History of CASS and the Hubble Space Telescope

The involvement of the Physics Department with what became the Hubble Space Telescope goes back to the very beginnings of UCSD. Prof. E. Margaret Burbidge had long supported the idea of a large optical telescope in space. Profs. Carl McIlwain and Larry Peterson laid the foundations of a space research group at UCSD which would go on to invent detectors and develop many successful instruments for space physics and X-ray astrophysics. In 1968 Carl McIlwain sketched out on a paper napkin (in the best tradition) the concept for a novel type of photon detector to his graduate student Ed Beaver. Ed developed the Digicon, a sort of 20,000 Volt photomultiplier with a linear solid state diode array in place of the phosphor screen and with elaborate magnetic steering for the photoelectrons. By 1971, Beaver had started taking this new detector off to various observatories. He was joined in 1974 by Richard Harms, another McIlwain student. By 1976 Beaver and Burbidge were on NASA panels, specifying instruments for a future Large Space Telescope (LST).

In 1977, NASA requested proposals to build scientific instruments for the 2.4 m LST. Four young scientists led by Richard Harms scored a coup in winning the Faint Object Spectrograph (FOS) contract, by showing a working detector. The scientific credentials of the FOS team were enhanced by the addition of Margaret Burbidge and others at various Co-Investigator institutions. Winning this large contract also set in motion the creation of the Center for Astrophysics and Space Sciences (CASS). Soon began 13 years of design, building, testing, integrating and waiting with a large team of engineers and software wizards at Martin Marietta and UCSD. Of these, most have left or moved on to other projects after the Hubble Space Telescope (HST) was finally launched in April 1990. Pam Capodicci is the official "project memory", Ron Lyons and Bill Baity provide science support, Ross Cohen and Vesa Junkkarinen are scientists who have been on the team since about 1985.

Rising to the challenge of the misfigured mirror, the team's first opportunity to sample HST data came in October 1990 with observations of the distant quasi-stellar object QSO UM 675 . At this time, target acquisition was not yet routine, and pumping adrenaline doubtless helped last-minute pointing corrections in the dead of night! With its large redshift, the spectrum of UM675 detected by the FOS reached into the far UV, down to 530A, beyond the resonance line of neutral helium. The absence of a dip in the spectrum here showed that nearly all the helium must have been ionized, early in time.

The spectral coverage of the FOS also allows study of ultraviolet spectral features in relatively nearby objects, features which could previously only be studied from the ground in distant QSOs, where the lines are redshifted into observable optical wavelengths. The high density of hydrogen absorption lines found by Vesa Junkkarinen and other team researchers at the lowest redshifts is one of the major discoveries so far with HST. This has indicated a fairly large number of "clouds" of intergalactic gas at the current epoch. These clouds are of interest because they may be remnants of primordial matter in the universe, the left-overs from the matter that collapsed to form the galaxies that we see today. The large number of these clouds at low redshift is probably due to a sharp decrease in the background intensity of ultraviolet light from the levels present when the universe was one third of its current age. Using Beaver's HST observations as well as ground-based observations of QSO 3CR 286, Ross Cohen and others have found that conditions in this high redshift galaxy are substantially different from those in our own galaxy; they are suggestive of a relatively un-evolved system at a fairly late time.

Current studies include the physics, distribution and velocities of the emission-line clouds around the central energy source of QSOs, and also using QSOs as probes of the gas in the halo of our own galaxy and other galaxies; most of the interesting lines fall in the UV, below the cutoff of Earth's atmosphere.

The team, now including post-docs Thanassis Diplas, Lin Zuo, Fred Hamann and recent graduate Tom Barlow, hopes that the correcting Hubble optics installed in the ambitious December 1993 Shuttle mission will allow the investigations originally planned.

These observations will lead to better understanding of the radiation and matter content in the spaces between galaxies, which will help choose between competing hypotheses linking these Ly a clouds to less primordial material in the very outermost regions of galaxies.

The FOS is a fully functional and well-calibrated international resource, also used on key investigations by CASS groups led by Profs. David Tytler and Art Wolfe. Its status as the most-requested instrument on the Hubble Space Telescope is a tribute to all those who worked on it, both here at the UCSD Physics Department and elsewhere. The 15-year mission to explore the distant universe is well on its way!

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