General Meeting Report
“A tour of the Universe with the Hubble Space Telescope”
A talk from Professor Duncan Forbes
Swinburne University, Centre for Astrophysics & Supercomputing
Professor Duncan Forbes is a New Zealander who obtained his PhD in Astronomy under an Isaac Newton Scholarship from the University of Cambridge in 1992. He worked as a Senior Lecturer in the School of Physics and Astronomy at the University of Birmingham in England, spent time at the Space Telescope Science Institute and Lick Observatory in California and in May 2000 moved to Australia to join the Swinburne team as a faculty member. Over the years he has worked on various aspects of galaxy evolution with a recent fondness for globular clusters in external galaxies.
As an active user of some of the world's best telescopes, including the Hubble Space Telescope, the Keck 10m and the Compact Array, Duncan is eminently qualified to talk about the subject. A quick tour through the history of the HST brought out some amazing statistics: It was deployed in low-Earth orbit (600 kilometres) by the crew of the space shuttle Discovery on 25 April 1990, weighs in at about 12 tons, is roughly the size of a Melbourne tram and orbits the Earth every 97 minutes, while its array of solar panels produce a steady 2,400 watt of electric power.
The current complement of science instruments include three cameras, two spectrographs, and fine guidance sensors: The Wide Field/Planetary Camera 2 (WFPC2) consists of three cameras in an L-shaped trio of wide-field sensors and a smaller, high resolution ("planetary") camera tucked in the square's remaining corner. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is a cryogenic instrument providing the capability for infrared imaging and spectroscopic observations of astronomical targets and detects light with wavelengths between 0.8 and 2.5 micrometers. The Faint Object Camera (FOC) consists of two complete detector system. Each uses an image intensifier tube to produce an image that is 100,000 times brighter than the light received. This system is so sensitive that objects brighter than 21st magnitude must be dimmed by the camera's filter systems to avoid saturating the detectors. The FOC offers three different focal ratios: f/48, f/96, and f/288. The f/288 field of view is 3.6 X 3.6 arc- seconds square, with resolution down to 0.0072 arc-seconds.
Everyone knows about the original flaw in Hubble’s mirror and the subsequent space-shuttle mission to repair the defect. But not so well known is the fact that regular maintenance/equipment upgrade visits are part of the HST program every three years, and the modular equipment design allows for easy replacement of most parts. Hubble has special grapple fixtures, 76 handholds, and is stabilized in all three axes. The Endeavour shuttle mission of December 1993 successfully restored the functionality of HST. Three attitude/pointing control gyros have been replaced during the 1997 visit, and camera upgrades effected in 99/2000.
Did you know Hubble’s slew rate is limited to approximately 6 degrees per minute of time? Thus, about one hour is needed to go full circle in pitch, yaw, or roll. As a result, large manoeuvres are costly in time and energy. So, whenever possible two scientific instruments are used simultaneously to observe adjacent target regions of the sky. Another restriction is, that due to interference from Earth, Moon, the Sun and magnetic anomalies, data acquisition time each orbit is limited to some 50 minutes, or just over half the total time.
Because of HST's location above the Earth's atmosphere its 2.4m reflecting mirror at f/24 optical system can produce high resolution images of stunning clarity, down to 0.1 arc-seconds, and the pictorial tour of the universe by Professor Forbes was breathtaking. Computer images and digital projection blended the program’s wide range seamlessly from our Solar System to the Orion Nebula and Eta Carinea; then to our galactic neighbours, globular clusters and colliding galaxies and out to the far corners of the Universe, where galaxies are counted by the thousands to the arc-minute and gravitational focussing extends the edge of the visible universe back to the beginning of time.
All the information and the images collected with the HST become public property after one year and are used extensively by educational facilities all over the world. Some were even on display at the rear of the hall. But, while the cosmic journey presented no new revelations it nevertheless was awe inspiring, and a lively question-time had to be cut short by the President. After all, this was St Valentines Day and the speaker anxious to get home to his family.