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The Webfooted Astronomer - September 2001


Minutes: Hubble, Hubble, Toil and Trouble?

By Greg Donohue

ABOUT 30 people attended the Aug 11 Poo Poo Point Picnic and Perseid Party at Tiger Mountain. Para gliders provided pre-night entertainment. Reports are that the south side is the better area for viewing. Congratulations are in order for Jerry West and his wife on the celebration of their 50th wedding anniversary, and for Steve Kulseth on the completion of his Messier certification! And the Orion Nebula Star Party at Table Mountain takes place September 13 16.

Dr. Bruce Balick, newly elected chairman of the University of Washington Astronomy Department, gave the evening's address. A member of the UW faculty since 1975, Dr. Balick's primary research involves the study of planetary nebulae, focusing on the late stages of stellar evolution from the hydrodynamics of the gas ejected by these aged stars.

Dr. Balick uses the Hubble Space Telescope (HST) for some of his research, and serves on the Space Telescope Science Institute's User's Committee. He is a member of the design team for the new WFC3 camera to be installed on the HST late in 2003, and his discourse focused on the technology of the Hubble Space Telescope and the plans for its future.

No single tool in the last 50 years has been more instrumental in advancing astronomy than the HST. It has benefited, and also upended, all major areas of astronomy and cosmology. Dr. Balick indicated that his own research was set back a decade by discoveries during Hubble's first year of operation. Basing telescopes in space eliminates the problem of atmospheric aberration. The HST operates continuously at better than 0.05 arc seconds resolution. Space-based telescopes are also able to study areas of the electromagnetic spectrum, particularly ultraviolet, to which our atmosphere is opaque. The image quality demands, combined with the requirement to do UV astronomy, meant Hubble's optical specifications were nearly 1000 times more stringent than ground-based instruments.

With a designed life expectancy of 15 years, the Hubble was scheduled to be decommissioned in 2005. But delays in the Next Generation Space Telescope (NGST) project led to a five-year reprieve for HST. Instruments on the HST are slowly wearing out, primarily due to cosmic rays, which are sputtering the mirror surfaces, degrading the CCDs, and causing noticeable pitting on some of the filters. By the end of the last of five Shuttle servicing missions (three of which have already taken place), nearly every part of the Hubble will have been replaced and upgraded.

The Wide-Field Planetary Camera 2 (WFPC2 or whiff pick) has been doing most of the optical imaging. The fourth servicing mission, scheduled for this December, will carry the new Advanced Camera for Surveys, which is almost 5 times more sensitive to many areas of the spectrum than is WFPC2, and has an 85% quantum efficiency compared to WFPC2's 20%. (An 85% quantum efficiency means the instrument records 85 out of every 100 photons striking it.) This mission will also carry a large refrigeration unit to cool the Near Infrared Camera and Multi-Object Spectrometer, whose brick of solid nitrogen coolant, expected to last 5 years, lasted only two.

The fifth and final mission, planned for late 2003, will carry with it the Wide Field Camera 3 (WFC3 or "whiff three") that Dr. Balick has helped design, to replace WFPC2. WFC3, a panchromatic instrument, has one camera optimized for far-red and ultraviolet (UVIS), and another for infrared (IR), making it sensitive to wavelengths from 200nm to 2000nm. The ultraviolet will be the best ever launched. The WFC3 camera contains next-generation CCDs that are far sturdier than those used in the ACS. Some think that cosmic rays will render ACS useless is only two years. A small mirror in the light path on WFP3 directs light to the UVIS CCD; when the mirror is retracted, the light goes to the IR detector.

Infrared imaging allows us to see past the interstellar dust that obscures many other wavelengths. For example, what looks like a fairly bland patch in the Orion nebula in optical light, turns out to be the most active infrared source in the entire nebula. Many very young stars, younger than those in the trapezium, are buried deep inside this dust region.

Similarly, in the Egg planetary nebula, the central star that powers the optical reflection nebula is invisible at optical wavelengths. But IR images show not only the star, but also warm dust that does not show up in the visible nebulosity. The central star is only a few thousand years from entering white-dwarfdom, and yet is shedding material in a non-spherical pattern. This is typical of other very old stars in other planetary nebulae, and current stellar theories are at a loss to explain this behavior. Dr. Balick is investigating magnetism as a possible mechanism for driving the beautiful and complex patterns that appear in these planetary nebulae.

Infrared imaging also reveals a great deal of additional information about galaxies, including information that helps track the history of interaction between galactic collisions and close encounters (such as the Antennae Galaxies). Ultraviolet images help reveal where new stars are currently forming in these interacting galaxies. Infrared images reveal the location of old red giant stars that existed long before the interaction. This helps give a before and after picture of the galaxies, allowing us to model how the gravitational interaction has progressed over time.

The WFP3 camera will be placed in one of the two off-axis bays on the Hubble Space Telescope during the HST SM4 mission. The current WFPC2 camera has three 800x800 pixel 0.1 arc second resolution arrays, and a single much smaller, but 0.046 arc second resolution array. This configuration is what yields the many stair-step images we have come to expect from Hubble. The ACS camera will have a much larger 4096x4096 array of pixels with a resolution as good as the small chip on the WFPC2. In addition to having large arrays with fine spatial resolution, the WFC3 camera has 72 filters. Many of these are very narrow band, some highly tuned to hot ionized gas, which make them very useful for Dr. Balick's study of planetary nebulae.

So as we toil to improve the operation of the HST through the end of this decade, the hope is that it will continue to cause trouble in every area of astronomy and cosmology, for that is how we expand our knowledge the most.

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