Why don't we build a giant telescope?

[CupOfWater]

Hi!

I've been thinking of something... We all want to know how distant things look like. And the further away a thing are, the bigger a telescope is needed. So why don't scientists/NASA build a giant telescope to photograph far and distant worlds? Wouldn't it be easier to see if a planet can support life if the planet can be studied in greater detail?

The way I imagine such a large telescope being built (maybe a km in diameter), is by first building the frame for the mirror in space. Then, they ship large amount of smaller mirrors up to the frame, and add them to the frame. When it's done, a gigantic mirror is built! Then they just need to add the needed hardware to make a telescope

Is this idea completely insane, or is there something to what I'm saying? Maybe a space-lift could be built to lower the costs of sending heavy mirrors up in orbit.

 

[aeroeng314]

A few questions:

How does a big mirror help?
How do you stop stuff from hitting the mirror?
How do you aim it?
What's the focal length of the mirror and where is the detector?

 

[nasher168]

How do you stop stuff from hitting the mirror?
How do you aim it?

It's in space, so it just does whatever it is they do with the Hubble space telescope.

 

[CupOfWater]

And a big mirror gathers more light :|

 

[ExeFBM]

I think that the problem is, is that to get a meaningful image you need a decent amount of light from the object. This can be achieved with distant objects by doing really long exposures. A planet is a lot smaller than a star or galaxy, emits a lot less light, and is constantly spinning around it's own axis, and it's parent star. Due to a planets motion, and the distances involved, a long exposure wouldn't give a detailed or particularly useful image of the planet.

 

[JacobEvans]

You mean like this? http://www.galaxiki.org/images/blog/arecibo.jpg

It's also extremely complicated and individual planets are almost always going to be obscured by the glare of their parent star.

 

[aeroeng314]

How do you stop stuff from hitting the mirror?
How do you aim it?

It's in space, so it just does whatever it is they do with the Hubble space telescope.

Except that the Hubble isn't 1 km in diameter. The Hubble's optics are all shielded inside of the structure itself and not floating in open space. Also, the moment of inertia scales up proportional to mass and the square of the length dimension, so it would have an absolutely massive moment of inertia. Also, with structures that size, the deformation of the structure itself becomes an issue. Because of the increased size, the structure can maintain vibrations that are lower in frequency than a small structure. This means that reorienting something that large could introduce low frequency vibrations which you'd have to damp somehow.

And a big mirror gathers more light

Yes, it does, but I don't know if this is really what the limiting factor is in astronomical observations. I was always under the impression that the issue with observing distant planets is not that we can't get enough light from them, but that we get too much light from the parent star. What I'm trying to get at is, what's the benefit of doing this instead of a bunch of small telescopes?

 

[ImprobableJoe]

What do you mean "we"?

 

[kf00kaha]

Apart from all the other issues, I'd say it would be really hard to set all the mirrors correctly. I mean, just look at the Hubble telescope, they had to "put glasses" on it, and Hubble's mirrors are not nearly as complicated as this mirror would be. If you want to get a clear picture of something it's crucial that all the mirrors are arranged exactly as intended, or you'll get a blur (more or less). This is easy to imagine, but really hard to do in reality, unfortunately.

And then nobody has even mentioned the costs...

 

[Netheralian]

A few points...

1. Meet the Overwhelmingly Large Telescope (gotta love scientists naming things)

2. You don't need a really big telescope to get good definition. Although the size of the aperture is VERY important (http://en.wikipedia.org/wiki/Airy_disk) there are other ways in which you can create a very large aperture without having to life thousands of tonnes of equipment into space. Aperture synthesis is the way to go as you don't need such an enormous mirror. See missions like Darwin and Terrestrial Planet Finder (The latter has been deferred indefinitely)

Edit: Darwin ESA link: http://www.esa.int/esaSC/120382_index_0_m.html
The only reference i see for the aperture size is "several hundred meters." I was working on a bid for this spacecraft a few years ago, but I can't remember the specific synthetic aperture size.

