaly (SAA). The SAA is a broad region, extending roughly from the west coast of South America, across the Atlantic ocean to the southern tip of Africa, in which the Earth's magnetic field dips downward to relatively low altitudes. This results in a much higher than average number of charged particles (mostly electrons) encountered by satellites in low Earth orbit as they pass through the SAA. This trapped particle radiation typically induces high levels of background noise in light sensors and other electronic components.

Scientists and engineers routinely plan spacecraft operations to avoid any ill effects from the SAA. For example, on Hubble Space Telescope, scientists never acquire scientific data during SAA passage. The CCD detectors WFPC-2 are routinely read out and the data stored or transmitted to the ground prior to entering the SAA, so that the data will not be corrupted. Hubble's Fine Guidance Sensors routinely break off their lock on guide stars prior to entering the SAA, and are programmed to reacquire the guide stars after egress from the SAA. Hubble passes through the SAA during approximately half of its orbits and the duration of each passage is on average about 20 minutes out of each 96 minute orbit.

Shortly after scientists commenced operations with STIS and NICMOS, they found that occasionally each instrument experienced unplanned "resets" of parameters within their electronic circuitry during passage through the SAA. The instruments themselves recognized that something was amiss and autonomously placed themselves in a "suspense" state, waiting for ground controllers to intervene. Goddard personnel are currently assessing the causes of these events and will revise their operations protocols as necessary to avoid them. One likely response will be simply to turn off the susceptible circuits prior to entering the SAA and turn them back on after egress from the SAA. In so doing they will take special care to gracefully turn on and off the high voltages of the STIS Multi-Anode Microchannel Array detectors, to minimize stress on them. In the long term scientists said they do not expect this "idiosyncrasy" of the new instruments to have a significant impact on their science return. In the short term, there may be a minor reduction in scheduling efficiency, pending development and refinement of new operational procedures.