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MegaPrime on the Canada-France-Hawaii Telescope
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Upper
End |
The new prime focus upper end
(PFUE) has been designed at CFHT with the help
of INSU-Division Technique: a new
base ring, a new set of spiders, and a prime focus base which will receive
all the other components of MegaPrime. The PFUE has been built on the West
Coast of the USA by L&F Ind.
In addition to its basic structure, the PFUE provides a temperature
controlled environment for MegaCam and its readout electronics. A temperature
controlled enclosure for the electronics of MegaPrime is installed on the
telescope "caisson central" .
Total weight of the structure itself: 3000 kg
Base ring: 2400 kg
Spiders: 1100 kg
Prime Focuse Environment base: 500kg
With all the Megaprime equipment: 5700 kg
Overall height from the base ring to the top of the cover: 6
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Wide
Field
Corrector |
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. The WFC
has been designed at HIA (Victoria, Canada).
The lenses have been fabricated by SAGEM/REOSC, which also built
the mechanical structure of the WFC and coated the lenses.
| Total weight: 660 kg
Overall height: 1.9 m
Four spherical lenses in BSL7-Y (enhanced UV transparency glass)
Lens diameter:
First lens: 81 cm
All others: between 50 and 56 cm
Image quality: designed to achieve better than 0.3" diameter at
80% encircled energy from u to z on most of the field.
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Image
Stabilizing
Unit |
The Image Stabilizing Unit (ISU) has been designed
and built at Observatoire de Paris. It is used
to produce small image position correction on the focal plane of MegaCam:
a
glass plate in the optical beam in front the camera can be tilted and it
produces a displacement of the image proportional to the small angle of
the tilt.
Tip-tilt plate: fused silica
Diameter: 480 mm
Overall weight (including electronics): 55 kg
Motion amplitude: +/- 1.2 degrees (or +/- 1 arcsecond on the focal
plane)
Image correction bandwidth: up to 5 Hz
Internal loop frequency: 50Hz |
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Focus
Stage/
Guiding
Focus
Sensing |
The Focus Stage Assembly
(FSA) is supporting the camera and allows its motion along the optical
axis in order to accommodate the focus variation due mainly to filter changes
and temperature induced telescope dilatation. Two guiders (GFSU) located
under the top plate of the FSA give a position and focus information from
two guide stars on the North and South edges of the MegaCam field of view.
The FSA and GFSU hardware have been designed and built at HIA,
while the the control has been designed and realized at CFHT.
Focus stage
Weight of the FSA itself: 260 kg
Weight supported (camera, shutter and cryogenics):
up to 250 kg
Repeatability of the motion along the optical axis:
0.01 mm
Motion speed: 1mm/second
Guiding/focus sensing
Limiting magnitude: ~15th magnitude
Guiding field area: 20' x 7' for each guider
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| CCDs |
Charge Coupled Devices
(CCDs) are the detector of choice in astronomy for observations in visible
light. Appeared in the early eighties, they have since replaced the photographic
plates or films used in astronomy for more than a century. The CCDs used
for MegaPrime have been built by e2v technologies.
| CCD type: CCD42-90
Number of CCDs: 40
Number of CCDs currently used: 40 (4 rows of CCDS, first and last rows have 9 CCDs, middle rows have 11 CCDs)
CCD size: 2048 x 4612 pixels
Pixel size: 13.5 micrometers
Pixel scale: 0.185 arsecond/pixel
Image size (whole mosaic): 378 Megapixels
Image size (current): 378 Megapixels
Operating temperature: -120 degrees Celsius |
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| MegaCam |
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). In addition to a cryostat housing the
mosaic, and its cryogenics system to maintain it cold, CEA built the camera
shutter, the filter jukebox and the electronics to acquire the image
and send it to a computer through fiber optics cables.
Overall mass: 350 kg
Mobile mass (moving with the FSA): 230 kg
Cryostat:
Cold plate temperature: -120 degrees Celsius
Readout electronics
Readout time: 30s
Readout noise: less than 5 electrons
Shutter
Type: Half rotating disk
Diameter: 1 m
Minimum exposure time: 1 second
Filter jukebox
Number of filters: 8
Filter change time (in any position of the telescope):
2 mn |
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