In the early eighties, astronomers using their brand-new Canada-France-Hawaii telescope might spend hours in chilling nights sitting in a tiny cage on top of the telescope, 15 meters above the main mirror. In the rarefied air of Mauna Kea, the highest point of the Hawaiian Islands, they would load a camera with a large photographic plate, open the shutter and let the light from remote stars or galaxies hit the emulsion.
To see something of the faint heavenly bodies, one would sometimes expose for hours, and be rewarded with images difficult to analyze: photography does not transform light intensity in gray levels for further analysis very effectively.
It does not come as a surprise that the first good digital detector to arrive at the observatory (a fraction of the size of the plates and with big pixels), quickly gained favor over the old plates: the counts in each pixels were proportional to the amount of light received, opening the door to nicely quantified information on the physics of the celestial objects observed. Back then, it was difficult to imagine that a day would come when one could do the same work in the warmth of a well equipped observing room, with a digital camera even larger than the old 20cm x 20cm plates, replacing the photographic camera in a remotely operated cage. However, that day has come, with MegaPrime beginning its operations this month at CFHT.
Before MegaPrime, CFHT had been pioneering the use of large contiguous detectors through a series of mosaics of increasing size and quality. The latest one, CFH12K, was made of 12 CCDs, or Charge Coupled Devices, the name given to the silicon chips used in digital cameras. CFH12K, the largest contiguous mosaic in use on an astronomical telescope, has been in operation for 4 years, serving many scientific programs and producing amazing pictures that have been published in books, magazines, on posters and on calendars. CFH12K and MegaPrime are part of the CFHT wide field imaging plan, which will exploit a niche larger telescopes cannot master: The observation of large areas of sky at very high resolution, while allowing scientists to see, simultaneously, the details of millions of faint objects, limited only by the specifications of the telescope and the condition of the atmosphere.
MegaPrime, a CFHT project, is a collaboration between CFHT and institutes in France and Canada, with three major industrial contractors. Within a myriad of capabilities, the principal mission of MegaPrime is to offer scientists a field of view of 1 degree by 1 degree, the size of four Full Moons, without compromising the resolution and the image quality.
At the heart of MegaPrime is MegaCam, a unique camera built by the "Département d'Astrophysique, de Physique des Particules, de Physique Nucléaire et de l'Instrumentation Associée" at the French "Commissariat à l'Energie Atomique" (CEA). To cover the 1 square degree field, CFHT ordered 40 CCDs from a company in the United Kingdom, e2v technologies, which specializes in the production of high quality detectors. CEA mounted these CCDs very precisely in a mosaic which central area, made of 4 rows of 9 CCDs, covers a square of 25cm by 25cm, or 1 degree by 1 degree on the sky.
When used for astronomical applications, CCDs have to be operated at very low temperatures to reduce the amount of thermic noise they generate during the long exposures (minutes to tens of minutes) typical of astronomical images. The mosaic is installed in a cryostat where the CCDs can be cooled to -120 degree Celsius. In order to minimize the thermal losses through temperature exchanges with the air, a high quality vacuum is maintained in the cryostat; the mosaic is cooled by a special cryogenics system based on pressure waves in pressurized helium, which extract the heat from inside the cryostat.
CEA also built the camera's shutter, a rotating half-disk able to uniformly open and close the camera for exposure times as short as 1 second; CEA also fashioned a filter jukebox which allows the observation of the sky in different colors, an essential device as the CCDs cannot disclose any color information by themselves.
The last key components built by CEA are the electronics needed to extract the image from the CCDs: each MegaCam image is currently made of 340 megapixels that have to be read quickly and carefully, without degrading the image. The MegaCam electronics designed by CEA can read the image in less than 35 seconds while maintaining a very low readout noise - this is, in fact, the shortest readout time ever achieved on a mosaic operated at CFHT!
As it was not possible to accommodate a 1 square degree field of view with the old cage of the early days (still in use for CFH12K), a new, upper end of the telescope had to be constructed. Designed at CFHT in collaboration with the "Division technique" of the French "Institut National des Sciences de l'Univers", it was built by a Californian company, L&F Industries, now a division of Erie Press Systems. 13 meters above the main mirror of the telescope, this upper end offers a platform ready to host the equipment needed to give to the camera a nice view of the sky.
The parabolic main mirror of the telescope does not produce, alone, a good image of the whole field of view; a wide field corrector (WFC) is installed in front of the camera. With four lenses 50 to 80 cm in diameter, in a structure two meters long for 660 kg, the WFC is an amazing piece of optics designed at the Herzberg Institute for Astrophysics (Victoria, Canada) and built in France by SAGEM/REOSC. The resultant pixel size is slightly less than 0.2 arc-seconds: a good resolution giving a reasonable sampling of the images even with good seeing (images of 0.5" were already observed several times). To complement the corrector, 5 filters were also fabricated by SAGEM. They follow relatively closely the Sloan Digital Sky Survey filter set, but for the blue filter, which makes good use of the superior transparency of the Mauna Kea sky and the UV enhanced WFC glass.
To accommodate the changes in focal length of the telescope with temperature, and the focus position changes induced by the various filters, the camera must be able to move along the optical axis of the telescope. The focus stage assembly (FSA) accommodates this motion, supporting the camera and its shutter on a motorized stage bolted on top of the upper end platform. In order to follow the apparent motion of the sky due to the Earth's rotation, two small cameras fix on stars outside of the field of view, providing automatic guidance of the telescope and measurements of the focal changes.
The guiding cameras ('guiders') are installed underneath the FSA, and like the FSA, were designed and built at HIA. To compensate for telescope oscillations due to windshakes or telescope tracking anomalies, an Image Stabilizing Unit (ISU) made of a tip/tilt plate is attached to the beam on top of the wide field corrector. Designed at "Observatoire de Paris", the ISU is servo-controlled through the guiders' signals.
The integration and overall control of MegaPrime, and all the utilities - helium lines, glycol hoses to eliminate all the heat sources on top of the telescope, electronics box housings, hundreds of yards of cables or optics fibers - and the development of the observing environment specific to MegaPrime, have been CFHT's work.
Nobody has been riding the cage for years now; neither do the observers come to the observatory. The Queued Service Observing, heavily used for CFH12K, is now the only option with MegaPrime. Astronomers from Canada, France and Hawaii prepare their observations from their own institutes. CFHT carries them when the sky conditions are optimal and even provide a pre-processing of the images, clearing them of the instrumental effects - a boon for the astronomers, especially with disk images of more than 700 Megabytes!
MegaPrime has already been operated in QSO mode in engineering nights and is now being released for scientific operation. Parallel to the engineering work done using nighttime to integrate the instrument, tune the optical alignments and make the guiders work, some scientific programs were performed to help assess the overall quality of the instrument. Among them: MegaPrime participated in the discovery of new satellites of Jupiter, spied on Near Earth Objects and checked wide areas of the sky to be observed on a future space mission. It prepared the observations of a large Canadian and French project, the CFHT Legacy Survey (500 nights in 5 years for studying the Kuiper Belt, the large scale structures of the universe) and observed Super Novae, in the continuing study of the evolution of the universe. As for amazing pictures, MegaPrime is already surpassing CFH12K...
The biggest digital imager ever installed on a telescope,
with at least 5 years of operation ahead, MegaPrime will be one of the
major instruments at CFHT and an important player in wide field astronomy.
All pre-processed images taken with MegaPrime will be archived at the Canadian
Astronomical Data Center and will be accessible to the worldwide community
in the future. They will be an important resource for data mining in the
framework of the Virtual Observatory.