by John McHale
GREENBELT, Md. — Engineers and scientists at NASA's Goddard Space Flight Center and the Space Telescope Science Institute in Baltimore recently completed upgrades of various electronics instruments on the Hubble Space Telescope, then after a three-week, in-orbit checkout declared the Hubble healthy and fit.
Astronauts performed the upgrades aboard the Space Shuttle Columbia earlier this year. Columbia journeyed to the Hubble for its fourth servicing mission in March. During a series of five spacewalks, astronauts installed new hardware and upgraded older systems during an 11-day mission, NASA officials say. The next Hubble servicing mission is set for February 2004, NASA officials say.
Major instruments that NASA experts upgraded were solar arrays, a power-control unit, a reaction wheel, the advanced camera for surveys, and a mechanical cooler, NASA officials say. Engineers custom-built the electronics at the NASA Goddard Space Flight Center in Greenbelt, Md., using off-the-shelf wiring and couplers, says Mindy Deyarum, a Hubble Space Telescope outreach specialist at Goddard.
Initial checkout of the spacecraft and instruments is finished, but calibrating the instruments will continue finish in two months, NASA officials say.
The two new sets of rigid solar arrays, coupled with the new power control unit, are working perfectly, generating 27 percent more electrical power than the old arrays, NASA officials say. This increase in power roughly doubles the power available to the scientific instruments. The new reaction wheel is operating normally, NASA officials say.
"The panels for the solar arrays were purchased from a commercial line of satellites — Iridium — and then assembled with custom made aluminum-lithium supports at Goddard," Deyarum says. "The former two sets of solar arrays were manufactured by the European Space Agency.
"The old power control unit has been on Hubble for the past 11 years, however, it was built in the early 1980s," Deyarum continues. "The new power-control unit is the original flight spare designed and built by Lockheed Martin Space Systems in Sunnyvale, Calif. The new unit was modified and retested to ensure optimal performance. This fresh unit will enable Hubble to remain healthy and productive throughout its lifetime. Without this change it is doubtful that the original 11-year old unit would have been able to sustain Hubble through its remaining years in orbit.
"Reaction wheels and their associated electronics are packaged as RWAs," Deyarum says. "The RWAs use rotational, or angular momentum, to move the telescope from one target to another — this motion is called 'slewing' — and to keep it pointed steadily once the target is acquired in the aperture of the observing scientific instrument."
The new advanced camera for surveys (ACS) is now undergoing its final optical alignment and focus checks, NASA officials say. The image quality of individual stars observed in a standard calibration field is excellent and the camera's light-sensing detectors are also working very well, NASA officials say. It is anticipated that the first early release observations of astronomical targets taken with the ACS will be available this month, NASA officials say.
The prime contractor on the new ACS is Ball Aerospace & Technologies, Corp. in Boulder, Colo., and the principal investigator is Dr. Holland Ford of Johns Hopkins University in Baltimore, Deyarum says. The new device "now occupies the location of the first-generation instrument — the Faint Object Camera (FOC)," she continues. "This camera, designed and built in the late 1970s to the mid 1980s, has been in flight since April of 1990. FOC was manufactured by the European Space Agency with the principal investigator being Duccio Macchetto.
"The primary purpose of this third-generation instrument is to increase the discovery efficiency of imaging with Hubble," Deyarum explains. "ACS will provide a combination of detector area and quantum efficiency surpassing that available from current instruments by a factor of 10. It consists of three independent channels with wide-field, high-resolution and ultraviolet imaging capability and an assortment of filters designed for a broad range of scientific goals.
"ACS will be able to survey a field with 2.3 times the area of the Wide Field and Planetary Camera 2 (WFPC2) currently on Hubble," she continues. "It will provide four times as much spatial information and up to five times the sensitivity of WFPC2. The ACS's wide field of view, high throughput mirrors with higher reflectivity and larger, more sensitive detectors dramatically improve the telescope's ability to deliver valuable science data.
"ACS is designed to provide Hubble with a deep, wide-field survey capability," Deyarum says. "The primary design goal of the ACS Wide Field Channel is to achieve a factor of 10 improvement in discovery efficiency compared to WFPC2. Discovery efficiency is defined as the product of imaging area and instrument throughput."
The new, high-tech mechanical cooler inserted by the Astronauts during the mission has been working continuously and properly since March 18, NASA officials say. The cooler's intended purpose is to attempt to resuscitate the dormant Near-Infrared Camera and Multi-Object Spectrometer (NICMOS), which depleted its expendable solid nitrogen coolant in January 1999. Although this new "refrigerator", dubbed the NICMOS Cooling System (NCS), has been reliably generating the amount of cooling power expected, Hubble engineers report that the NICMOS instrument is cooling down more slowly than originally expected, NASA officials say. Because it will take longer to reach the proper operating temperature, below approximately 80 degrees Kelvin, the initial checkout and scientific observations with NICMOS will be delayed for several weeks, NASA officials say.
The NICMOS Cryocooler was manufactured and tested at Goddard and engineers at Creare Inc. of Hanover, N.H., developed the turbo-alternator, Deyarum says
"This type of mechanical cooler allows longer operational lifetimes than current expendable cryogenic systems," Deyarum says. "The attempt to revive NICMOS with NCS is viewed as an experimental application of a promising new technology. There is no guarantee that NICMOS will return to full, normal science operation. However, the importance of the science enabled by NICMOS makes the 'experiment' well worth the effort."
NCS has three fluid loops — the circulator loop, the primary cooling loop, and the capillary Pumped Loop (CPL), Deyarum explains. "Gas circulates first in the circulator loop between the cooling system and the inside of the NICMOS cryo-stat, carrying heat away from the cryostat and keeping the detectors at their operating temperature of 73 degrees Kelvin.
"The primary cooling loop is the heart of the NICMOS cryocooler," she continues. "It contains a compressor, a turbo-alternator, and two heat exchangers. This loop implements a reverse-Brayton thermodynamic cycle, providing the cooling power for the entire system. Generating this cooling power also produces a significant amount of heat — as much as 500 watts. The CPL carries the heat away from the primary cooling loop. It connects the main heat-generating component, the compressor, with an external radiator that radiates the heat into space. Evaporating ammonia on the hot end of the CPL and recondensing it at the cold end remove the heat.
"The Electronics Support Module (ESM) controls the major functions of the NCS," Deyarum says. "It contains an 8051 microprocessor that implements control laws for cooler functions, including compressor, turbo-alternator, and circulator speed. It also controls the CPL reservoir temperatures, regulating the quantity of heat transported to the radiator. In the background, the ESM collects and monitors critical NCS telemetry and general housekeeping telemetry, and relays commands to the NCS subsystems."
Routine science observations have now resumed with the Space Telescope Imaging Spectrograph and the Wide Field and Planetary Camera 2, the two instruments that were operating on Hubble prior to Servicing Mission 3B, NASA officials say. Also, a gyro — Gyro 3 — that had not been performing as well as it should prior to the mission resumed perfect operation after it was turned off and re-started while Hubble was in Columbia's payload bay, NASA officials say.