AAO Newsletter October1996 - Page 12
some of the history of this detector and its role in the AAO.
The IPCS was commissioned in Nov 1976 replacing the 3 Stage EMI intensified photographic system on the RGO Spectrograph. It joined the Wampler Image dissector scanner as a panoramic photon counting system. Despite its meagre 18000 pixels it was utilised for approximately 50% of the total allocated telescope time. The arrival of the external memory in 1978 relieved the limitations of the Interdata Model 70 memory by permitting windows of up to 192K pixels. The AAT had its first two dimensional detector. This also allowed the real time display of the integrating image and made the on-line data reduction easier. Keith Shortridge paid a visit in 1979 to move IPCS from the CHAOS operating system to RTOS easing the support of the system. The team from UCL arrived for another upgrade in August 81. John Fordham and Keith Shortridge expanded the external memory to a megabyte allowing 512k pixels, and introduced an increased X (spatial) resolution. The introduction of image tube scanning was introduced in 1985 to help minimise the effect of image tube granularity and to increase the effective yield rate of EMI image tube production runs. EMI had long supported the astronomical application of its detectors by continuing to develop the device and offer technical support with the operation.
The IPCS has a long history of innovation and modification. As well
as the basic upgrades to memory, a strobing mode was developed almost immediately
for time resolved field photometry. The detector also saw considerable
use direct at the f/15 focus for photoelectric imaging in the normal integration
mode. In 1991 there was an attempt to implement an analogue mode intended
to overcome the count rate limitation of the IPCS, but technical difficulties
and ultimately the developments in CCDs prevented the mode entering regular
service. The analogue front end was used later for speckle interferometry.
The ability to scan the external memory led to its application to instruments
such as the loaned TAURUS I. This application and others led to the detector
hitting a peak utilisation of over 67.6% of allocated telescope time. Though
the first CCD arrived in 1981, the IPCS still had advantages of blue sensitivity,
area, geometry and temporal resolution. 1983 saw the arrival of a team
from ROE with a polarimeter module for the RGO/IPCS. The polarimeter used
a Pockels cell modulator capable of adjustable switching rates and exploited
the multiplexing ability of the exmem and hardware data acquisition control.
Another example of the flexibility of the system was the introduction of
the ASPECT mode which enabled scanning of the spectrograph slit synchronously
with the data acquisition for area spectroscopy. The addition of a High
Speed mode allowed time tagging of photon times to allow of the use of
the detector in a speckle interferometer and for high time resolution spectroscopy.
The planned arrival of UCLES required the construction in house of a second
detector head and rack in 1987.
An extensive IPCS jargon entered the vocabulary of observers over the
period, along with a substantial body of folklore. Imagine the uninitiated
decoding the following. ..
"Unfortunately we slewed through pop just before the the trip.
We were already battling the jitter to flatfield this one when the DCR
switched to local on a flow failure. We had carefully checked the ECD for
worms fearing coincidence loss and beam bending, but later noticed a strange
feature in the flatfield where every fourth increment had a 5% increase
in counts, except for the ringing affected increments. The effect disappeared
when the NA accidentally ran his chair into the desk which terminated the
run prematurely and caused the status lamps on the desk to go into Christmas
tree mode. We switched the DCR to local, cycled the IPC and then noticed
that the scan correction had gone off..."