150 – The European Extremely Large Telescope
In this episode we talk with Jochen Liske from the European Southern Observatory about the E-ELT Telescope for which construction is about to start. We discuss the engineering challenges of building a 40m mirror and the associated telescope, as well as the science that is planned to be addressed with the E-ELT once it is finished. We also discuss a number of issues around optical astronomy in general that were not covered in our episode about the LBT.
Joe Liske is also the host of both the HUBBLEcast and the ESOCast, two video podcasts about astronomy you may want to check out.
- European Southern Observatory (ESO)
- Dr. Jochen Liske
- E-ELT (ESO)
- E-ELT (WP)
- Very Large Telescope (ESO)
- VLT (WP)
- Overwhelmingly Large Telescope (WP)
- NRAO Very Large Array
- VLA (WP)
- HubbleSite – Out of the ordinary…out of this world.
- Hubble Space Telescope (WP)
- Resolution (WP)
- Large Binocular Telescope Observatory
- Interferometry (WP)
- Aperture (WP)
- ALMA Observatory
- Adaptive optics (WP)
- Active optics (WP)
- Gran Telescopio CANARIAS
- Gran Telescopio Canarias (WP)
- Nasmyth telescope (WP)
- Subaru Telescope
- Subaru Telescope (WP)
- Cassegrain reflector (WP)
- Anastigmat (WP)
- ESO – NTT
- NTT Telescope (WP)
- W. M. Keck Observatory
- Keck Telescope (WP)
- Laser guide star (WP)
- General relativity (WP)
- Gravitational wave (WP)
- Exoplanet (WP)
- Biomarker (WP)
- Fine-structure constant (WP)
- Standard Model (WP)
- Absorption spectroscopy (WP)
- String Theory (WP)
- Coronagraph (WP)
- Spectrograph (WP)
- James Webb Space Telescope
- James Webb Space Telescope (WP)
- The Square Kilometre Array SKA Home
- Square Kilometer Array (WP)
- LSST: The New Sky
- Large Synoptic Survey Telescope (WP)
- ESO – Science Archive Facility
Just discovered this podcast, absolutely love it! Really enjoyed this episode.
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Thanks Michael :-)
Congratulations Dr Völter !
Hi Markus. Congratulations on acquiring your Phd! I’ve been listening to this podcast for a couple years and absolutely love it.
Thanks Caleb :-)
Congrats on the degree and the doctorate. I enjoyed this one immensely, and the nice varied subjects you find interesting guests to discuss, with such enthusiasm and detail.
I discovered this whilst I was browsing ESO’s Twitter profile. I am very glad I did, excellent podcast. Thank you MaNo and Jochen Liske — this was really great to listen to. ^_^
Cheers from the U.S.
Another excellent episode! I learned a lot about the physical layout of the telescope, but I still don’t understand the need for five mirrors. Also, I am wondering if each of the ~800 component mirrors of the primary mirror is flat.
Thanks Sean :-)
Thanks Brian! Be sure to check out our other astronomy episodes!
I also don’t really understand the need for 5 mirrors. Check out this URL http://www.eso.org/sci/facilities/eelt/telescope/mirrors/ it has some more details on the mirror architecture. It looks like each of the 800 is flat, but the image may be misleading.
Here’s another picture where the segments look roughly flat: http://www.eso.org/public/germany/announcements/ann12013/
Here’s a reasonably intuitive description of why 5 mirrors are necessary:
Having 1 mirror places your instrument in a very inconvenient place. Having 2 mirrors, in a Cassegrain configuration where the secondary is closer to the primary than the prime focus, helps “fold” the telescope and keep it short. Relative to the huge diameter, keeping the telescope as short as possible is important both for the structure of the telescope itself and to limit the size of the dome it fits in.
However, with 2 mirrors, there is a limit to the size of the (well-corrected) field of view. As the viewing angle increases, there are various distortions that creep into the image. Basically, it’s because the distance from primary to secondary mirror depends on the cosins of the angle the light beam is travelling at, so if the mirrors are perfectly in focus for straight-on light, there’s a slight focus problem at an angle. Every camera has these, but astronomers are particularly sensitive to them. Pretty much every practical 2-mirror design has been studied, and the tradeoffs are known.
