Instrument Description: the various components



MegaPrime on the Canada-France-Hawaii Telescope



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 m



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.
 



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



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
 



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: 36 (a square of 4 rows of 9 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): 340 Megapixels

Operating temperature: -120 degrees Celsius



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