[ATM] Re: Large Amateur Telescope

[vladimir sacek <vladis.2@juno.com>]

I don't see the rationale for building a multiple aperture
Cassegrain-like system 
with a single secondary. The primaries would have to be off-axis sections
of larger parent mirror, kept aligned to a sub-wavelength precision. It
wouldn't be 
cost-effective, whether primaries are made separately, or cut out of a
larger mirror
(tilted primaries would produce aberrations that no single secondary
could correct).

An array of separate Cassegrain-like telescopes  would also require 
high-precision 
alignment plus beam combiner - technology not commonly available to ATMs.

Folded Gregorian (also called "Johnsonian") may be the only two-mirror
arrangement
that offers realistic chances of successfully making a multiple aperture
telescope to advanced 
ATMs. The basic design would consist of 3 to 6 primaries, each with
separate secondary 
placed out of the incoming light, receiving diverging cone from the
primary after it reflects from 
a small flat in primary's focal zone. Secondaries would reflect
converging cones towards the axis of
the array, to reflect off a cluster of flats towards common focus behind
the set of primaries.
The cluster of sets would have to have  high-precision alignment
capability for both both, 
bringing the separate converging beams to a common focus (by adjusting
flats' tilt) and to 
a common phase ( by adjusting flats' axial position). The cluster of
flats would be, in effect, a beam-combiner.
Far from easy, but may be doable. The effective F# would be lower than
that of each primary, 
and determined by the tilt of the cluster of flats. A set of, say, six
10" primaries, arranged so that 
edge-to edge span is 26", should perform close to ~25% obstructed 26"
aperture level. 
A set of 16" primaries with edge to edge span of 40", 
would perform close to a 40" aperture level.

Vlad
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