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Fraser
2003-Jul-13, 03:17 AM
The advances made with ground-based astronomy has been tremendous in the last few years. One technology, called adaptive optics allows a telescope to adjust its mirror to account for variations in the Earth's atmosphere creating images which are much clearer. A second technology is interferometry, where the light from several telescopes is merged together to act as a single, gigantic telescope.

The Gemini telescopes are already producing images that match the Hubble Space Telescope in some situations, and they're way easier to fix and upgrade.

One exception, I think is situations where the Earth's atmosphere blocks the waves entirely, like X-rays, but what about the rest?

Is the age of space-based telescopes over?

awvance
2003-Jul-15, 05:58 PM
I don't think we even begun to us space effectively for celestial observation. I think we would get a lot more observation time if the telescope was in a solar orbit or on the moon.
The moon is such a great platform for so much science. Low earth orbit is the worst place to do science. Itís expensive just to stay in orbit. Youíre flying in the atmosphere and corroding all your equipment with atomic oxygen. Thereís a thousand thing going on in low earth orbit that are contra to the science. Put a telescope on the back side of the moon and just keep it on all the time.

The Bad Astronomer
2003-Jul-15, 06:42 PM
One thing where space beats ground-based is in sky background. Without air, the images taken in space have much lower background light, allowing them in principle to see fainter objects.

There are three (and a half) advantages for space:

1) No turbulent air, so resolution is better. This gap is closing as ground-based instruments improve.

2) Lower sky background, as noted above.

3) The ability to detect wavelengths that are absorbed by air, as Fraser noted too. This should not be discounted; no amount of tech will allow, say, far infrared to get past the CO2 and water in our air! Here is a great place to read about this (http://imagine.gsfc.nasa.gov/docs/science/know_l2/emspectrum.html)!

3.5) Long baselines for interferometry. The bigger the distance between two telescope, the higher the resolution (and smaller objects) which is theoretically possible by combining the data from the two (or more) 'scopes. Having spacecraft a LONG way apart can ideally yield incredible resolution. Many planned space missions are working on this, including the Space Interferometry Mission (http://planetquest.jpl.nasa.gov/SIM/sim_index.html), the Terrestrial Planet Finder (http://planetquest.jpl.nasa.gov/TPF/tpf_index.html), the Laser Interferometry Space Antenna (http://lisa.jpl.nasa.gov/), Constellation-X (http://constellation.gsfc.nasa.gov/), and probably others I am forgetting. :D

Joe
2003-Jul-16, 12:55 PM
The posters have brought up some good points.
I'm a little biased, myself, since HST provided me with more than a few years worth of employment ... :rolleyes:
As wonderful as it's been, HST has was tremendously expensive (over $2 B US, I believe) and late. Very late. (Not all of the delay was due to the project. The Challenger Explosion contributed to the delay). The engineering difficulties were tremendeous, even before the flaw in the mirror was uncovered. HST may be justified as a "full employment act for scientists and engineers", but the nation will do only so much of that without at least the promise of practical results.

From a purely scientific point of view, space based telescopes (we'll include Chandra here) I think have provided something quite different from what was expected. That's a good thing. HST forced us to do science differently; at least, the data gathering and organizing is now done differently. Data mining didn't exist at all when I was scanning Palomar plates for HST guide stars.

Do we need more data? Maybe. For now, only in specific areas.
Do we need more projects, for the sake of the nation? Probably.
Do we need more telescopes? I don't know. Better ones, cheaper ones, surely. The advantages of telescopes in space (over ground based) have decreased, not increased during the lifetime of HST.
Joe

Fraser
2003-Jul-16, 03:34 PM
We need a telescope capable of determining if Earth-sized planets orbiting other stars have life on them. Like the Darwin mission. It would be great if they could build that from the ground or in space, but in my opinion, whatever gets us there first. If I was running NASA/ESA, etc, that would seriously be my top priority - spare no expense!

Duane
2003-Jul-17, 03:10 AM
Another plus to space-based telescopes is their ability to stay on one specific target or area for extended periods, something earth-based telescopes cannot match due to the earth's rotation.

Hopefully the Terrestrial Planet Finder will actually launch somewhere close to the projected launch date. At the least it should provide some new areas for exploration that we haven't even considered yet.

As for low earth orbit--yes it does have some disadvantages, but don't forget the advantages--upgrades, the ability to repair problems, etc. If the HST had been at one of the Lagrange points when the mirror flaw had been discovered--well you get the point.

Joe
2003-Jul-17, 03:23 AM
Duane,
It's not quite so easy. HST cannot view several objects of interest for more than a fraction of an orbit (the Earth itself is in the way for half an orbit!). And even for those objects never obscured by the Earth's disk, HST observations have been interrupted since day one twice every orbit, every time in goes in and out of Earth Shadow, by "thermal snap". That's the effect of the sudden change in temperature when the solar panels go in and out of shadow. At first, the whole thing jiggled for nearly 10 minutes each time, 20 minutes out of each orbit were useless! I'm not sure what the characteristics are for the new set of solar panels are, but one of the reasons they were changed was this thermal snap problem.
Of course, with a space telescope at a lagrange point, that problem goes away.

