Best Place for a (Really Big) Telescope?

Adaptive optics have allowed Earth-based telescopes to rival the
performance of space telescopes in some respects. But it is still true
that this is an imperfect measure, and it certainly can't do anything
about the fact that some wavelengths are blocked by Earth's atmosphere.

Placing a telescope in Earth orbit gains the advantages of full access
to all forms of starlight in the vacuum of space. But a telescope in
space has to be positioned either using flywheels or through the
consumption of propellant. Avoiding vibration, therefore, is a critical
element of space telescope design.

It's been suggested that a better place for a really *large* telescope
would be the surface of the Moon. Unlike a space telescope, one does
have the disadvantage of only seeing half the sky at one time. But one
has the entire Moon beneath the telescope, so it can just be pointed
from place to place with electric motors, in the same manner as an
Earth-based telescope. However, the Moon's gravity, at one-sixth that of
Earth, still sets some limit as to how large a telescope can be made.

Thus, if one wanted to design a telescope with perhaps (near) infrared,
optical, and (near) ultraviolet detectors, that was in the 1200-inch (or
30 metre) class, one way to avoid the need of consumables for
positioning, but not have a gravity like the Moon's to contend with,
would be to place it on an asteroid.

But what asteroid? An Earth-crossing asteroid, or one in the asteroid
belt, might hit something. Using hydrogen bombs to change the path of an
asteroid, so it would be captured by the Moon as an Earth satellite
would be ideally convenient, but too demanding; it might be done later,
but that capability would not be available in time for the *first* space
telescope in this class.

As it happens, there are already small asteroid-sized objects in a nice
stable orbit around a planet. The planet isn't Earth, but it is nearby.
And a larger orbit than that of the Earth is a nice plus for more
accurate stellar parallaxes.

Deimos, in addition to being the moon furthest from Mars, and thus more
completely avoiding its atmosphere, is also smaller, thus it seems to be
clearly the best choice to be home to a giant space telescope. It is
tidally locked to Mars, just as our own Moon is to Earth, with a
sidereal rotation period of just over 30 1/2 hours. (As this is slightly
over the length of a Martian day, it moves very slowly through the
Martian sky.) That is a bit of a disadvantage, since such a telescope
would have to slew about as rapidly as an Earth-based telescope to
follow any given celestial object, unlike one on the Moon, which has a
sidereal rotation period of about 27 1/3 days, or a space telescope,
where the issue does not arise.

Perhaps, further in the future, a good site for a telescope in the Solar
System would by Iapetus. It has a radius of 718 km, so it is still much
smaller than Earth's Moon. It is 3.5 million km away from Saturn; this
should put it outside Saturn's radiation belts, and, as it is
tidally-locked to Saturn, this gives it a long rotational period of 79
1/3 days.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://www.quadibloc.com/index.html
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In article ,
lid says...
Adaptive optics have allowed Earth-based telescopes to rival the
performance of space telescopes in some respects. But it is still true
that this is an imperfect measure, and it certainly can't do anything
about the fact that some wavelengths are blocked by Earth's atmosphere.

Placing a telescope in Earth orbit gains the advantages of full access
to all forms of starlight in the vacuum of space. But a telescope in
space has to be positioned either using flywheels or through the
consumption of propellant. Avoiding vibration, therefore, is a critical
element of space telescope design.

It's been suggested that a better place for a really *large* telescope
would be the surface of the Moon. Unlike a space telescope, one does
have the disadvantage of only seeing half the sky at one time. But one
has the entire Moon beneath the telescope, so it can just be pointed
from place to place with electric motors, in the same manner as an
Earth-based telescope. However, the Moon's gravity, at one-sixth that of
Earth, still sets some limit as to how large a telescope can be made.

Thus, if one wanted to design a telescope with perhaps (near) infrared,
optical, and (near) ultraviolet detectors, that was in the 1200-inch (or
30 metre) class, one way to avoid the need of consumables for
positioning, but not have a gravity like the Moon's to contend with,
would be to place it on an asteroid.

But what asteroid? An Earth-crossing asteroid, or one in the asteroid
belt, might hit something. Using hydrogen bombs to change the path of an
asteroid, so it would be captured by the Moon as an Earth satellite
would be ideally convenient, but too demanding; it might be done later,
but that capability would not be available in time for the *first* space
telescope in this class.

As it happens, there are already small asteroid-sized objects in a nice
stable orbit around a planet. The planet isn't Earth, but it is nearby.
And a larger orbit than that of the Earth is a nice plus for more
accurate stellar parallaxes.

Deimos, in addition to being the moon furthest from Mars, and thus more
completely avoiding its atmosphere, is also smaller, thus it seems to be
clearly the best choice to be home to a giant space telescope. It is
tidally locked to Mars, just as our own Moon is to Earth, with a
sidereal rotation period of just over 30 1/2 hours. (As this is slightly
over the length of a Martian day, it moves very slowly through the
Martian sky.) That is a bit of a disadvantage, since such a telescope
would have to slew about as rapidly as an Earth-based telescope to
follow any given celestial object, unlike one on the Moon, which has a
sidereal rotation period of about 27 1/3 days, or a space telescope,
where the issue does not arise.

Perhaps, further in the future, a good site for a telescope in the Solar
System would by Iapetus. It has a radius of 718 km, so it is still much
smaller than Earth's Moon. It is 3.5 million km away from Saturn; this
should put it outside Saturn's radiation belts, and, as it is
tidally-locked to Saturn, this gives it a long rotational period of 79
1/3 days.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://www.quadibloc.com/index.html
_________________________________________
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It could be used for a very small part of the sky and it would be a very
distorted image but what you might want to consider is that it is
possible to use the sun as a lens. A probe in position say at 1000 AU
(way beyond Pluto) could give us resolution greater then a million times
better then our best telescopes!

--
It takes a lot of study to figure out how Euclid's geometry is based on
*self-evident* truths.

Observations of Bernard - No 90

Wasn't it John Savard who wrote:

But what asteroid? An Earth-crossing asteroid, or one in the asteroid
belt, might hit something.

The probability of a telescope on the Earth, on the Moon, or in Earth
orbit being destroyed by asteroid impact is greater than the probability
of a telescope mounted on a near Earth asteroid being destroyed by the
asteroid hitting something.

The difference is that the Earth and Moon have gravity fields which can
convert an encounter that would have been a near miss into a hit. If it
were not for that effect, a telescope on a near Earth asteroid would
have the same hit probability as one on the Earth or Moon.

A telescope on a main-belt asteroid would have a higher hit probability
because there are more objects out there.

A telescope on Phobos or Deimos would have a higher asteroid hit
probability because Mars is closer to the main asteroid belt.

In any case, these hit probabilities are really tiny, and not worth
bothering about.

--
Mike Williams
Gentleman of Leisure

Put the telescope on the moon. Someone mentioned gravity limiting size.

Thats not really a issue,

Just like in Chile. Use hundreds of small scopes, that once forcused
electronically are the equivalent of a solid one. perhaps inside a big
crater.

this could probably be done in space, but control over pointing would
be a killer

"John Savard" wrote in message
...

Placing a telescope in Earth orbit gains the advantages of full access
to all forms of starlight in the vacuum of space. But a telescope in
space has to be positioned either using flywheels or through the
consumption of propellant. Avoiding vibration, therefore, is a critical
element of space telescope design.

It's been suggested that a better place for a really *large* telescope
would be the surface of the Moon.

I think it's funny when I see people say a telescope should be placed on the
moon rather than in orbit and profess to a concern about the propellant
requirements for pointing the telescope. What about the propellant
requirements for a TLI burn, the braking into low lunar orbit, the braking
of that orbit down to the surface, to hover, and then to soft-land? And not
just once one-way for the telescope itself, but both ways every time anyone
needs to visit the telescope, either to repair a component or to upgrade a
system?

To see someone fuss about the few pounds of propellant needed to point here
and there, while at the same time considering the many tons of propellant
needed for all the above to be no big deal, suggests to me more that they're
pulling for the telescope to go on the moon because they desire lunar
development for it's own sake.

--


Regards,
Mike Combs
----------------------------------------------------------------------
Member of the National Non-sequitur Society. We may not make
much sense, but we do like pizza.

wrote in message
oups.com...

this could probably be done in space, but control over pointing would
be a killer

Why a killer? NASA has been successfully using orbital telescopes for many
years now.

--


Regards,
Mike Combs
----------------------------------------------------------------------
Member of the National Non-sequitur Society. We may not make
much sense, but we do like pizza.

On Mon, 12 Dec 2005 13:15:07 -0600, "Mike Combs"
wrote, in part:

I think it's funny when I see people say a telescope should be placed on the
moon rather than in orbit and profess to a concern about the propellant
requirements for pointing the telescope. What about the propellant
requirements for a TLI burn, the braking into low lunar orbit, the braking
of that orbit down to the surface, to hover, and then to soft-land? And not
just once one-way for the telescope itself, but both ways every time anyone
needs to visit the telescope, either to repair a component or to upgrade a
system?

There's a distinction between capital costs and operating expenditures.

I should have perhaps explicitly noted that I was thinking in terms of a
design with an MTBF of a thousand years or so. Repairs and upgrades,
therefore, would not be an issue, whereas propellant over the life of
the telescope would be.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://www.quadibloc.com/index.html
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John Savard wrote:

Adaptive optics have allowed Earth-based telescopes to rival the
performance of space telescopes in some respects. But it is still true
that this is an imperfect measure, and it certainly can't do anything
about the fact that some wavelengths are blocked by Earth's atmosphere.

Placing a telescope in Earth orbit gains the advantages of full access
to all forms of starlight in the vacuum of space. But a telescope in
space has to be positioned either using flywheels or through the
consumption of propellant. Avoiding vibration, therefore, is a critical
element of space telescope design.

It is anyway. You have just as many problems with vibration on the
surface of an object as you do in space.

The proper place for a really large telescope is in space, where there
are no size limits whatsoever. Even on a small moon or asteroid, the
telescope can't be larger than the object it's on!

A good, quiet place where the telescope won't get human interference and
won't get in the way, but still is relatively easily accessible, would
be the Earth-Moon L2 point. If the entire Solar System is accessible,
then just put it in distant orbit around one of the larger gas giants,
or just in solar orbit not near any colonized planets or asteroids

--
Erik Max Francis && && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
And I'd rather be damned if I don't.
-- Robert S. MacNamara

: Erik Max Francis
: It is anyway. You have just as many problems with vibration on the
: surface of an object as you do in space.
: The proper place for a really large telescope is in space, where there
: are no size limits whatsoever. Even on a small moon or asteroid, the
: telescope can't be larger than the object it's on!

Sure it can, if the object has small enough gravitation.
But of course, then the object is more aptly said to be
attached to the telescope than the other way 'round.

But... why wouldn't a large doughy mass help with damping vibration?


Wayne Throop http://sheol.org/throopw

One of the best places is next to a space habitat
this way propellant, instruments and coatings can be maintained easily
and if the thing were to be scrapped, then at least it could be easily used
for say, reaction mass, or comms

another place is anywhere in space, perhaps L2, provided the thing can be
robotically serviced

of course this would fly in the face of the current practice of planned
obsolescence of one off jewells,
which throws away perfectly serviceable equipment for the sake of political
expediency and jobs programs

Some of the best American inventions - the production line, and real
entrepreneurial spirit, have been ignored when it comes to the exploitation
of space, to keep the porkbarrel rolling for the big corps

the current 'exploration' of space is merely the exploitation of the
taxpayer, to keep the neo-feudalist corporations and the political numbers
men happy

"John Savard" wrote in message
...
Adaptive optics have allowed Earth-based telescopes to rival the
performance of space telescopes in some respects. But it is still true
that this is an imperfect measure, and it certainly can't do anything
about the fact that some wavelengths are blocked by Earth's atmosphere.

Placing a telescope in Earth orbit gains the advantages of full access
to all forms of starlight in the vacuum of space. But a telescope in
space has to be positioned either using flywheels or through the
consumption of propellant. Avoiding vibration, therefore, is a critical
element of space telescope design.

It's been suggested that a better place for a really *large* telescope
would be the surface of the Moon. Unlike a space telescope, one does
have the disadvantage of only seeing half the sky at one time. But one
has the entire Moon beneath the telescope, so it can just be pointed
from place to place with electric motors, in the same manner as an
Earth-based telescope. However, the Moon's gravity, at one-sixth that of
Earth, still sets some limit as to how large a telescope can be made.

Thus, if one wanted to design a telescope with perhaps (near) infrared,
optical, and (near) ultraviolet detectors, that was in the 1200-inch (or
30 metre) class, one way to avoid the need of consumables for
positioning, but not have a gravity like the Moon's to contend with,
would be to place it on an asteroid.

But what asteroid? An Earth-crossing asteroid, or one in the asteroid
belt, might hit something. Using hydrogen bombs to change the path of an
asteroid, so it would be captured by the Moon as an Earth satellite
would be ideally convenient, but too demanding; it might be done later,
but that capability would not be available in time for the *first* space
telescope in this class.

As it happens, there are already small asteroid-sized objects in a nice
stable orbit around a planet. The planet isn't Earth, but it is nearby.
And a larger orbit than that of the Earth is a nice plus for more
accurate stellar parallaxes.

Deimos, in addition to being the moon furthest from Mars, and thus more
completely avoiding its atmosphere, is also smaller, thus it seems to be
clearly the best choice to be home to a giant space telescope. It is
tidally locked to Mars, just as our own Moon is to Earth, with a
sidereal rotation period of just over 30 1/2 hours. (As this is slightly
over the length of a Martian day, it moves very slowly through the
Martian sky.) That is a bit of a disadvantage, since such a telescope
would have to slew about as rapidly as an Earth-based telescope to
follow any given celestial object, unlike one on the Moon, which has a
sidereal rotation period of about 27 1/3 days, or a space telescope,
where the issue does not arise.

Perhaps, further in the future, a good site for a telescope in the Solar
System would by Iapetus. It has a radius of 718 km, so it is still much
smaller than Earth's Moon. It is 3.5 million km away from Saturn; this
should put it outside Saturn's radiation belts, and, as it is
tidally-locked to Saturn, this gives it a long rotational period of 79
1/3 days.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://www.quadibloc.com/index.html
_________________________________________
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More than 140,000 groups
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