
Telephoto Camera Lens Assists Dark Matter Find - retupmoc01
https://www.quantamagazine.org/20160928-off-the-shelf-lens-assists-dark-matter-find/
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antognini
Small telescopes are very useful for searches for extremely faint, diffuse
galaxies because it turns out that these searches are surface brightness
limited rather than being magnitude limited like most surveys. Surface
brightness is a little counterintuitive because the minimum surface brightness
a survey can find is actually independent of telescope size. Using a 8-m
telescope is no better for finding a diffuse galaxy than a telephoto lens. The
only thing that can improve your signal to noise ratio is observing time and
observing time is much cheaper on a small telescope.

The basic reason for this is that as you increase the size of your telescope,
not only does the collecting area increase, but the area of the image
increases as well. So if you're looking at a diffuse source like a galaxy, the
increased number of photons gets spread out over a larger area and the total
number of photons per pixel remains the same. This isn't a problem for point
sources (like stars) because even as the image size increases, all the photons
from a point source still fall in the same pixel so the source appears
brighter to the detector.

As peter303 also mentioned, refracting telescopes don't suffer from the same
artifacts that reflecting telescopes do, which is why these surveys use small
telephoto lenses rather than small reflectors.

~~~
sandworm101
So would you get similar surface brightness performance out of a larger
telescope by decreasing the size of the detector, by adding a couple lenses to
focus the light onto a smaller area? It's been a while since I took astronomy
courses and I try to keep up with the basic physics.

~~~
antognini
Yes and no. In principle it would be possible to do something like that, but
you could only do it for a part of the image and you would lose the focus.
You'd basically be trying to transform an extended source into a point source.
In practice it's easier just to use a smaller telescope for a longer period of
time.

Incidentally, this is related to a puzzle: can you burn a paper with moonlight
if you have a sufficiently big magnifying glass? It turns out that you cannot
(at least as long as you're using a normal magnifying glass that brings light
to a focus), because the highest temperature you can produce is the
temperature at the surface of the moon. In the limit of having an infinitely
big magnifying glass, the view from the sheet of paper would be the same as
the view from the moon, and so the temperatures would be the same as well [1].

[1]: Making various assumptions about radiative thermal equilibrium, etc.

~~~
tgb
Very cool. I don't have any questions to ask you but I want you to keep
talking since this is all very interesting.

Edit: this xkcd does a wonderful elaboration on the fire - by - moonlight
topic [https://what-if.xkcd.com/145/](https://what-if.xkcd.com/145/)

~~~
antognini
I had not seen that xkcd article before! Thanks! It's the most lucid
explanation of the problem that I've seen.

The only other interesting surface-brightness-related fact I can think of at
the moment is that surface brightness fluctuations can be used to measure the
distance to galaxies. This is one of the cases where it helps to have a really
big telescope.

The basic idea is that although galaxies are extended sources, they're really
just a collection of point sources --- they're just a bunch of stars grouped
together in an area of the sky. Now suppose you have two similar galaxies (at
least they have similar stellar densities) and one of them is close by and the
other is far away. The nearby galaxy will have relatively few stars per pixel
and the distant galaxy will have many more stars per pixel. Since the stars
are randomly distributed, the number of stars in any given pixel will be given
by a Poisson distribution. A consequence of this is that in the nearby galaxy
there will be a lot of variation in the flux from one pixel to the next,
whereas in the distant galaxy the image will be much more smooth. So even
though the average flux per pixel from both galaxies is the same, you can
still tell which one is close and which one is distant based on the surface
brightness fluctuations.

~~~
sandworm101
But without actually imaging stars, how do you know that two galaxies have
similar stellar densities? The OP was about low-star/high-darkmatter galaxies.
I guess you would have to hope that the entire thing was rotating, and at an
angle, that doppler effects could determine the size/mass.

~~~
antognini
That's where you have to make some assumptions given the galaxy type. As with
a lot of distance techniques in astronomy, there is a lot of uncertainty. This
is one of the reasons that surface brightness fluctuations are not a very
popular method.

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acqq
The official page about the Dragonfly telescope with both the news coverage
and the academic papers:

[http://www.dunlap.utoronto.ca/instrumentation/dragonfly/](http://www.dunlap.utoronto.ca/instrumentation/dragonfly/)

An example with the details as it used only 8 lenses:

[http://inspirehep.net/record/1278457](http://inspirehep.net/record/1278457)

Previously on HN with some good comments:

[https://news.ycombinator.com/item?id=10991628](https://news.ycombinator.com/item?id=10991628)

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peter303
An additional explanation I heard is that the more common reflector telescope
possesses diffraction artifacts inherently caused by the mounts of secondary
mirrors. These artifacts interfere with seeing the faint magnitude of galaxy
halos. Refractor telescopes don't have these artifacts.

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dekhn
I've worked across a wide range of quantitative science fields and one thing
I've noticed is that while the bulk of the funding goes to massive facilities,
often a bit of clever engineering and $100K can make a huge difference. This
is true in computing; one of the things that's interesting about computer
science is that it can turn a complicated physics equation that is O(n __4) or
worse into an O(nlogn) approximation that is "good enough" to justify not
using brute force.

