May 8, 2009
Scientists and engineers now creating new technologies for NASA's James Webb Space Telescope (JWST) have realized that these advancements can be used to enhance the Hubble's Advanced Camera for Surveys (ACS) in the upcoming servicing mission.
One piece of new technology astronauts will be installing in Hubble during Servicing Mission 4 (SM4) — scheduled to launch May 11, 2009 — is a small, specialized integrated circuit called an Application-Specific Integrated Circuit (ASIC). This device enables an entire circuit board's worth of electronics to be condensed into a small package. It will be a part of the new Charge-Coupled Device (CCD) Electronics Box (CEB) that astronauts will install to repair the failed ACS instrument. The ASIC is used to read the voltages generated by instrument detectors and to convert them to digital signals that are processed by the spacecraft's electronics for transmission to the ground.
The ASIC design is the same as the one already developed and tested for the JWST; however, the electronics packaging for Hubble is different because of the different operating conditions such as temperature and electronics environments.
How different are the thermal environments? Hubble is in low Earth orbit, approximately 350 miles (560 kilometers) above Earth's surface, while the JWST will have an orbit one million miles from Earth. In addition, the JWST uses infrared- based cameras that will be adversely affected by heat given off by circuitry. ASIC consumes very low power, which helps keep the JWST's science instruments cold. Hubble's ACS is sensitive to electromagnetic radiation at visible and ultraviolet wavelengths, but not infrared radiation. Its CCD detectors must also be cooled to minimize the background thermal noise produced by the warmth of the detector assemblies themselves; however, the CCD's are not nearly as sensitive to heat as the JWST infrared sensors.
The ASIC's technology has already been ground-tested, as four of the tiny circuits were installed in early 2007 in the University of Hawaii's 7.2-foot (2.2 meters) telescope on Mauna Kea. Since then, they have been collecting science data and giving scientists experience in operating ASICs. Now, they're ready for their first trip into space on Hubble.
SM4 will be the last trip astronauts make to repair and upgrade the Hubble Telescope. Over the course of five spacewalks, astronauts will install two new instruments, repair two inactive ones, and perform the component replacements that will keep the telescope functioning at least into 2014.
The installation of WFC3 and Cosmic Origins Spectrograph instruments on Hubble is one of the SM4 mission's priorities. The WFC3 will study dark energy, the populations of stars in other galaxies beyond the Milky Way, and remote galaxies previously beyond Hubble's vision. The capabilities of the ACS instrument are complementary and, to some extent, unique from WFC3. Having both instruments function together would give Hubble an extremely powerful imaging capability.
Smaller, new technology is important and useful to the repair of the ACS because space is limited. In order to repair ACS in space, astronauts must first remove four printed-circuit electronics cards that make up the original CEB. The new CEB circuit boards have to be smaller than the originals they replace and very easy to install by space-walking astronauts. ASIC enables an entire circuit board's worth of electronics to be condensed into a small package.
But size isn't everything when it comes to ASIC. High-performance capability, controllability, and availability are other benefits of using this JWST technology in Hubble's ACS. The ASIC's performance and control benefits are in the fact that they can be completely reprogrammed by commands from controllers on the ground, and the ACS can be fine-tuned for best performance. Because the ACS repair task was a rapid development for the SM4 mission, the availability of an existing design that met the electronic requirements became an instant enabler.