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There are many large reflecting/optical telescopes around the world, just to name a few: the Very Large Telescope, Large Zenith Telescope, and European Extremely Large Telescope.

My question is does actually building larger telescopes improve visibility of the universe, quality of an image, and various other attributes of a telescope?

Well it does seem kind of obvious that a larger telescope may have more magnification and better imaging, however if that is true than why don't we just build a super large one (think really really big, unlike the size of the current large ones) so we can see the universe in HD - so to speak?

Since we haven't done this yet (I think) is it because of financial issues or is it because simply building a bigger telescope won't improve much to visibility and image quality?

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    $\begingroup$ Side note: The European Extremely Large Telescope isn't really a telescope yet. It won't have first light until in ten years or so. $\endgroup$ – pela Sep 6 '15 at 20:01
  • $\begingroup$ @pela yes, I've noticed that, also the one their building in South America, I believe its the GMT, won't be done until 2026 (i think). Why does it take so long to build a telescope? $\endgroup$ – koolkid Sep 6 '15 at 20:14
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    $\begingroup$ Good question. There are practicalities such as foreseeing whether the current technology will be severely outdated by future technology before the telescope is finished, but unfortunately I think it's mostly a question about money and management. $\endgroup$ – pela Sep 7 '15 at 12:46
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    $\begingroup$ I'm not really into telescope building plans, but much of the work leading up to deciding whether or not a given telescope should be built is done by people who both have other duties, and who are employed for typically two years before moving on to something else. $\endgroup$ – pela Sep 7 '15 at 12:46
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    $\begingroup$ @kiilkid: The Giant Magellan Telescope has 7 mirrors which are spin-cast. There is one facility that can make them and it seems to take about a year to get a cool, finished mirror blank. Then it needs to be ground and figured. Ignoring the first mirror which took 7 years because it was the pathfinder, it looks like it will take ten or so years to cast and figure the other six (plus one spare). $\endgroup$ – Mark Olson Jun 12 '18 at 21:02
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Magnification is not the main purpose of a telescope. A very small telescope will have a lower resolution than a larger one, but for larger ones, the atmosphere limits the amount of detail visible.

Large telescopes capture more light than small telescopes. More light means that dimmer objects are visible, and there is more light to be analysed. A very dim object, such as a planet orbiting another star, needs a very large mirror to collect enough light to analyse spectroscopically. And this is the reason why larger telescopes are made. Of course larger telescopes cost a lot, and cost is the reason that telescopes like the "Overwhelmingly large telescope" was not built.

Much of the development in telescopes in the last 25 years has been not in ever larger mirrors, but in adaptive optics, or using two telescopes together as an interferometric telescope.

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  • $\begingroup$ Thank you for your answer. So essentially a larger telescope would be better right? I had a feeling costs played a role and it sucks that it is the reason why we haven't built an "Overwhelminingly large telescope" yet. $\endgroup$ – koolkid Sep 6 '15 at 19:12
  • $\begingroup$ For some things, bigger is better. But as there are limited resources, there are choices to make. A 100m telescope could do some fantastic things, but I'd rather have a functional JWST (6.5m) than a OWT. Obligitory xkcd: xkcd.com/1294 $\endgroup$ – James K Sep 7 '15 at 18:09
  • $\begingroup$ @JamesK how does the Relight criterion play a role here. Say for a large optical reflecting telescope what would $D$ and $\theta = 1.22\frac{\lambda}{D}$ be? More light is one of the reasons for a large mirror, but the resolution should also be affected by it. The focal length $f$ should be also somehow made small, since the spatial resolution is $l = 1.22\frac{\lambda \cdot f}{D}$ $\endgroup$ – Alexander Cska Dec 3 '17 at 13:49
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You've pretty much answered it yourself.

Very large telescopes are expensive to build, and there are diminishing returns for large instruments operating under Earth's atmosphere.

Air turbulence (what is known as "seeing") limits the resolving power of the telescope - its ability to distinguish small details and make high resolution images. It's an essentially random phenomenon, so sometimes it gets better, but generally speaking large instruments are more strongly affected by turbulence.

Light pollution is the glow caused by all artificial light sources (city lights, industrial lights), which are making it difficult to see very faint, very distant objects. It is drowning the faint objects in artificial glow.

The bigger the telescope is, the better its resolving power, and the better its light gathering ability (the ability to see faint objects). But seeing and light pollution are affecting both. You could fight seeing with adaptive optics, with decent results. You could fight light pollution by installing the instrument far from cities. Or you could fight both by launching the instrument into space.

All these solutions cost money. Large instruments are expensive. Adaptive optics are expensive. Running an observatory in the middle of nowhere also adds costs. Finally, launching things into space is not cheap either.

However, each one of these methods is used in one case or another. For each kind of application, astronomers need to make a decision for what methods they could use to improve performance, and then match that up with the money available to the project. It's a complex overall decision that takes a long time, lots of discussions, often politics gets involved, etc.

So what gets built is a compromise between what astronomers want, and what is actually doable from a financial, political, and technological standpoint.

BTW, "magnification" is not a parameter that astronomers use to define an instrument. Any instrument can give you any magnification you want - just use a different eyepiece. The relevant parameter here is the resolving power - the ability of the instrument to distinguish fine details. This depends on the instrument size - the diameter (or "aperture") of the telescope. Bigger scope = better resolving power (all else being equal). As I said above, seeing (air turbulence) is a major limiting factor here.

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I'd like to add more inputs on the physics behind telescope resolution. The telescope resolution is given by its primary mirror size which produces a "quasi" punctual image for each light source it sees, that is called a airy disk. The airy disk size is inversely proportional to the telescope primary mirror diameter and like pixels on a LCD screen the lower the pixel size the better the resolution. However, the limiting factor on the ground is the atmospheric turbulence, this is why huge telescope of few meters wide cannot see better than a 500mm amateur telescope resolution wise (see Fried parameter). So in order to reach the telescope full resolution potential we need to build adaptive optics or better we could send the telescope to space.

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  • $\begingroup$ Keep in mind that atmospheric turbulence is a random phenomenon. Under normal conditions, a very large telescope cannot see better than a smaller one - however, periods of calm occur randomly, and therefore the image could improve dramatically once in a while. Of course, it's not entirely predictable when this happens. So, yes, adaptive optics or space based telescopes are the true answer to this issue. $\endgroup$ – Florin Andrei Jul 18 '16 at 18:30
  • $\begingroup$ You are right Florin this is why lucky imaging technique can be used to grasp these elusive moments of stable air flow. Still larger telescope are more affected by turbulence than smaller ones, just imagine how chaotic looks a tiny airy spot shaken by air currents compared to a larger one. $\endgroup$ – Carthusianorum Jul 20 '16 at 5:25
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BIG Telescopes afford two luxuries More refined mirrors spread over a larger area permit Greater image resolution. They capture MORE light, even the faintest weakest light. And being bigger means the telescope get's more comprehensive views of the night sky.

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Just a small supplementary -- there is no point in building a larger telescope unless you can manufacture a mirror that big without any kind of flaw or deformation and then mount it so that it doesn't change shape as you point it in different directions. This is a whole pile of really hard engineering problems which have also limited the size of telescopes that could be built at various times, or at the very least shifted the price-performance tradeoff.

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