Michel Gros

Evidence for gravitational microlensing by dark objects in the Galactic halo

THE flat rotation curves of spiral galaxies, including our own, indicate that they are surrounded by unseen haloes of ‘dark matter’1,2. In the absence of a massive halo, stars and gas in the outer portions of a galaxy would orbit the centre more slowly, just as the outer planets in the Solar System circle the Sun more slowly than the inner ones. So far, however, there has been no direct observational evidence for the dark matter, or its characteristics. Paczyński3suggested that dark bodies in the halo of our Galaxy can be detected when they act as gravitational ‘microlenses’, amplifying the light from stars in nearby galaxies. The duration of such an event depends on the mass, distance and velocity of the dark object. We have been monitoring the brightness of three million stars in the Large Magellanic Cloud for over three years, and here report the detection of two possible microlensing events. The brightening of the stars was symmetrical in time, achromatic and not repeated during the monitoring period. The timescales of the two events are about thirty days and imply that the masses of the lensing objects lie between a few hundredths and one solar mass. The number of events observed is consistent with the number expected if the halo is dominated by objects with masses in this range.

MegaCam: the new Canada-France-Hawaii Telescope wide-field imaging camera

MegaCam is an imaging camera with a 1 square degree field of view for the new prime focus of the 3.6 meter Canada-France-Hawaii Telescope. This instrument will mainly be used for large deep surveys ranging from a few to several thousands of square degrees in sky coverage and from 24 to 28.5 in magnitude. The camera is built around a CCD mosaic approximately 30 cm square, made of 40 large thinned CCD devices for a total of 20 K x 18 K pixels. It uses a custom CCD controller, a closed cycle cryocooler based on a pulse tube, a 1 m diameter half-disk as a shutter, a juke-box for the selection of the filters, and programmable logic controllers and fieldbus network to control the different subsystems. The instrument was delivered to the observatory on June 10, 2002 and first light is scheduled in early October 2002.

Development of MegaCam, the next-generation wide-field imaging camera for the 3.6-m Canada-France-Hawaii Telescope

MegaCam is the new wide-field imaging camera currently being built for the new prime focus of the 3.6m Canada-France- Hawaii Telescope. The camera will offer a 1 square degree field of view and is built around a mosaic of 40 2K by 4.5K CCD devices. The delivery of the CCDs is proceeding along the schedule, the project passe dits final design review and the realization phase started, for an expected delivery to CFHT in Summer 2001.

The CFHT MegaCam 40 CCDs camera: cryogenic design and CCD integration.

MegaCam is an imaging camera with a 1 square degree field of view for the new prime focus of the 3.6 meter Canada-France-Hawaii Telescope. In building the MegaCam mosaic we encountered unprecedented challenges from both the large size of each CCD device (2K x 4.5K with 13.5 micron square pixels each) and the large size of the mosaic in which 40 devices have been assembled in a nearly 4-buttable edge manner on a cold plate. The CCD mosaic flatness of ± 16 μm has been optically checked at its nominal functioning temperature. The CCD mosaic is cooled at 153 K with a cryogenic unit; a close cycle pulsed tube with a power of 90 W at 140 K. A cold capacity, allows a slow warm-up during cooling shutdowns and a thermal dispatching leads to a temperature uniformity better than 3 K on the whole mosaic. The camera cryostat is designed in order to have easy access to the CCDs. The vacuum needed to avoid CCD contamination, leaded us to the use of low out-gassing materials in the cryostat. The instrument was delivered to the observatory on June 10, 2002 and first light is scheduled in October 2002.

40 CCDs of the MegaCam wide-field camera: procurement, testing, and first laboratory results

The 40 CCDs produced at Marconi Applied Technologies are dispatched at CEA to be tested and characterized before being inserted into the MegaCam camera mosaic. We describe here briefly our CCD test bench, the measurements we perform with it and the results obtained so far. A model has also been developed to interpret the quantum efficiency results.

The Electronic Controller of the 40 CCDS MegaCam Mosaic

MegaCam is the last generation wide-field imaging camera, which is presently mounted at the prime focus of the Canada France Hawaii Telescope. It is based on a mosaic of 40 2K×4.5K backthinned CCDs.

The CFHT MegaCam filter, shutter and roll pitch mechanisms.

MegaCam is an imaging CCD camera with a 1 square degree field of view for the new MegaPrime prime focus of the 3.6 meter Canada-France-Hawaii Telescope. This CCD camera is fixed on an aluminum structure, called Camembert for its shape, housing a shutter, a filter system and a roll pitch system to tune the CCD mosaic plane. The shutter is made with 1 meter diameter honeycomb half disks that rotates to covers or exposes the CCD mosaic. On this shutter a calibration source is fixed to monitor the CCD and its electronics. The filter system is made of a jukebox with a capacity of eight 30 cm square filters and of a loading arm to place them under the field of view. The instrument was delivered to the CFHT observatory on June 10, 2002 and first light is scheduled in October 2002.

The CFHT MegaCam control system: new solutions based on PLCs, WorldFIP fieldbus and Java softwares

MegaCam is a wide-field imaging camera built for the prime focus of the 3.6m Canada-France-Hawaii Telescope. This large detector has required new approaches from the hardware up to the instrument control system software. Safe control of the three sub-systems of the instrument (cryogenics, filters and shutter), measurement of the exposure time with an accuracy of 0.1%, identification of the filters and management of the internal calibration source are the major challenges that are taken up by the control system. Another challenge is to insure all these functionalities with the minimum space available on the telescope structure for the electrical hardware and a minimum number of cables to keep the highest reliability. All these requirements have been met with a control system which different elements are linked by a WorldFip fieldbus on optical fiber. The diagnosis and remote user support will be insured with an Engineering Control System station based on software developed on Internet JAVA technologies (applets, servlets) and connected on the fieldbus.