AAO Newsletter October 1996 - Page 13

Optical designers probably will not mourn the demands for generous backfocal distances, and similarly the mechanical engineers for the requirements of supporting the not inconsiderable weight of the detector head. The IPCS requirements dominated a generation of spectrograph camera designs.

Dick Hunstead and Max Pettini were allocated the IPCS for an observing run in November 1995. It was not intended to be the final run of the IPCS, but the general feeling developed that this was likely to be the last one.

Tests in preparation for the last run indicated that the detector was delivering its specified performance levels after nearly 20 years and numerous modifications. Factors that help explain this include the following. Professor Alec Boksenberg and the IPCS engineering design team selected proven available technology and, where necessary, developed novel technical solutions to overcome instrumental effects. In an age of fast track instruments and short life cycles, the value of 20 years continuity of support from original members of the design team such as John Fordham of UCL and earlier Tony Jordan of RGO should not be underestimated.

The last night of the IPCS was spent with the detector doing what it was designed for, observing absorption lines in quasars. The integration was started and with the detector slowly accumulating counts, we left the control room to view the comet Schwassinann-Wachmann 3 from the catwalk. It was a dark clear night after a hot Australian day with the warm breeze and a beautiful comet hanging in the western sky it proved to be an appropriate night for the IPCS to end its observing career.

Here are a number of scientific highlights from two decades of observations with the IPCS:

• Using the IPCS in direct mode, the Vela pulsar was identified with a 24th magnitude star.

• SS433 was identified with a Molonglo radio source, leading to the discovery in California that gas was being ejected at one quarter the speed of light.

• The IPCS was used as the detector to measure the redshift of many of the "record holding most distant QSOs" including PKS 2000 - 330. At z = 3.78, it was then the most distant quasar known breaking an 8-year drought on such discoveries. Eventually the first QSO with z > 4 was found with the IPCS.

• Observations of the ISM towards SN 1987A at the highest resolution ever achieved were made using the IPCS with the PGS (or wooden spectrograph, especially built for the project). The spectra showed cold clouds of gas located between the Galaxy and the Large Magellanic Clouds, which could not have been ejected by either galaxy.

• Spectra taken with UCLES of QSO 2206-l99N revealed the presence of an intervening galaxy at z = 2 with extremely low abundances, indicating very little evolution from the primordial material.

• The IPCS was used with FOCAP and Autofib in a number of projects. It was found that globular clusters in Centaurus A were similar to those in our own galaxy; that the SMC is being tidally disrupted; and that QSOs were more luminous in the past and are clustered.

The IPCS was a major optical detector for nearly 20 years, and the only panoramic optical photoelectric detector available for spectroscopy for 10 years. It is not likely that any future detector will match this record. The absence of readout noise and large format maintained the popularity of the IPCS well after the efficiency of the detector was superseded. A number of the projects listed above could not have been achieved with the CCD detectors available at the time as they had significant readout noise.