I spy with my little fibre, ten million or so galaxies

Lawrence Berkeley National Laboratory boffins are getting ready to point thousands of optical fibres at the night sky, starting with a 10-robot system proof-of-concept. The ProtoDESI the boffins are wiring up will use robots to aim optical fibres at distant galaxies, a light-gathering trick the DESI (Dark Energy Spectroscopic …

[frank ly]

If you want precise positional information about a distant object, surely you'd need the individual sensors to be as far apart as possible. So why are these fibre optic heads placed so close together that they have to be careful not to make them collide with each other when adjusting their aiming line?

[Flocke Kroes]

How it works

The instrument starts with a big mirror, like a normal telescope, but instead of putting a CCD in the focal plane, there is a large number of ends of fibre optic cable. Point the big mirror at patch of sky, and move it to counter the rotation of the earth, then each optical fibre gets the light from a different galaxy. Presumably the other ends of the fibres are lined up pointing at a diffraction grating to split the light into colours, and the result goes to a CCD so you get a complete spectrum of a bunch of galaxies in the same patch of sky in one go. Every time you pick a different patch of sky all the fibres have to be moved, and that appears to be the tricky bit.

[Boothy]

Re: How it works

The Sloan Digital Sky Survey (SDSS) basically did the same process a few years back (the fibre bit), but without the robots, so was a very manual process. Basically drilling holes in plates for the fibres, that lined up with the galaxy positions in a specific area of the sky.

I remember watching a documentary about it a few years ago, took them days to set up a single plate.

This is basically automating the process, to ramp up the numbers (massively).

A couple of pics and some further links/reading here.

[Boothy]

@ frank ly

Quote : "If you want precise positional information about a distant object, surely you'd need the individual sensors to be as far apart as possible...".

The galaxies are too far away to use trigonometry to measure distance. We can use that for neighbouring stars, but once you get past a certain distance, it becomes very inaccurate.

So this about grabbing the spectrum of the galaxies, which gives us the red-shift, which tells us how far away the galaxy is far more accurately at these distances than trigonometry would.

We've done this already on a smaller scale, so this is about doing it on mass.

Quote: "So why are these fibre optic heads placed so close together that they have to be careful not to make them collide with each other when adjusting their aiming line?"

Don't think aiming, think more filter. It's one fibre per galaxy.

Imagine a disk (i.e. a disk of aluminium about a meter across), now drill holes in that disk that precisely match with the relative positions of galaxies in a specific area of the sky. Stick this plate at the end of a telescope (where the camera normally is), and point it at that section of the sky, so that the light from each galaxy lines up exactly with the holes on your drilled plate.

Now direct via fibre optics the light from each hole to a sensor, and you can measure the spectrum of each galaxy, one fibre being the light from one galaxy.

This was basically the Sloan Digital Sky Survey, check out wikipedia etc for some pics.

This new work is to automate the process, so rather than drilling metal plates, and fitting the fibres, a plate for each section of sky, you just move the fibres around via the 'robots'.

[frank ly]

Thank you everyone, for your well written and informative replies. This is just one of the great things about The Register.

[Michael H.F. Wilkinson]

Ten million galaxies is nice, but it is all about spectra

The Sloan Digital Sky Survey release 12 has far more objects: 208,478,448 galaxies to be precise. It only contains spectra for 2,401,952 galaxies and 477,161 quasars, so DESI becomes particularly interesting because it captures spectra of so many objects.

Pointing an optical fibre directly at the sky is of course pointless, as the aperture is pathetically small. The trick is to get a big mirror (4 m in this case), which gathers a lot of photons, and use fibre optics to guide the light to (multiple) spectroscopes. The more usual trick is to use a slit, which captures spectra from a little stripe across the image plane. By using carefully placed fibre optics, the project (as I read it) wants to tap into more of the optical plane, to record spectra of far more objects simultaneously. The robots pick out the right bits of the image from which to obtain spectra. Sounds very interesting

[Christopher Lane]

And as the last galaxy was counted...

...Overhead, without any fuss, the s̶t̶a̶r̶s̶ galaxies were going out.

[xeroks]

Re: And as the last galaxy was counted...

as long as we don't start counting the names of God..

[DubyaG]

Re: And as the last galaxy was counted...

Well, maybe there are more then nine billion of them, what with over 208 billion galaxies.

[G.Y.]

Re: And as the last galaxy was counted...

I believe Arthur Clarke's billion is a Eurpoean billion (=10^12)

[Anonymous Coward]

Rather than build the whole 5,000-robot kit and wreck it with a software bug

Well, they could DevOps the whole idea! A neighbor of a cousin of a colleague of my ex-wife told me that a DevOps expert was able to "orchestrate" half a trillion robots with six lines of TCL code.

[ravenviz]

3D map of the universe out to 10 billion light years - wonders will never cease!

[Graham Cunningham]

wonders will never cease!

There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.

There is another theory which states that this has already happened. - DNA

[Mark 85]

Re: wonders will never cease!

There is another theory which states that this has already happened

It obviously has, otherwise how would explain both front runners for the US Presidential election?

[allthecoolshortnamesweretaken]

Re: wonders will never cease!

If only we could harness the power of stupidity. That would be one hell of an energy source.

[allthecoolshortnamesweretaken]

Re: wonders will never cease!

There is also a third theory that suggests that the first two theories were made up by an editor of THHGTTG in order to boost sales. (also DNA, in the H2G2 radio version)

Anyway, re this bit of astroboffinry: very clever indeed.

[Swarthy]

Re: wonders will never cease!

There is another theory which states that this has already happened. More than once. - DNA

[VeganVegan]

Why don't they just use a very dense array of fibers?

And let the software figure out which fibers are pointing at which galaxies (versus you run-of-the-mill stars and other objects).

[oldcoder]

Re: Why don't they just use a very dense array of fibers?

That would require several thousands times more fibers... and more spectrographs to match

[Mark Exclamation]

"....swinging the fibre optic heads under computer control on two axes to aim them is difficult...." - indeed, those things used to chop wood would be difficult to utilise in this instance, unless you wanted to chop it all up. Axis?

[Mark Exclamation]

Just eaten a HUGE plate of humble pie. Axes is the correct word. Oooops, sorry!

[Anonymous Coward]

A fairly accessible paper on the engineering of a large survey multifibre spectrograph is here

http://magnum.anu.edu.au/~TDFgg/Public/Publications/lewis_2dF.pdf