These telescopes are huge (E-ELT, OWL, VLT)

Hi,
today I will write more about telescopes. In a previous post I already mentioned why radio telescopes like Arecibo are so huge, its because of the long wavelength. Today though I will concentrate on another type of telescopes and those are the ones that sit on Earth and collect information from visible light (those are called optical telescopes).

Comparison of various telescopes. Note OWL, the big circle in the background and even bigger white Arecibo.


I already mentioned Hubble telescope and James Webb Telescope (JWT) that is planned for launch next year. Those are in space so they have quite limited size. Down here we can build bigger ones. Right now in building phase is the E-ELT (European Extremely Large Telescope). Its primary mirror will have 39 meters [1] making it the largest optical telescope. It has to be so big because otherwise it could not match the ones in space. This is because we have our lovely atmosphere in the way and it makes harder for telescopes to distinquish small objects (though these days we have software that is able to account for that).

VLT aka Very Large Telescope is already working optical/infra-red telescope. It consists of 4 telescopes each of them with primary mirror of 8.2 meters in diameter. They can work together to make images of angular resolution 0.001 arcsecond. In one post I said that we are not able to take an image of star other than just point like source of light but apparently that is not true so I apologize for it:

First Direct Photo of Alien Planet Finally Confirmed

First confirmed image of an exoplanet. The ones that we can see are bigger than Jupiter and usually quite far away from their star. Credit: Gemini Observatory

VLT is second, right behind Hubble in the amount of scientific papers that its work produced (in the field of visible light/infrared telescopes).

There are many other telscopes that I might mention in the future like Thirty Meter Telescope but I will end it with OWL – Overwhelmingly Large Telescope. It was supposed to be the largest telescope ever, with primary mirror of 100 meters! The price was estimated to be about 1.5 billion euro and because of that it was decided that its not worth it. If we do not kill ourselves we might see giants like those in the future though I have no idea how the scientists, or whoever does it, will name them (UGHT – Unimaginably Giantic Huge Telescope).

Dragallur

[1]Just as JWT, the mirror is made from smaller segments. In the case of E-ELT it is because the mirror would be too heavy and we do not have the technology to build it and in the case of JWT it is because you have to somehow get it into orbit.

Comparison of telescopes: By Cmglee – Own workiThe source code of this SVG is valid., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=33613161

Why are radio telescopes so large?

Hi,
take a Hubble telescope, it has primary mirror 2.4 meters tall. Now take this:

Arecibo Observatory Aerial View.jpg

The giant Arecibo radio telescope

I mean the thing above which is Arecibo radio telescope has diameter of 305 meters. Both things are for the same thing, to observe the Universe.


So the answer lies in the name. Arecibo is a RADIO telescope which means that it works on quite different wavelengths, actually magnitudes bigger wavelengths since the wavelength of for example visible light for Hubble is 550 nanometers which is quite small while radio waves can have wavelength of hundreds of meters.

This is essentially the key. If you want to see clear image in light (that we can see) you need just a small telescope. Both work the same way though from what I understand you need larger area to collect all of those waves and reflect them on the focus which is above. The equation shows it clear:

θ=1.22*λ/D

Where θ shows how close two points can be to each other without you being able to distinquish them. λ is the wavelength of the light and D is the diameter of your telescope. So you will see best when wavelength is small and diameter is huge since this will lower the angle that you are not able to distinquish. Of course that there is huge difference when you insert meters instead of nanometers so you must compensate it with the diameter of the telescope.[1]

If you want to have a clear image in radio waves, well you have to build Arecibo.. really? Isn’t there another option?

Yes there is! You can build a lot of small radio telescope that would alone be very weak but if you take lot of them you can have a Diameter of kilometers. Such a device is called interferometer which means that is “operates by myltiplying the data from each pair of telescopes together to form interference patterns”.

There is more of them and this one is ALMA observatory.

So those are huge fields or rows of smaller (even 60 meter) discs that collect data. They have to be extremely accurate what is time concerned (atomic clocks).

Dragallur

[1]1.22 is just an empirical value.