Edit2: Shit - rereading point 2 its not very clear. Bigger the mirror, the more light you collect (lest observational time). Bigger the aperture the better the resolution you can theoretically achieve. (See Airy Disk link).

 

[Netheralian]

Apart from all the other issues, I'd say it would be really hard to set all the mirrors correctly.

True, but still possible. If you want to see precision positioning then you should look at mission like LISA with positioning accuracy of picometers. Edit: thats pico meter accuracy from spacecraft about 5000 kms apart.

I mean, just look at the Hubble telescope, they had to "put glasses" on it, and Hubble's mirrors are not nearly as complicated as this mirror would be. [

Hubbles mirror problems was due to a manufacturing error (http://en.wikipedia.org/wiki/Hubble_Space_Telescope#Flawed_mirror)

If you want to get a clear picture of something it's crucial that all the mirrors are arranged exactly as intended, or you'll get a blur (more or less). This is easy to imagine, but really hard to do in reality, unfortunately.

Segmented mirrors are commonly manufactured. See the link on the OWL in my post above as an example. Also see http://en.wikipedia.org/wiki/Segmented_mirror

 

[Salv]

Can someone explain why radio astronomy wouldn't be the way forward here?

We can add huge arrays of radio telescopes on land without the worry of huge mirrors being shipped into space. Very newb question here. Why couldn't a radio telescope pick up higher frequency waves, I mean much higher, like way above the radio spectrum? Couldn't we build a microwave telecope? I suppose we could, we build infradead telescopes. What I meant to say, couldn't we somehow use arrays of infrared telescopes, somehow adapt the technology from the radio telescopes?

The kepler telescope as far as I know is the best way to find other planets and the ATLAST, the Advanced Technology Large Aperture Space Telescope, will probably be best at observing these planets.

ATLAST is envisioned as a flagship mission of the 2025 - 2035 period, designed to address one of the most compelling questions of our time. Is there life elsewhere in our Galaxy? It will accomplish this by detecting "biosignatures" (such as molecular oxygen, ozone, water, and methane) in the spectra of terrestrial exoplanets.

From stsci

Hmm, I though ATLAST was going to be deployed around 2020?

 

[kf00kaha]

True, but still possible. If you want to see precision positioning then you should look at mission like LISA with positioning accuracy of picometers. Edit: thats pico meter accuracy from spacecraft about 5000 kms apart.

Well, I didn't say it's impossible, right? Cool, the LISA project looks really awesome! What I understand they will measure the minute displacement of the lasers due to the gravitational waves, correct? Not as many parts as for this mirror, but hey, it's all about time and effort, I'll agree with that.

Hubbles mirror problems was due to a manufacturing error (http://en.wikipedia.org/wiki/Hubble_Space_Telescope#Flawed_mirror)

True, and one could argue that this could happen to some of the mirrors, more likely since there would be so many of them. But then they should have learned from the Hubble project you would imagine...

Segmented mirrors are commonly manufactured. See the link on the OWL in my post above as an example. Also see http://en.wikipedia.org/wiki/Segmented_mirror

Agreed (as above), but 1 km in diameter..? Oh well, time and effort/money... I give.

 

[Netheralian]

What I meant to say, couldn't we somehow use arrays of infrared telescopes, somehow adapt the technology from the radio telescopes?

See above link on Aperture Synthesis - there are missions already proposed to use this technology in Vis/NIR. See above on Darwin

 

[Netheralian]

Well, I didn't say it's impossible, right? Cool, the LISA project looks really awesome! What I understand they will measure the minute displacement of the lasers due to the gravitational waves, correct? Not as many parts as for this mirror, but hey, it's all about time and effort, I'll agree with that.

No one said it wouldn't be hard. I worked on Darwin for a while and some of the issues that come up were incredible. E.g. the difference in temperature of the spacecraft from one side to the other can be enough to create a force that affects the positioning for the interferometry (everything needs to be perfectly aligned for the nulling interferometry to work - i.e. fractions of a wavelength stable for long duration imaging between 4-5 spacecraft). Formation flying at its extreme.