Joe

Duane
2003-Jul-17, 03:47 AM
That is interesting Joe, never knew that!

Thing is, weren't both the north & south deep space shots a 10 day "photo"?

Joe
2003-Jul-17, 12:37 PM
Duane,
I was writing too late last night. I wasn't very clear!

The "Thermal Snap" problem I noted was due to the solar panels flexing (more like flapping, actually) with the temperature changes. It was a little unexpected by the engineering people, I recall.

Yes, the deep space photos were indeed *very* long exposures. But they were taken over several sessions, not one continuous session. I've always been amazed at how well they can point HST to create multiple exposure observations like that.

It's sure a good thing that CCDs don't suffer from the same kind of "fogging" that photographic film does!

Joe

Duane
2003-Jul-17, 02:23 PM
Thank you for clearing that up :)

Arramon
2003-Jul-17, 06:56 PM
Aren't there talks of relay satellites that may be used to transmit data back towards Earth? Say, if there were some set up in orbit just beyond Mars, or past the asteroid belt farther out...
The pictures and data from combinations of Earth/Moon-based & relay satellites could become invaluable to humankind...
'course... i'm thinking just alittle ahead of our time... but who doesn't? =)
...also... money is always a ?? that sticks upon the mind...
Who would pay for such an endeavour?
Combined resources would be the best bet, i think...
It would be for the benefit of all of us, and our decedants... so like someone else said... Spare no expense!! Just take the steps needed to progress... little steps, big steps... baby steps... leaps!! Anything to advance our knowledge...

I, for one, enjoy ALL of the telescopes currently in use... For, never have i ever been more drawn to space than when i can actually SEE the images that are out there...

Here's a good site to visit... :)

Astronomy Picture of the Day (http://antwrp.gsfc.nasa.gov/apod/archivepix.html)

'til next time!

. ..-={Arramon}=-.. .

PLEIADES 7
2003-Jul-18, 06:40 AM
Originally posted by fraser@Jul 13 2003, 03:17 AM
The advances made with ground-based astronomy has been tremendous in the last few years. One technology, called adaptive optics allows a telescope to adjust its mirror to account for variations in the Earth's atmosphere creating images which are much clearer. A second technology is interferometry, where the light from several telescopes is merged together to act as a single, gigantic telescope.

The Gemini telescopes are already producing images that match the Hubble Space Telescope in some situations, and they're way easier to fix and upgrade.

One exception, I think is situations where the Earth's atmosphere blocks the waves entirely, like X-rays, but what about the rest?

Is the age of space-based telescopes over?
;) Should scientists be able to use interferometry not with 2, but 4or 6 telescopes, or more,we would finally find out what we have been looking for : nothing but the universe, once again ! God knows how hard astronomers work to reach Him by fostering them to go ahead; that is why He is so close to us. :)

Andrew

kashi
2003-Jul-21, 08:50 AM
I sent an e-mail to an astronomer by the name of Michael-Lloyd Hart about adaptive optics. He works for the Center for Astronomical Adaptive Optics in Arizona.

This is what I asked him:



I understand that new Earth based telescopes in the future may be equipped with optical systems that will counteract the ever-changing refraction of the Earth's atmosphere to deliver pictures with far greater resolution than was previously possible using Earth based telescopes. My question is, will this be done digitally (i.e. will images be taken and then manipulated by computer programs) or will the mirror physically move ever so slightly to counteract this distortion. Does the distortion that occurs in our atmosphere occur within a predictable range (like a normal distribution curve or something)? If it did, would it be possible to take say 10000 photos of a particular object, and use a computer program to detect which photos were distorted (and in what way), and to then combine this information to generate an image that was accurate?


Below is his response:



Adaptive optics systems are operating now on a handful of what today counts as a big telescope (8 metres or so) in Hawaii, Arizona, and Chile. Plans for the future have us building telescopes of around 30 m or so, which would certainly be equipped with adaptive optics too. The way it works is that a deformable mirror in the telescope's optical beam train moves in response to signals from a gadget called a wave-front sensor, which is basically a box of optics with a detector looking at a star. We know what the image of the star ought to look like; we compare that to what it does look like instantaneously, and from the difference compute how to bend the mirror. The mirror is bent so that it takes an equal and opposite shape to the optical distortion introduced in the starlight by the atmosphere. All this has to be done very fast - the atmosphere changes, and the mirror must be updated about 1000 times per second to keep track. Stars by themselves are generally not very interesting astronomical objects (my stellar colleagues might disagree!) but by correcting the light from the star, objects close to it in the sky will be seen with greatly improved resolution too.

