

The Billion-Dollar Telescope Race - dnetesn
http://nautil.us/issue/11/light/the-billion_dollar-telescope-race

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antognini
One of the reasons that there wasn't much in the way of development of larger
telescopes in the late 1970s and 1980s was the development of the CCD. Prior
to the CCD, astronomers used photographic plates to take their images, and
photographic plates have horrendous efficiencies. A typical photographic plate
will only register a few percent of the incident photons. A CCD, however, has
an efficiency very close to 100% (except at really low wavelengths). Almost
immediately after its invention, new instruments with CCDs were built to be
put on existing telescopes. [1] Because the efficiency was a factor of ~30
larger, it was as though you had quintupled the diameter of your telescope.
There wasn't much of a reason, then, to invest in huge new telescopes when you
could effectively have a huge new telescope just by building a CCD detector.

Also, as a graduate student at Ohio State I would be remiss if I neglected to
mention that the Large Binocular Telescope was originally planned to be built
at the same time as Keck. It was originally conceived to be larger and with
OSU providing a large fraction of the funding. OSU's president, Gordon Gee,
cut funding for the project in the early 90's, however, which threw the
project into disarray. Most of the faculty in the Ohio State astronomy
department resigned in protest which is why Ohio State has had a department
with relatively young faculty. Due to problems funding the project and
technical difficulties (putting two very large mirrors on a common mount
turned out to be harder than anticipated), the telescope had to be scaled back
and it only saw first light in 2005.

[1] As an aside, astronomy has been one of the major drivers of improvements
in CCD technology. CCDs in consumer devices are generally used in photon-rich
environments and the users can tolerate a lot of noise in their images.
Astronomers, however, demand very low noise and use their CCDs in photon-poor
environments, and the production of these kinds of devices has required major
improvements in CCD technology.

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brave-new
I have a 130mm (5") Newtonian Reflector telescope, do you have an advice on a
good CCD detector to use with it?

~~~
reportingsjr
If you have a DSLR you can get pretty good results with that. You will
absolutely have to learn how to use a stacking program for great images as
qwerta said. If you could afford it you would want a super chilled (80K is
common due to LN2 cooling) detector for low noise.

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batbomb
You can check out the following SVG to see a comparison of telescope sizes:

[http://upload.wikimedia.org/wikipedia/commons/c/c5/Compariso...](http://upload.wikimedia.org/wikipedia/commons/c/c5/Comparison_optical_telescope_primary_mirrors.svg)

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melling
Several years ago I backpacking in the Atacama desert in Chile. That's where a
lot of telescopes are built because it's so dry. I stopped in one city and I
tried to find an observatory through my hostel that took visitors but had no
luck. Guess whatever tour was available wasn't popular enough. Has anyone had
any luck in Chile?

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welterde
ESO offers guided tours of their Paranal and La Silla observatories.

[http://www.eso.org/public/about-
eso/visitors/](http://www.eso.org/public/about-eso/visitors/)

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millioncents
That's pretty impressive. Given the size and importance of the mirrors, I
wondered how they actually clean them. Came across this:
[https://www.youtube.com/watch?v=CkV8RRRu7gE](https://www.youtube.com/watch?v=CkV8RRRu7gE)

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mrfusion
Will any of these new scopes be able to image exoplanets?

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onion2k
The GMT telescope has mirrors 8.4 meters across with a fault tolerance of 19
nanometers. If you were to scale that mirror up to the size of Earth, the
biggest imperfection would be just 2.8 centimetres high. That is _amazing_.

~~~
IanCal
For another perspective on that size, it's a mirror between 4 and 5 times your
height, to a fault tolerance of less than 200 hydrogen atoms (or about 60 SiO2
molecules., if I have my calculations right)

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ars
Does the mirror have to be round?

Can you make an extremely long, but not very tall mirror? Seems like that
would be easier to build.

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smeyer
Why would that be easier to build?

Edit: I'm asking because I'm pretty sure it wouldn't be.

~~~
ars
Less weight to support per ground area, the mirror would not sag as much.

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smeyer
Do you mean building a 5 meter wide oblong mirror instead of a 5 meter wide
circle? That might be easier but wold be a far inferior telescope. The amount
of light collected goes as the area of the mirror. If you're suggesting
keeping the same area but being more oblong, then I don't see any advantages.

~~~
ars
I did mean the same area. The advantage is that since it is longer it has a
larger footprint.

Meaning more support, which to me seems easier to build (mechanically, don't
know if the optics themself will be more complicated - that was my question).

~~~
smeyer
I think a big challenge would be that it would be much harder to grind the
mirror. Round mirrors have the advantage of being somewhat close to a
parabola, so you can spin them and use gravity and then just refine from
there. The optics also get more complicated.

One key thing that I think would help you here is that the hard parts of
building a telescope are the mirror and the instruments, not the mechanical
telescope to support it. Supporting a ten meter wide mirror is not too hard
compared to making a near-perfectly ground mirror that is ten meters wide or
building top shelf instruments to collect and study the light. It's not cheap
or anything, but it's not really what's holding back telescopes. You can see
this in the new generation of 20-30 meter scale telescopes with multi-mirror
designs. The GMT uses several 8 meter mirrors because we can't really make
useful mirrors larger than about 10 meters.

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ohitsdom
Disappointed the article didn't discuss the pros/cons of earth-based
telescopes versus space based. Why build earth based telescopes when you have
the atmosphere in the way?

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welterde
It's way cheaper to build ground-based telescopes than it is to build space-
based ones (and you can easily upgrade it too). Thanks to advances in adaptive
optics you can now get most of the properties of space-based telescopes with
ground-based ones (even existing ones).

One major limit here is what the atmosphere absorbs.. so for X-Ray, Far-IR,
etc. you will still need space-missions.. that's where the focus is now set
mostly at ESA/NASA/etc..

