Next Generation CFH Facility: Some ideas...

1/ Off-axis Filled-aperture 15-m Telescope

In order to have a instrument suitable for high-performance coronography, it is of prime importance to have a filled pupil.

For manufacturing reasons, a spherical primary is prefered rather than more difficult and probably very expensive
off-axis aspheric segments. Labeyrie proposed in the 90's (not published as far as I know) to build 15-m mirrors using
segments cut in now standard 8-m blanks. This allows to have a 15m primary mirror made of 7 segments (figure below).
Secondary, tertiary and quaternary mirrors are all aspheric. They correct the huge spherical aberration from the
primary and the 3 mirrors are all off-axis also. But in fact, these mirrors are 'half mirrors'
and maybe they can be cut in 2 parts after their polishing, if the residual stress in the blank is small enough and if
active optics is forseen in the telescope. Only M1 is segmented (in 7 segments).The combination of a spherical M1
+ M2+ (M3 M4 corrector) is now almost a classic when 15-m or larger telescopes are discussed (see for example
the abstracts of the papers presented at the ELT workshop hold in Sweden in 1999,


2/ FAST (Filled Aperture Smart Telescope)

Based on a MMT concept, with filled entrance and output pupils.

A 50-m telescope made of 36 telescopes of 8-m in diameter on a (huge) single mount has been presented last June 99 at the Extermely Large Telescope Workshop in Sweden.
Should be scaled to fulfil the Next Generation CFH facility size requirement.
Paper reference is:
«  A Multiple-Mirror-Telescopeconcept for a very compact 50-m Extremely Large Telescope »,
L. Arnold, Proc. of Bäckaskog Workshopon Extremely Large Telescope, Andersen et al Editors, Sweden, 1-2June 1999.
Below 2 figures from the paper showing the concept:
M1/M2 array of Gregory-Mersenne telescopes with hexagonal primaries.
M3/M4/M5 block (on a stiff structure) to re-image the pupil on M6 (M3/M4/M5 can be replacedby 2 flats if a field lens is put at M1/M2 focus).
M6: in pupil plane.Array of adaptive flats, also deformed as a reflecting Schmidt plate.
M7: spherical mirror of the recombining telescope (a Schmidt telescope).



3/ A 2-ring 30m visible/IR interferometer

Let's call it the CFHI, for Canada-France-Hawaii Interferometer.

CFHI concept : Big diameter, comfortable collecting array.


The 30m outer ring is made of 62 1.5m Gregorian-Mersenne (afocal) telescopes. 41 telescopes form the inner 20m ring. The collecting area is equivalent to a 17m telescope.

Each telescope is made of 3 mirrors + a lens:
a 1.5m primary,f/1.7 parabola (M1)
a 160mm secondaryf/1.7 parabola (M2), providing a 1500mm/160mm=9.4 beam compression
a flat tertiary (M3)
a field lens (FL) at the M1/M2 common focus.
All telescopes are installed on a virtual steep 30m parabola.
All M3 feed the M4 array, made of flat mirrors, each being in a pupil plane thanks to the FL. The M4 mirrors, each 160mm in diameter, are adaptive (M2 can also be adaptive for multi-conjugate AO). In order to have equal optical paths between all arms of the CFHI, the M4 mirrors are on a parabola having the same focal point than the primary 30m parabola. Thus, the 1.5m-telescope and the M4 arrays act as a flat-flat Cassegrain-Mersenne giant telescope.
The pupil on the M4 array is homothetic to the entrance pupil, therefore the instrument is a Fizeau interferometer providing a large coherent field. The re-imaged outer ring on M4 has a diameter of 3.4m, and the venerable CFHT can be used as the recombiner telescope if its optics is refigured (pupil not on the 3.6m mirror itself).
The 2 rings form a mobile structure supported by a kind of upside down rocking chair mount on a azimuth plateform.

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Created March 8, 2000