Limits to telescope size

Henry Spencer wrote:

In article ,
apparently there are practical advantages in having a
single filled aperture rather than a wide scattering of smaller mirrors.

I suspect that apodisation is very much easier with the filled aperture.

In sci.space.policy wrote:
Just seen some pictures of Hubble, and The "Very Large Telescope"
(original name) in Chile. It made me think.

With a reasonable space based industry, moon mining, metal working,
aluminium and glass production, precision engineering, how big an
optical telescope could be built in zero-g?

What are the limits? Could a 100m diameter optical telescope be built?
what would it see?

Hmm.
100 light years = 3*10^8m/s * 3*10^7s * 100 = 9*10^17m.
For 1m resolution at that range, in the visible you're looking at about a
diameter of 10^12m, or an area of around 10^24m^2.

For an average thickness of 100nm, this is 10^17m^3, or for aluminium,
or a cube of about a thousand kilometers across.
I'd guess this to be about the aluminium content of the moon, plus a few
other bodies.

This would take a fairly reasonable space industry.

As somebody else mentioned, a large baseline optical interferometer is
likely to give more info for a much smaller price. So, what is th
elargest baseline optical interferometer we can build?

In article . com,
wrote:
As somebody else mentioned, a large baseline optical interferometer is
likely to give more info for a much smaller price.

No, it depends on what sort of information you are after. In particular,
optical interferometers are useless for spectroscopy and are usable for
imaging only on fairly bright objects (because you need a fair number of
photons per second to form detectable interference fringes). For many
astronomical purposes, there is just no substitute for lots of mirror area.

Current preference seems to be to build 2-4 quite large telescopes within
maybe 100m of each other (e.g., the two Kecks and the four-telescope ESO),
so they can be used individually for general-purpose astronomy and
experimentally together for interferometry.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Henry Spencer wrote:
In article . com,
wrote:
As somebody else mentioned, a large baseline optical interferometer
is
likely to give more info for a much smaller price.

No, it depends on what sort of information you are after. In
particular,
optical interferometers are useless for spectroscopy and are usable
for
imaging only on fairly bright objects (because you need a fair number
of
photons per second to form detectable interference fringes). For
many
astronomical purposes, there is just no substitute for lots of mirror
area.

Erm, interferometers are not really "useless" for spectroscopy.
Interferometry makes spectroscopy a whole hell of a lot more
difficult, but it's still doable. If the only way to observe
the target is through deep interferometric nulling, for
example, then you accept the difficulties and move on.
Specifically, what you want is to be able to sample the
configuration space (by rotation the interferometry assembly
along an axis which points at the target, for example) to a
degree that allows you to create a processed, properly
calibrated spectrum that you can trust.

For this specific example, the collection of the spectra relies
on the interferometry. This takes advantage of the deep
nulling ability of interferometry to allow the observation of
very dim targets close to very bright targets (e.g. planets
near stars). However, in general, spectroscopy does not rely
on the features of interferometry, so in most cases spectroscopy
and interferometry live in different worlds. For example, while
interferometry can create higher spatial resolution views, doing
so necessarily lowers the surface brightness "per pixel" of the
image, which is not what you want for spectroscopy (which
almost always prefers more signal over finer spatial coverage).
And, of course, in general the whole idea of interferometry is
to use parlay a small multiple in mirror area (e.g. 2x, 5x)
into a large multiple in resolution through separation (or a
high degree of nulling). So, in general, as has been pointed
out, most spectroscopic targets benefit more from greater
light collection area than from anything else.

Interestingly, there have been a lot of very innovative
alternatives to interferometry in regard to deep nulling and
such like which have been proposed for various projects,
such as terrestrial planet finding, recently. For example,
the leading contender for NASA's TPF mission is, I believe,
a coronograph rather than a nulling interferometer.
Reflecting, I think, the many advances in overall "old
school"ish more or less classic-design type designs which
have taken place in the last decade or so as well as advances
in instrumentation (dynamic range, etc). Compare, for
example, the sophistication and capabilities of, say,
Chandra, WMAP, and Spitzer versus say Einstein, COBE, and
ISO. Where once the TPF specs. were beyond the state of
the art for conventional telescope technologies, those
technologies are starting to catch up to the point where
just a, mostly, plain jane telescope can meet the specs.
As above, one of the great advantages of a coronograph
style design is that it would greatly simplify spectra
aquisition and improve spectra collection throughput.

An even more interesting, and certainly more innovative,
concept would be the gigantic pinhole camera*, which
would have somewhat similar problems to an interferometer
but could offer much deeper nulling and finer resolution
at significantly less cost.


(*)
http://www1.nasa.gov/vision/universe/newworlds/new_worlds_imager.html

Henry Spencer wrote:
There's no reason why you couldn't build an OWL in space, although it
would be an expensive project if it used current infrastructure.

Speaking of big telescopes and space, would operation in space simplify
(if nothing else) the optical design of an OWL-scale segmented
telescope?

Mike Miller

In sci.space.policy wrote:

Henry Spencer wrote:
There's no reason why you couldn't build an OWL in space, although it
would be an expensive project if it used current infrastructure.

Speaking of big telescopes and space, would operation in space simplify
(if nothing else) the optical design of an OWL-scale segmented
telescope?

Yes, for one thing, you can relax the specs on the deformable mirror bits
of the system, as they now only have to cope with surface inaccuracies and
thermal movement, not the atmospheric.
However.
While what's needed of most of the mechanics would be a lot simpler, as
there is less weight load, ..., the overall system would not be.

On an earth based telescope, if mirror positioner 34 dies, you say oh ****,
as you've lost observations for the night, and have to send Harry up to fix
it.

How about this: putting 3 or 4 Hubble-sized (for redundancy & time-use
issues) craft in Jovian Trojan orbits would give you a 10 1/2
AU-equivalent instrument! That would almost see planets in Andromeda!

In article .com,
wrote:
How about this: putting 3 or 4 Hubble-sized (for redundancy & time-use
issues) craft in Jovian Trojan orbits would give you a 10 1/2
AU-equivalent instrument! That would almost see planets in Andromeda!

Only if you could hold the distance between them stable to within a
fraction of a wavelength of light, *and* beam the light gathered by one
to another across that distance without losing much of it. That...
presents problems, to put it mildly.

People are still struggling to make imaging interferometry work well at
distances of a hundred *meters* with both telescopes resting on solid rock.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

In article ,
Henry Spencer wrote:
In article .com,
wrote:
How about this: putting 3 or 4 Hubble-sized (for redundancy & time-use
issues) craft in Jovian Trojan orbits would give you a 10 1/2
AU-equivalent instrument! That would almost see planets in Andromeda!

Only if you could hold the distance between them stable to within a
fraction of a wavelength of light, *and* beam the light gathered by one
to another across that distance without losing much of it. That...
presents problems, to put it mildly.

In principle, there's another option, analogous to the way radio
astronomers do inteferometry with telescopes thousands of miles
apart; it only requires that you be able to record phase information
as well as brightness information.

But while this means recording data at a few gigahertz for 20cm radio
work, this would mean doing so at about 10^14 Hz, which also...
presents problems, to put it mildly. But this particular problem
may be more solvable in the long run.

cheers,

Steven

--


"M-Theory is the unifying pachyderm of the five string theories."
- Brian Greene, _The Elegant Universe_