radio telescopes

[g99]

Several new radio telescopes are coming online that are gigantic. Some are built into vallys and others are stung up along whole regions. How does the size of a radio telescope matter? Would a large one not get the same signal strength as a small one since radio waves propogate in all directions at the same strength?

Also to send out a signal does size matter more or power? Or both? Like with the same amount of power, would a radiotelescope with a dish 1 km wide or 100 m wide send out a signal of greater strength?

Sorry for all the questions to answer at once.


The reason i am asking is that i am reading the new Area 51 book. In the book, the aliens build a gigantic radio telescope out of Olympus mons on Mars. They say that it is 100 times larger than the Arecibo telescope.
They say that it will be able to use the advance tech of the aliens to send and recieve signals to/from across the galaxy (due to faster than light communication) and that they would need something that big to do that.

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<font size=-1>[ This Message was edited by: g99 on 2003-02-07 21:59 ]</font>

[sacrelicious]

radio telescope need to be large because radio waves are very faint. the smaller the dish, the more individual waves will likely miss it. since radio is a spectrum of light, lets think of it in those terms:

we all know the basics of how a camera works: light is let in through a lens, and strikes a chemically treated membrane that has a reaction when exposed to light. now since very little light is needed, the hole is small, and there is a shutter that opens and closes at high speed. in older cameras, the chemical emulsions for the film were very primative, so the subject and camera would have to remain still for a long period of time (several minutes) as more and more light particles strike the film.

now, as we all know, if you expose the film with a lens the diameter of, say, a frisbee, then it would quickly over expose the film (you would need a shutter that moves at unprecidented speeds in order to avoid that outcome). furthermore, we all know from personal experience that if we squint our eyes, very little light comes in, but enough to make out shapes and colors, and when we open them wide, alot of light comes in. we also know that our pupils will widen in the dark so that we can take in the maximum amount of light, thus allowing us to see more.

now radior waves being a whole lot fainter than the visible spectrum of light are going to be best seen with a "wider pupil", so that the maximum amount of radio light will be taken in. to give you an idea, the amount of radio energy taken in by even a large telescope is over time only equal to the impact of a single snowflake!

and to think if I didn't own Carl Sagan 'Cosmos' boxed set on DVD I would never have been able to answer you question! that thing pays for itself (well, that is if I got paid for my answer. you aren't planning on paying me for my answer are you?)

[g99]

Depends, how much does the force of a snowflake cost? [img]/phpBB/images/smiles/icon_smile.gif[/img]


Wow! Thank you for such a well written and clear answer. It makes alot more sense to me now. So in essence it acts on the same principles at a regular light telescope.

So how about sending out signals. Does it work the same way?

You want to catch as many radiowaves as possible, makes sense. So, why build a very large radio telsecope made of a bunch of smaller telescopes that cover a gigantic area (i am thinking of the VLA. http://antwrp.gsfc.nasa.gov/apod/ap000530.html )? Does not the risk of you loosing the signal in the intermittent spaces overide the effect of the large array? would it not be cheaper to build one large stationary array in the earth than 20 or so smaller and probobly more expensive?

_________________
"Hi!!" - Some person, somewere, at some time.
"It takes Thousands to fight a battle for a mile, Millions to hold an election for a nation, but it only takes One to change the world." - Dan Sandler 2002

<font size=-1>[ This Message was edited by: g99 on 2003-02-07 22:52 ]</font>

[sacrelicious]

On 2003-02-07 22:52, g99 wrote:
Depends, how much does the force of a snowflake cost? [img]/phpBB/images/smiles/icon_smile.gif[/img]


Wow! Thank you for such a well written and clear answer. It makes alot more sense to me now. So in essence it acts on the same principles at a regular light telescope.

So how about sending out signals. Does it work the same way?

You want to catch as many radiowaves as possible, makes sense. So, why build a very large radio telsecope made of a bunch of smaller telescopes that cover a gigantic area (i am thinking of the VLA. http://antwrp.gsfc.nasa.gov/apod/ap000530.html )? Does not the risk of you loosing the signal in the intermittent spaces overide the effect of the large array? would it not be cheaper to build one large stationary array in the earth than 20 or so smaller and probobly more expensive?

_________________
"Hi!!" - Some person, somewere, at some time.
"It takes Thousands to fight a battle for a mile, Millions to hold an election for a nation, but it only takes One to change the world." - Dan Sandler 2002

<font size=-1>[ This Message was edited by: g99 on 2003-02-07 22:52 ]</font>

well you'd have to ask someone with more specific knowledge about the arays. I would suspect that they are intended for a different dedicated purpose, such as focusing in on specific details that bear interpolation better. or perhaps it is purely a cost compromise (several smaller dishes might be easier to maintain and repair than one giant one, for instance)

[JS Princeton]

Interferometry is the keyword, g99. I will not do it justice, but I bet google will.

[David Hall]

Astronomers have two things they want to maximize, light collection power and resolution (there are others, but I won't get into them here). There are also two ways to go with any telescope, a single element arrangement, or an interferometer. There is a tradeoff between the two.

