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
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:
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