The CFH12K CCD mosaic camera mounts in the CFHT prime focus cage.
The prime focus consists of the main structure (initially designed
to accommodate the presence of an astronomer in the early time
of the telescope when photographic plates were still in use),
the prime focus bonnette and the wide-field corrector.
Figure 1 (left):
The CFHT at night. South view looking towards the |
celestial pole. More on this image on the CFHT image of the week site.
Figure 2 (right): The CFHT in prime focus configuration: the top-end
ring supports the whole prime focus structure (black).
Photographs by J.-C. Cuillandre, Copyright 2000 CFHT
CFH12K is mounted in a look down
configuration on the bonnette. The bonnette
includes the focus unit, the guiding camera and
the central mirror (to center the field). The
wide-field corrector is made of three lenses
and offers a 1 square degree field of view.
Not shown on this diagram are all the power
supplies and cabling that actually fill up the top
space of the cage.
The prime focus cage has benefited from major improvements to reduce
the scattered lights. All surfaces (entry cone, wide-field corrector
sides, mounting structures, ...) have been blackened with very low
reflectance sturdy black velvet. Compared to its CCD mosaic predecessors at
prime focus (MOCAM in 1994 and UH8K in 1995), these improvement have
greatly improved the flat-fielding quality and reduced the sky background.
CFH12K image with the bonnette rotated during the exposure.
Position of the rotation center on CCD02.
The bonnette rotation is disabled because the native brakes
of the bonnette were not strong enough to avoid the slight rotations
(fraction of a degree) at high airmass caused by the large torque induced
by weight of the CFH12K. The CFH12K weight is not centered on the bonnette
rotation axis because of the off-centered filter wheel.
A very solid clamp has been permanently installed to block the bonnette.
The bonnette is set at the precise angle 89.92 degrees (coming from the lower angle to avoid hysterisis effects) and is repetivive from one observing run to another to within 0.5 degrees. The long axis of the mosaic (along the lines) is aligned with the East-West axis within 0.5 degrees.
However, the center of the mosaic does not align exactly with the optical axis (center of rotation of the bonnette). The actual center, where a precise telescope pointing will put a given object for some given coordinates, will fall on the top corner of CCD02: around [X=1990,Y=3970] when CCD02 is read from output A (1999A), around [X=60,Y=3970] when CCD02 is read from output B (since 1999B).
For observers needing a very precise pointing (when planning to come back to a given field on two different nights or observing runs), the central mirror should be used to align precisely with a nearby SAO star (request to be made to the CFHT Observing Assistant piloting the telescope).