Light collection ability depends on the area of the primary. A single primary element will collect all of the light hitting it and reflect it into the reciever. So this is good for making out extremely faint objects. But there is a practical limitation to the size of the telescope you can make in this way.

Resolution, OTOH, depends on the total diameter of the primary. The wider the scope, the more detail you'll see at any given light level. An interferometer gets around the size limitation of a single scope by combining the light from several scopes into one. That way you have the practical diameter of a single scope with the same distance across as the distance between the furthest interferometer scopes. This is absolutely great for resolution. But the downside is that there is very little light collected, maybe only one or two percent, compared to a single scope of the same size.

Intereferometers have long been used in radio astronomy because it's relatively easy to combine the long-wave radio waves from several scopes. Visible light interferometry is only just now reaching practicality because the shorter wavelengths require much finer precision in the alignment of the scopes to get a clear picture.

I think nowadays, resolution is becoming more important to astronomers than light collection, so interferometry is really being pursued vigorously.

Caveat: IANAA, so if I said anything wrong here, someone will likely take me to task. [img]/phpBB/images/smiles/icon_smile.gif[/img]

_________________
"If we die, we want people to accept it. We're in a risky business and we hope if anything happens to us it will not delay the program. The conquest of space is worth the risk of life." -Gus Grissom

<font size="-1">(fiksed a cuple uv tpyos.)</font>

<font size=-1>[ This Message was edited by: David Hall on 2003-02-08 03:59 ]</font>

[g99]

So could you not combine the inferometer and a large single scope?

Say use the inferometer to get a location of a interesting source of light/radio waves and then use the giant single scope next to it to zone in on it and get a better picture with more information?

[David Hall]

On 2003-02-08 14:49, g99 wrote:
So could you not combine the inferometer and a large single scope?

Say use the inferometer to get a location of a interesting source of light/radio waves and then use the giant single scope next to it to zone in on it and get a better picture with more information?

The whole point is that interferometers by nature expand the potential size of the telescope. You can set up the biggest single scope technically possible, and then add other scopes to it for an even bigger interferometer. The size of the interferometer is limited only by the technical ability to synchronize the wavelengths of light involved (or something like that). In radio wavelengths the VLBA (very long baseline array) interferometer connects radioscopes from all around the world, creating an interferometer pretty close to the diameter of the Earth itself. Can't get much larger than that without going orbital. [img]/phpBB/images/smiles/icon_smile.gif[/img]

In optical wavelengths, we already have the twin Keck scopes working together, and construction is underway on the 4-scope VLT for even more firepower. These are already some of the largest single scopes in operation, and combined they should be incredible!

[Charlie in Dayton]

So how about sending out signals. Does it work the same way?

To a degree. The idea is similar, except for outbound signals we can control the signal strength to the limits of the amplifiers and the electrical currents the equipment will handle.

You want to catch as many radiowaves as possible, makes sense. So, why build a very large radio telsecope made of a bunch of smaller telescopes that cover a gigantic area (i am thinking of the VLA. http://antwrp.gsfc.nasa.gov/apod/ap000530.html )? Does not the risk of you loosing the signal in the intermittent spaces overide the effect of the large array? would it not be cheaper to build one large stationary array in the earth than 20 or so smaller and probobly more expensive?

No, you wouldn't lose the signal in the intermittent spaces. The spaces between the elements of the VLA can work out to be small percentages of the size of the radio wave, depending on the frequencies being monitored.
As a matter of fact, the timing differences between signal acquisition for each antenna (while very small) are measurable, and these differences help in all the array elements tracking together.
Why 20 smaller ones instead of one big one? Believe it or not, 20 smaller ones come out to be cheaper, because of off-the-shelf parts, so to speak. In addition, the one big one is stationary, so it only points to a particular source as the source passes overhead through the focal cone of the antenna in the sky. The smaller ones can AIM -- so that the dishes can track a source whenever and wherever it's above the horizon. We're talking the difference between minutes and hours here between the stationary and trackable dishes. Additionally, the 20 element array in theory could track twenty relatively strong signals simultaneously -- or whatever combinations are needed to track multiple signals. And, if the big dish fries a microtron rotary frambulator, the whole thing's down; but on the multiple element array, you've still got 95% capability with one dish inoperative. The ideas of flexibility and multitasking are not limited to the computer world.

The same idea applies in the new generation of multi-element adaptive optic telescopes...each element (dish or mirror) catches its own signal, which can be analyzed (and cleaned up if necessary). The idea of stacking a dozen clean signals/images has resulted in information intelligibility that is orders of magnitude above what can be gotten from a single source/image. The odds of the same error being in all the signals at the same time at the same place is negligible . It works with radio waves the same way it does with light waves - you just can't tune in "Arcturus's Top 40" with your eyeballs...

_________________
I am Dyslexia of Borg; prepare to have your *** laminated. - Lori Martin

Yours in cogno-intellectualism
Charlie in Dayton



<font size=-1>[ This Message was edited by: Charlie in Dayton on 2003-02-09 23:31 ]</font>

[g99]

Thanks alot charlie. You answered my questions exactly. And thank you everyone else too who helped. [img]/phpBB/images/smiles/icon_smile.gif[/img]