Next Generation CFH
to have a instrument suitable for high-performance coronography, it is
of prime importance to have a filled pupil.
manufacturing reasons, a spherical primary is prefered rather than more
difficult and probably very expensive
aspheric segments. Labeyrie proposed in the 90's (not published as far
as I know) to build 15-m mirrors using
cut in now standard 8-m blanks. This allows to have a 15m primary
made of 7 segments (figure below).
tertiary and quaternary mirrors are all aspheric. They correct the huge
spherical aberration from the
and the 3 mirrors are all off-axis also. But in fact, these mirrors are
maybe they can be cut in 2 parts after their polishing, if the residual
stress in the blank is small enough and if
optics is forseen in the telescope. Only M1 is segmented (in 7
combination of a spherical M1
(M3 M4 corrector) is now almost a classic when 15-m or larger
are discussed (see for example
2/ FAST (Filled Aperture
Based on a MMT concept, with filled
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
99 at the Extermely Large Telescope Workshop in Sweden.
Should be scaled to fulfil the Next
CFH facility size requirement.
Paper reference is:
Below 2 figures
from the paper showing the concept:
M1/M2 array of
Gregory-Mersenne telescopes with hexagonal primaries.
(on a stiff structure) to re-image the pupil on M6 (M3/M4/M5 can be
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
Let's call it the CFHI, for
CFHI concept : Big
comfortable collecting array.
The 30m outer ring
is made of 62 1.5m Gregorian-Mersenne (afocal) telescopes. 41
form the inner 20m ring. The collecting area is equivalent to a 17m
is made of 3 mirrors + a lens:
a 1.5m primary,f/1.7
a 160mm secondaryf/1.7
parabola (M2), providing a 1500mm/160mm=9.4 beam compression
a flat tertiary
a field lens (FL)
at the M1/M2 common focus.
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
FL. The M4 mirrors, each 160mm in diameter, are adaptive (M2 can also
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
same focal point than the primary 30m parabola. Thus, the
and the M4 arrays act as a flat-flat Cassegrain-Mersenne giant
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
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
on a azimuth plateform.
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March 8, 2000