We are continued the construction and commissioning of the STACEE detector. The baseline design calls for three groups of 16 heliostats, with each group viewed by a telescope comprising a secondary mirror focussing light onto a 16 element camera. A schematic of the apparatus is shown in figure 1 and a photograph of one of the telescopes is shown in figure 2.
The telescope design was based on a prototype which had been deployed during 1996. In December, 1997 we upgraded this prototype by installing improved mounting devices for the phototubes in the camera and by substituting new light concentrators. These concentrators are called dielectric total internal reflection concentrators (DTIRCs) and are a compact substitute for the more commonly used Winston Cones. Their primary function is to limit the field of view of each phototube in such a way as to maximize signal from the target and minimize night sky background. The DTIRCs were made at McGill using UV transmitting acrylic obtained from the SNO collaboration.
In February, 1998 we installed the new data acquisition (DAQ) package. The STACEE prototype work had been carried out using a PC based system with limited sophistication. The new system runs on a unix work station. As such, it can multi-task and is able to separately acquire data, adjust high voltages and timing delays and monitor the state of the experiment.
In April, 1998 we installed the first of the production model telescopes. This views the western group of 16 heliostats and, together with the prototype, doubled the number of channels in STACEE to 32. It also expanded the collecting area of the experiment to the point where a large fraction of the Cherenkov light pool is now being sampled and various analysis strategies based on shower smoothness can now be brought to bear. The production telescopes are based on a 1.9 meter diameter spherical mirror composed of seven hexagonal facets mounted on a `spider' as shown in figure 3. The mirror has a focal length of 2 meters. The facets are made from soda lime glass which is slumped to the approximate shape and then ground, polished and aluminized. These were obtained commercially but the mounting and positioning devices were designed and constructed at McGill and Chicago.
From December to February we ran STACEE-16 for physics, tracking the Crab and in April and May we used STACEE-32 to acquire data on Mrk 421 and Mrk 501. The performance and reliability of the heliostats and the detector were remarkable and preliminary analysis of the data is encouraging.
STACEE was shut down during the summer since the nights are short, weather is unstable and our initial targets are not in our acceptance during the summer months.
In September and October, 1998 we replaced the prototype with a production version telescope. Extensive alignment and surveying were carried out, culminating in a successful `moon campaign' wherein the optical paths and acceptances of both the east and west telescopes where checked by imaging the moon on each camera element. We also installed a calibration system based on a laser and optical fibres. This system was modelled on the system developed by one of us for the ZEUS experiment and is being provided by the McGill and Barnard College part of the STACEE collaboration. The custom parts required for the system were made by the McGill experimental particle physics team.
The future of STACEE over the next three years is the following.
We will run STACEE 32 for the entire 1998-99 season, ending observations in May, 1999. Meanwhile we will complete the third telescope to allow expansion of the detector to 48 heliostats. This will be carried out during the early part of 1999 and it is planned that STACEE-48 will take preliminary data before the end of the current season.
The final component of the detector is the flash ADC electronics. These are necessary to provide mulitple-hit timing and charge information. We have chosen a supplier (ETEP, in France) who are beginning to produce a 1 GHz digitiser. We have ordered prototype models and the VME crates they require and will perform acceptance tests in early 1999. The entire complement of channels will be purchased (by Chicago) over the period 1999-2000.
With the installation of the FADC electronics, STACEE will be complete as designed. It is in this configuration that it will achieve its lowest threshold and highest sensitivity. As alluded to above, this will happen in time for the 2000-2001 observing season. We already have a very powerful instrument, even if it is only equipped with conventional electronics and must therefore operate at a higher threshold. Operation of the detector for science has already begun and our program is well defined. This year we are concentrating on a solid detection of the Crab nebula in its steady state mode. This will establish the credentials of STACEE since the Crab is the `standard candle' of the field. We have equipped the readout with a GPS clock time stamping device that will allow us to know the absolute time of arrival of each gamma ray to an accuracy of better than 1 microsecond. This will enable us to search for a pulsed signal in the data.
In the late winter and in the spring the Crab will no longer be in our acceptance. However the two AGNs, Mrk 421 and Mrk 501, will be visible and it is our intention to track them.
This pattern will be repeated in following years as experience and new electronics allow us to improve the experiment. Additional sources such as Gamma-Cygni in our galaxy and more distant AGNs from the EGRET catalog will be added to the observing list based on their time of appearance and expected flux levels based on extrapolations from EGRET data.
It is the stated aim of the collaboration to run for at least three years at design sensitivity. Modifications to improve or enlarge the detector will depend on the outcome of the first observing period.
Douglas M. Gingrich (gingrich@ualberta.ca) This page last updated: