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TBerk wrote: > Seems like a good idea off the top of my head. Lets see:
> - Barely any atmospheric interference.
Check.
> - 'Dark Side' of the Moon will have less Earthshine to deal with.
?? But still sunshine. Well suited for radio astronomy, however.
> - Lesser gravity allows for different construction and perhaps larger > optical elements.
Check.
> - Lunar material could be used for building stuffs, given a decent > smelter.
Bzzzzzzt! Wrong answer. That answer calls for hundreds or possibly thousand of humans and dozens of factories and millions of tons of support for those humans. That's the SF way. Cost: incalculably large. Relatively simple robots do the work with materials prefabricated on the Earth.
> We could collect the images 'locally' and beam them back to Earth via > laser (or microwave or whatever).
Check. We do have some considerable experience sending imagery across long distances. I vote for radio so that we can get our images on cloudy days as well as sunny :-)
> There could even be a superbig Lunar version of the Arecibo Radio > Telescope (if you saw the movie 'Contact'...) that was built in Puerto > Rico.
Do you mean the one they're getting ready to shut down for lack of funding? That doesn't bode well for spending a trillion $ or so to build observatories on the Moon, does it?
> Just blue-sky daydreaming, but sketching out some brainstorms might > lead somewhere, someday.
Someday. Maybe.
Davoud
-- I agree with almost everything that you have said and almost everything that you will say in your entire life.
On Sat, 21 Nov 2009 23:48:32 -0800 (PST), TBerk <bayareab...@yahoo.com> wrote:
>Seems like a good idea off the top of my head. Lets see:
A radio telescope on the lunar farside would seem to make a lot of sense. I can't see that the Moon really provides any advantages for optical telescopes that can't be realized much cheaper from a space-based platform, though. For that matter, it probably doesn't provide such a huge advantage over Earth-based optical telescopes, which can be made arbitrarily large fairly easily, can be maintained and upgraded easily, and are increasingly able to cancel most of the effects of the atmosphere (the highest resolution images today come from ground-based telescopes, not the HST.) _________________________________________________
In article <221120091233039915%s...@sky.net>, Davoud <a...@bbb.ccc> wrote: >TBerk wrote: >> There could even be a superbig Lunar version of the Arecibo Radio >> Telescope (if you saw the movie 'Contact'...) that was built in Puerto >> Rico.
>Do you mean the one they're getting ready to shut down for lack of >funding? That doesn't bode well for spending a trillion $ or so to >build observatories on the Moon, does it?
There is an Arecibo successor being built in central China at the moment, in a natural depression of the same sort of shape as the Arecibo one; called the Five Hundred Metre Spherical Telescope.
Perhaps it too will be shut down for lack of funding in another fifty years.
On Sun, 22 Nov 2009 18:31:11 -0800 (PST), palsing <pnals...@gmail.com> wrote:
>> The "dark" side gets more light than the side that faces us.
>OK, I'll bite.
>This is not intuitive to me. Perhaps you could provide a little >explanation.
It's not such an easy analysis. The near side is occasionally in the Earth's shadow, which would bring down the overall average amount of light for that side. But the far side never sees Earthshine, which would reduce its total light.
Realistically, both effects are going to be negligible compared with the total light received from the Sun, I would think. _________________________________________________
Chris L Peterson wrote: > On Sat, 21 Nov 2009 23:48:32 -0800 (PST), TBerk <bayareab...@yahoo.com> > wrote:
>> Seems like a good idea off the top of my head. Lets see:
> A radio telescope on the lunar farside would seem to make a lot of > sense.
Only a very large one working at frequencies where the Earth is radio bright at due to artificial emissions. There are enough gaps in the radio spectrum window for most astrophysics to be done from the ground.
Although whenever a new spectral gap opens even for an instant astronomers will have a look in it to see if there are any new discoveries to be had. This is occurring in the switchover to DTV as some analogue frequencies are freed up and unoccupied for a while. eg
> I can't see that the Moon really provides any advantages for > optical telescopes that can't be realized much cheaper from a > space-based platform, though. For that matter, it probably doesn't > provide such a huge advantage over Earth-based optical telescopes, which
There is an advantage to space based scopes in that they can use wavelengths that are not available on the ground.
> can be made arbitrarily large fairly easily, can be maintained and > upgraded easily, and are increasingly able to cancel most of the effects > of the atmosphere (the highest resolution images today come from > ground-based telescopes, not the HST.)
Increasingly light grasp for spectroscopy is more important than resolution as such. So big scopes will always be in demand.
In article <udsOm.37377$6c2.25...@newsfe03.iad>, Martin Brown <|||newspam...@nezumi.demon.co.uk> wrote:
>Increasingly light grasp for spectroscopy is more important than >resolution as such. So big scopes will always be in demand.
A lot depends on the source; if you're hunting planets, a light-bucket is perfect and you can spend all your money on the spectrometer. But there's also interesting science to be had in very crowded fields (the Galactic centre most obviously), where you might want to get distinct spectra for each of a hundred stars at the core of omega Centauri to see what the dynamics are like, and for that you need resolution and spectral grasp.
There are some interesting recent arxiv papers doing dynamics on M31's satellites by using multi-object spectrographs on a sample of several hundred red giants.
(out of curiosity, who here reads arxiv astro-ph? A lot of it is some way over my head, and some of the applied-relativity is miles over my head, but I can pretty well understand the exoplanet papers now; I suppose it's my main amateur astronomic activity, I have no car and my neighbours recently put in bright security lights)
_ wrote: > On Mon, 23 Nov 2009 03:01:49 GMT, Sam Wormley wrote: >> A spinning orb in the sunlight has a side (other than the poles) >> that gets more light? Care to explain?
> Lunar eclipses; though Peterson's (valid) point means that more strictly > speaking the dark side gets more SUNlight.
Somehow, I can't help but think you learned something new (under the sun)! Maybe not--Far Side not Dark Side! <smiling>
On Nov 22, 11:10 am, Chris L Peterson <c...@alumni.caltech.edu> wrote:
> I can't see that the Moon really provides any advantages for > optical telescopes that can't be realized much cheaper from a > space-based platform, though.
There is one advantage nobody mentioned so far. When you point the telescope somewhere, all you have to do is point the telescope. You don't have to spin up big flywheels, and eventually use up propellant to keep things in balance.
Having the ground under one's feet is a convenience.
On Tue, 24 Nov 2009 18:48:51 -0800 (PST), Quadibloc <jsav...@ecn.ab.ca> wrote:
>There is one advantage nobody mentioned so far. When you point the >telescope somewhere, all you have to do is point the telescope. You >don't have to spin up big flywheels, and eventually use up propellant >to keep things in balance.
>Having the ground under one's feet is a convenience.
I'd say having the ground under one's feet is a huge inconvenience when it comes to telescopes. On the ground, a huge part of the telescope is a complex and expensive mounting and pointing system. In space, a flywheel and propellant system is comparatively simple. Sure, you need propellant, although there are systems that can last as long as the instrument is designed for.
All you have to do with any system is just "point the telescope". That's a LOT easier with a space-based platform. _________________________________________________
However, it's a whole lot easier to fix hardware when it's on the ground. Reaction wheels eventually fail (Cassini is keeping a close eye on theirs, and even Hubble has had its replaced), and of course thrusters eventually run out of fuel.
But you don't have flexure in space, and you don't have seeing to contend with. Nor atmospheric absorption at UV and most IR wavelengths.
But you also can't launch huge light buckets, at least not yet, and we've got adaptive optics.
I have to conclude that both ground- and spaced-based scopes have their place.
As for lunar -- the original topic of this thread -- it seems to me that you'd have all the problems of space, *and* most of the problems of ground, plus others (moon dust anyone?), and the only real benefits would be a longer tracking time and lack of atmosphere.
Chris L Peterson wrote: > On Tue, 24 Nov 2009 18:48:51 -0800 (PST), Quadibloc <jsav...@ecn.ab.ca> > wrote:
>> There is one advantage nobody mentioned so far. When you point the >> telescope somewhere, all you have to do is point the telescope. You >> don't have to spin up big flywheels, and eventually use up propellant >> to keep things in balance.
>> Having the ground under one's feet is a convenience.
> I'd say having the ground under one's feet is a huge inconvenience when > it comes to telescopes. On the ground, a huge part of the telescope is a > complex and expensive mounting and pointing system. In space, a flywheel > and propellant system is comparatively simple. Sure, you need > propellant, although there are systems that can last as long as the > instrument is designed for.
> All you have to do with any system is just "point the telescope". That's > a LOT easier with a space-based platform. > _________________________________________________
Quadibloc wrote: > On Nov 22, 11:10 am, Chris L Peterson <c...@alumni.caltech.edu> wrote: >> I can't see that the Moon really provides any advantages for >> optical telescopes that can't be realized much cheaper from a >> space-based platform, though.
> There is one advantage nobody mentioned so far. When you point the > telescope somewhere, all you have to do is point the telescope. You > don't have to spin up big flywheels, and eventually use up propellant > to keep things in balance.
> Having the ground under one's feet is a convenience.
It is also a huge inconvenience in that to keep the optical figure good at all elevations requires very complex engineering. Radiotelescope dishes do it with a design that stays parabolic but changes focal length with altitude. This doesn't hurt interferometry.
Optical wavelengths and finer require very rigid supports and or clever active compensation for the changing direction of gravity as the scope tracks. It is a huge advantage for a big scope to be weightless.
In article <Q5iPm.36778$ky1.26...@newsfe14.iad>, Martin Brown <|||newspam...@nezumi.demon.co.uk> wrote:
> Optical wavelengths and finer require very rigid supports and or clever > active compensation for the changing direction of gravity as the scope > tracks. It is a huge advantage for a big scope to be weightless.
I was going to post something of that form, but I was concerned that the advantage only appeared if you were able to build the scope in weightlessness; any scope that you build in the Lockheed Martin or Bell Aerospace clean-room has to be able to hold together in 1G for years as you build it (indeed, to hold together for a few minutes at 6G as the Ariane goes up), and I suspect the documentation for the testing gets an awful lot lighter if the scope is able to focus correctly in a 1G environment at at least one orientation, and lighter still if you manage more than one.
JWST has active optics (little motors behind the mirror segments) and the task of getting the mirror segments phased seems quite a difficult one; I suspect any large space scope really wants to have active optics, partly so that you can test it with gravity correction and partly so that you can afford to make the mirror light and floppy to get a bigger mirror to fit in the very finite payload allowance.
On 26 Nov 2009 00:33:01 +0000 (GMT), Thomas Womack
<twom...@chiark.greenend.org.uk> wrote: >JWST has active optics (little motors behind the mirror segments) and >the task of getting the mirror segments phased seems quite a difficult >one; I suspect any large space scope really wants to have active >optics, partly so that you can test it with gravity correction and >partly so that you can afford to make the mirror light and floppy to >get a bigger mirror to fit in the very finite payload allowance.
You still avoid dealing with a mount that outmasses the telescope itself. And designing a scope that operates in a microgravity environment has got to be simpler than one that operates at one (or one-sixth) G, even if it has higher storage and transport requirements. _________________________________________________
On Wed, 25 Nov 2009 09:37:37 -0800, Bill Owen <w...@jpl.nasa.gov> wrote: >I have to conclude that both ground- and spaced-based scopes have their >place.
>As for lunar -- the original topic of this thread -- it seems to me that >you'd have all the problems of space, *and* most of the problems of >ground, plus others (moon dust anyone?), and the only real benefits >would be a longer tracking time and lack of atmosphere.
Agreed. Pretty much what I said in my initial post <g>. _________________________________________________
> You still avoid dealing with a mount that outmasses the telescope > itself. And designing a scope that operates in a microgravity > environment has got to be simpler than one that operates at one (or > one-sixth) G, even if it has higher storage and transport requirements.
Chris, do you or anyone know how space telescopes are guided when taking deep-space photographs?
Controlling accuracy with an equatorial mount for very long photos seems to be a challenge to amateurs. So, suppose the Hubble needs to take a 1-2 hour worth of photons of a DSO.
What is the procedure for moving, centering and then keeping it on target without the ground as a reference point?
On Thu, 26 Nov 2009 06:25:15 +0200, "I.N. Galidakis"
<morph...@olympus.mons> wrote: >What is the procedure for moving, centering and then keeping it on target >without the ground as a reference point?
Actually, the HST uses the same reference that ground-based telescopes do: guide stars. The telescope is positioned by rotating about its CG using reaction wheels- flywheels on motors. That's a pretty simple system. To manage the reaction wheels (you don't want them to get up to their maximum speed, or you lose control) magnetic torquers are used. These operate against the Earth's magnetic field, and can provide a low but continuous torque to offset the reaction wheels. Again, a simple system. It works as long as the mechanics last and the system has electricity. There is no dependence on chemical propulsion.
I don't mean to trivialize the system- certainly there is a complex control system to manage the feedback from gyroscopes and the star tracker, and to drive the reaction wheels and magnetic torquers. And the mechanical and electronic redundancy is sophisticated. But these systems nevertheless represent very mature, safe technology, which is relatively easy and inexpensive to implement (as much as anything space-based can be easy or inexpensive <g>). _________________________________________________
Thomas Womack wrote: > In article <Q5iPm.36778$ky1.26...@newsfe14.iad>, > Martin Brown <|||newspam...@nezumi.demon.co.uk> wrote:
>> Optical wavelengths and finer require very rigid supports and or clever >> active compensation for the changing direction of gravity as the scope >> tracks. It is a huge advantage for a big scope to be weightless.
> I was going to post something of that form, but I was concerned that > the advantage only appeared if you were able to build the scope in > weightlessness; any scope that you build in the Lockheed Martin or > Bell Aerospace clean-room has to be able to hold together in 1G for > years as you build it (indeed, to hold together for a few minutes at > 6G as the Ariane goes up), and I suspect the documentation for the > testing gets an awful lot lighter if the scope is able to focus > correctly in a 1G environment at at least one orientation, and lighter > still if you manage more than one.
ISTR that most big space telescope mirror designs will only behave as correctly figured on Earth when pointing vertically upwards on appropriate support structures (and not on the flight structures). There is a big conflict between minimum weight and optimum rigidity. Part of what caught out the HST was that there was no possibility for a full system imaging test on the ground. Which was doubly ironic as the defects in the primary mirror as manufactured would easily have been seen.
> JWST has active optics (little motors behind the mirror segments) and > the task of getting the mirror segments phased seems quite a difficult > one; I suspect any large space scope really wants to have active > optics, partly so that you can test it with gravity correction and > partly so that you can afford to make the mirror light and floppy to > get a bigger mirror to fit in the very finite payload allowance.
Given that some of the more exciting experiments are now looking for planets near stars then being able to put a deep null on the stars location by tweaking the mirrors phase contributions becomes useful.
On Thu, 26 Nov 2009 06:25:15 +0200, "I.N. Galidakis"
<morph...@olympus.mons> wrote: >What is the procedure for moving, centering and then keeping it on target >without the ground as a reference point?
Spacecraft have used stars for orientation for decades. The HST Guide Star Catalog was created for pointing the Space Telescope. The Hubble uses gyroscope-like electrically powered reaction wheels to point the telescope. They use conservation of angular momentum to skew the telescope and maintain position once on target. The telescope's frame of reference is the telescope itself.
Terrestrial observatories also are oriented to the stars rather than the ground. The ground just supports the telescopes.
> Seems like a good idea off the top of my head. Lets see:
> - Barely any atmospheric interference.
> - 'Dark Side' of the Moon will have less Earthshine to deal with.
> - Lesser gravity allows for different construction and perhaps larger > optical elements.
> - Lunar material could be used for building stuffs, given a decent > smelter.
> We could collect the images 'locally' and beam them back to Earth via > laser (or microwave or whatever).
> There could even be a superbig Lunar version of the Arecibo Radio > Telescope (if you saw the movie 'Contact'...) that was built in Puerto > Rico.
> Just blue-sky daydreaming, but sketching out some brainstorms might > lead somewhere, someday.
> berk
Fine and dandy for rad-hard robotics. However, why would the extremely electrostatic charged moon offer any less radiation than our GSO of <2e3 Sv/year while shielded by 5/16" aluminum?
> On Nov 21, 11:48 pm, TBerk <bayareab...@yahoo.com> wrote:
> > Seems like a good idea off the top of my head. Lets see:
> > - Barely any atmospheric interference.
> > - 'Dark Side' of the Moon will have less Earthshine to deal with.
> > - Lesser gravity allows for different construction and perhaps larger > > optical elements.
> > - Lunar material could be used for building stuffs, given a decent > > smelter.
> > We could collect the images 'locally' and beam them back to Earth via > > laser (or microwave or whatever).
> > There could even be a superbig Lunar version of the Arecibo Radio > > Telescope (if you saw the movie 'Contact'...) that was built in Puerto > > Rico.
> > Just blue-sky daydreaming, but sketching out some brainstorms might > > lead somewhere, someday.
> > berk
> Fine and dandy for rad-hard robotics. However, why would the > extremely electrostatic charged moon offer any less radiation than our > GSO of <2e3 Sv/year while shielded by 5/16" aluminum?
> ~ BG- Hide quoted text -
> - Show quoted text -
Brad Why don't you discuss the evidence now available showing how wrong you are on the moon landings
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