 

[Salv]

I remember reading about OWL quite a few years ago. Thanks for pointing out the aperture synthesis Netheralian.

 

[kf00kaha]

No one said it wouldn't be hard. I worked on Darwin for a while and some of the issues that come up were incredible. E.g. the difference in temperature of the spacecraft from one side to the other can be enough to create a force that affects the positioning for the interferometry (everything needs to be perfectly aligned for the nulling interferometry to work - i.e. fractions of a wavelength stable for long duration imaging between 4-5 spacecraft). Formation flying at its extreme.

I bow to the expertise Not really my field but should have realized that fitting the mirrors would be the least of the problem. Thanks for correcting me

I suppose there are some kind of compensation for small movements? I usually work with a gas-FTIR that's using an IR laser to analyze the content of a gas flow. This instrument has a mirror that vibrates back and forth to compensate for pollutants on the KBr-windows, particles or scratches on the mirrors in the gas-cell etc. Do you know if something like that would be used in this case too?

 

[Netheralian]

I suppose there are some kind of compensation for small movements? I usually work with a gas-FTIR that's using an IR laser to analyze the content of a gas flow. This instrument has a mirror that vibrates back and forth to compensate for pollutants on the KBr-windows, particles or scratches on the mirrors in the gas-cell etc. Do you know if something like that would be used in this case too?

Your field sounds interesting! As to your question, not as far as I am aware. I am just a thermal engineer so some of the intricacies go straight over my head but I don't think so. Space, in a way, makes things really simple as the magnitude of the forces are really small so it tends to drift rather than move quickly. Although that does introduce the problem of creating a thruster that can output nanoN of force for control. However, I guess you can post process a lot of the crap data out anyway by dumping data that was collected during misalignments.

I eagerly await something like Darwin being launched. Direct imaging of an exo-solar planet will be very cool...

As an aside, I have seen a few random proposals for liquid mirrors on the moon. Here is some info: http://en.wikipedia.org/wiki/Liquid_mirror#Moon-based_liquid_mirror_telescopes and just so I have some sort of respectability http://www.nasa.gov/centers/ames/news/releases/2007/07_36AR.html

 

[kf00kaha]

Space, in a way, makes things really simple as the magnitude of the forces are really small so it tends to drift rather than move quickly. Although that does introduce the problem of creating a thruster that can output nanoN of force for control. However, I guess you can post process a lot of the crap data out anyway by dumping data that was collected during misalignments.

Hmm, when I think about it post processing seems easier, to an extent of course. Probably it will be something in between...

:idea: Just realized that we're talking about space, where a vibrating mirror probably would make a disturbance in the whole space craft and really cause some serious trouble keeping the signal.

Oh well, I'm better off with after treatment of trucks Any questions about that and I'm your man :geek:

 

[Master_Ghost_Knight]

From an engineering stand point, it doesn't woth it.

First of all it is to expensive (it is a better cost effective solution to a system of new telescopes in orbit)
2nd It is technologically to dificult (and thus falling into problem 1) to make it precise enough.
3rd to observe extremely distant objects you will need to have a good tracking mechanism (which leads to problem 4) and it would need to have very few vibration susceptibility (which is simply not possible on earth, because even a car driving a mile away is still going to make allot of vibration), even if we ignore the previous whatever you could possibly gain by building a giant telescope on earth would be completly made uselles due to the fact that we have an atmosphere that simply screws whatever image detail you could possible hope to get, (that is why there are plans to build a giant disc on the moon)
4th Something so big would be to hard to move on a planet, so you get to be limited to whatever the telescope happens to be pointed at the time (which is a total waste), trying to fix this would only increase problem number one absurdly.

There might be other issues, but with this alone if I was the project manajer it would be suficient for me to fail it. And that is why we don't do it, there are better and cheaper solutions then that.