Can you see a coin from 400 kilometers?

Hi,
no, probably not with naked eye, I think, though if it would be some special coin, maybe? The point is that you can use some cool things to see such a coin, for example Hubble Space Telescope!


Ok, I found some page with physics problems and one of the first was to calculate what is the smallest angle that Hubble Telescope can distinquish. I calculated that it is roughly:

0.0127”

This means 0.0127 arcsecond!!!

How far away football has to be to have the angular diameter of one arcsecond

What about a human eye how good is it? Well eye can distinquish only things that are 1′ away from each other [1], which means one arcminutes, one degree has 60 of those so it is pretty good but not so much as Hubble. But anyway, back to the title, how small is a coin?

I measured the second largest Czech crown to have 2.55 centimeters.

Now how far away do you need to be to not be able to see it with naked eye? We will use this formula to get the diameter:

θ=D/r

Where the diameter (D) is 0.0255 meters. The distance (r) is what we are trying to find and the angle eye can distinquish is θ (in radians). [2]

r=D/θ

If all the calculations went right it should be:

r=87.6625 meters

But beware this is not counting air, humidness and so on so you will most certainly not be able to see coin on this distance.

What about the Hubble telescope? We can use the same equation but for θ we will insert much smaller value!

And yes, you can see the coin from 414,153.744827 meters! Which means that you can see one freaking coin from London 50 kilometers behind Paris! One coin![3]

This is so cool.

Dragallur

[1]You can try this by drawing two dots on paper and then moving away from it, at one point when you are far enough you should be able to see only one.

[2]This equation is simplified but it should work for small angles.

[3]Again, this works only in space because there is nothing that would block your sight.

 

Telescope of this generation

Hi,
this title is kind of weak, I know that, but I did not want to write “rusty, old piece of metal”. First because rusty could be a little bit misleading, rust only occurs when there is oxygen and there is only little of that in space, more accurately there is 1% of all the atoms in the Universe which is quite few tons but at the same time Hubble Space Telescope is flying in near vacuum when there are just a couple of hydrogen atoms per cubic meter, maybe in his lifespan he even touched few oxygen atoms. Second because he just does not deserve it. (Check out the post about James Webb Space Telescope)


His lifespan? Well it has been a long for such an instrument which are usually meant to work for much less years. For now the telescope has been working for 25 years, 9 months and 29 days. It is assumed that it might work for another 20 years or so, it could be even longer but it is slowing down and as I explained in post about orbits, it will eventually fall down and burn up in atmosphere. Not that this space telescope would be half immortal or so, but it was able to work for so many years because there were actually missions to repair some of its tools and power systems, otherwise it would not work by now [1].

When Hubble was launched and first images sent to NASA they were good, but not good enough which was because of the mirror. It was polished to a little bit wrong shape so they had to repair that. Before they did it NASA used some complicated image processing to get the best out of those pictures.

The repair itself was very complicated since replacing mirror was simply impossible and to bring the telescope back was too expensive. They solved it by making the same mistake but “of opposite direction” on another part of the telescope so it was totally canceled out, you can check out on the next picture to see how much it made a difference:

 

One of the images taken during the misson for some repair.

Pillars of creation, by Hubble

NASA even decided to borrow some of Hubble’s time for amateur astronomers, of course it was not easy to get that time but it was definitely worth it I am sure.

Hubble is not justing taking those beautiful pictures but it also helped to measure Hubbles constant on whole next level and also through his data we found out that the expansion of Universe is actually increasing, phenomenon which is not well understood but most people think that it is because of dark energy.

This is monstrosity, almost all of those bright points are galaxies, galaxies which we will most probably be never able to search through.

Hubble also helped with some observations of black holes and also as you can see above, Hubble made a whole new picture of the youngest parts of Universe, so extremely far away.

Depiction of progress in the detection of the early Universe

How far can we actually see?[2]

Dragallur

[1]Actually it is the first such thing in space that was repaired by humans which is actually pretty cool.

[2]Check out this post to read about red shift, the x axis on the picture.

All pictures from here and here