Typical motions of the mirror are only a few microns, but the resulting improvement in the sharpness of the telescope's images can be a factor of ten or more, so it's a hot topic in the community. The atmospheric distortion is governed by statistical processes that are reasonably (but imperfectly) understood. The theory of turbulent fluctuations in an unconstrained medium was worked out decades ago by two Russians - Tatarskii and Kolmogorov - and that work is usually relied upon be the designers of adaptive optics. It says that the amplitude of refractive index fluctuations in the air (responsible for image degradation) entrained by the turbulence will follow a specific power law as a function of spatial scale.

There is also a very active industry in post-detection image reconstruction. This is less useful in one crucial way to astronomers though: once light from an object has been recorded on a detector, there is no way to remove the effects of noise in the detection process. Noise comes from two main sources: photon noise, described by quantum mechanics, and read noise associated with thermal effects in the detector material. By concentrating the light into a sharp image _before_ detection, adaptive optics offers a less noisy way to achieve high resolution than post-detection image processing. That translates into an ability to detect fainter objects.

There are also groups that do the "cherry picking" you describe: take a lot of short exposures, and combine the best of them into a high-quality image. That approach though suffers from a severe faintness limit too.

Kashi

jsc248
2003-Jul-21, 11:55 AM
<_< Hi Fraser,
I can well understand the point you are making and yes ground based telescopes through advanced optics and interferometry are making remarkable progress. Why has this not been taken to the space level yet? If ground based telescopes are linked together to form one big optical base line, why not another hubble space telescope linked to the existing one to make the biggest interferometer yet. The images acheived by such a huge space based distance line, free of atmospheric disturbance, would surely be worth the money spent on such a venture. Does any one else agree.
jsc248

kashi
2003-Jul-21, 01:30 PM
Space based inferometry using two or more satellites would be hugely expensive because of the difficulty in keeping track of their positions with sufficient precision. Such space based inferometers are planned. Darwin for instance, which will orbit Jupiter I think (to be further from our sun&#39;s glare).

It&#39;s all a matter of money. The costs of building larger mirrors fitted with adaptive optics are still probably much less then sending a smaller mirror (or two or more as you suggest) into space.

Kashi

anjulpa
2003-Jul-22, 06:04 AM
why not another hubble space telescope linked to the existing one to make the biggest interferometer yet


the biggest interferometer has been built with SOHO and an earth based telescope
srry &#33; ;)

Adian
2003-Jul-22, 10:36 PM
With our filthy atmosphere getting worse every decade that passes, itís a good idea to keep more than one egg in the basket.

setiman
2004-Oct-16, 10:53 AM
;) My position on space telescopes is already over-stated here, but as to expense for more advanced space telescopes that are unfettered (outside Earth orbit), it would be easily doable if we took just half of the funding we waste annually in this country and applied it to this effort. So, it is really a matter of priorities rather than dollars.

Most all of our astro-physical research is international in nature, including the ground based systems, but HST was the first to really open the door much wider for combined intellectual efforts. We need to continue and to expand this accord and opportunity. It is is one tenous but important thread that holds us all together.

A short word about wasted funds and priorities. It is hard to be objective when you are hungry and right now so much of our planet is hungry. We need to balance solutions of that problem with support of expanded astro-physics research. We gain little if we discover a second Earth and there is no one here to cheer&#33; :)

Arramon
2004-Oct-18, 02:39 PM
Whoa.. out of the dust comes this thread..&#33; ;)

But, it seems the same matters still persist, even today.
During the debates, I heard narely a word concerning programs other than those about reform and terror and what this person voted on or what that person voted on...

What about the real matters at hand? Like how are humans going to progress smoothly into the next millenium? Stuck in terror, wars, oil price hikes, famine, pollution, poverty... or will we move into another era of innovative progression that steps us beyond the above woes...?

To me it seems our government (and others) don&#39;t really concern themselves with the total picture... Why are we still fighting over land? Oil? Religion? Why is half the world still living in the 2nd century? I thought our purpose was to explore and learn? Not nurture our children with thoughts of violence, sex, immorality and hatred... ? ? ? What the heck happened to our Money is no longer needed, Food is free for everyone, and there&#39;s no more poverty...?

Science seems to be the only thing making leaps right about now. So maybe they (governments) should put more effort into helping fund new and innovative ways of healing the world by expanding our knowledge of Universe, discovering new cures, medicines, blah etc yata yata... why is my car still not flying?

"You need land of your own? Here&#33; Take some on the moon, mars, titan, europa, pluto even..&#33; Make yourself comfortable.. JUST STOP FIGHTING FOR IT&#33; (Oh.. and you&#39;ll need this to breathe.. ;))"

On topic:
We need satellites because they help us to see what&#39;s really there. And if we could have a thousand satellites conveyed into a net structure throughtout the L2 region of our solar system (or somewhere out there), then we could truelly map our home here in space. And keep track of any *ahem* flying saucers that may be lurking about. :roll eyes:

*shivers*

burrrr.. it&#39;s getting cold&#33;

Happy Halloween.. =)