To get a sharp image across a wide field of view, a third curved mirror is required. (The LSST uses three mirrors for the same reason.)
Okay, now we have three mirrors. But because that’s an odd number, the light is travelling up, to a point in front of the telescope. That’s an awkward place for an instrument! The LSST simply accepts this difficulty for its one instrument, but the E-ELT is intended for multiple instruments.
The basic solution is to have a flat diagonal “fold mirror” to divert the light off to the side. (A non-flat fold mirror doesn’t work because the focal point isn’t well-defined.) The best place for that is on the elevation axis, so the telescope can tilt up and down and the image stays in one place. (Otherwise, it’s necessary to lift and lower a multi-ton instrument in exact synchronization with the telescope, which is extremely difficult.) This is called a “Naysmith configuration”, and is very common. All the VLT unit telescopes (2 curved mirrors plus one flat folding) work this way.
So can we get away with 3 curved mirrors plus one flat? It turns out that the elevation axis where the fold mirror needs to go is too close to the bottom of the telescope where the third curved mirror goes. To capture the entire image, it would have to be so big it would cover the third mirror and the whole thing wouldn’t work.
So a fourth flat folding mirror was added, higher above the tertiary, which reflects the light back down to the folding mirror to the instruments.
Now, most telescopes with adaptive optice make the secondary mirror adaptive. But the fourth flat mirror ends up being an excellent place to put the adaptive magic. When making a much larger and more complex adaptive mirror than ever before, doing it to a nice simple flat mirror greatly simplifies the engineering.
So although the optical design has many interactions and tradeoffs, if I simplify a bit, each of the five mirrors is there for a specific purpose:
Primary: Collect light
Secondary: Keep telescope short and thus buildable
Tertiary: Give wide field of view
Quatenary: Adaptive optice to compensate for atmospheric distortion
Quinary: Flat folding mirror to direct light to instruments beside the telescope. Also provides rapid tip/tilt to compensate for telescope shaking in the wind.
This is oversimplified, but hopefully it gives some feel for the issue. You lose a few percent of the light on each reflection, but the wide field of view makes up for that. Collecting twice as big an image means you need half the number of images, so you can spend twice as long taking each. Which more than makes up for the slight loss of light.
Wow, thank you very much for the detailed explanation!
Oh, people were speculating that the primary mirror segments were flat. Not even close! While the primary is made of 6 identical pie-shaped regions, within each of those regions, each of the 133 mirror segments is a different curved shape.
The plan is to make 7×133 = 931 mirror segments, so there is one spare copy of each segment shape. Then each day, a few segments are removed from the primary, replaced with freshly-coated spares, and the old segments are cleaned and coated to become the new spares.
I presume they’ll also have a few “black segments” which can be installed instead of a mirror if a segment can’t be made to work for some reason. For example, if the active support motors get stuck in a way that takes time to order replacement parts for. Or if one of the 133 segments shapes gets severely damaged (e.g. dropped) and a spare isn’t available during recoating. Such a black segment would introduce some diffraction artifacts, but that’s preferable to the glare you’d get off a mispositioned segment.
Dear Doctor Markus,
Greetings from Perth Western Australia.
Congratulations on your PhD and also on another brilliant interview. The technology behind the EELT is absolutely mind-boggling. And thank you for providing all us mono-linguists with excellent technical podcasts in a language other than your native one! It is very much appreciated.
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Hi Robert, thank you for your nice feedback. Always appreciated :-)
I really did appreciate this technical, astronomical engineering kind of episode again!
It is a very interesting project with a whole lot of brain in it and I am keen to see it work in a few years. The conversation with Jochen Liske, to whom I was tuned in with his ESOCast and HubbleCast, was very good readable for my ears as a german listener and amusing in terms of talking of things behind the scenes.
And thanks to Mr Adam Baynes for the explanation of the mirror design which now makes senses also for me.
Looking forward to the next episode… Thanks again for your dedicated work!
Thanks Jochen :-)
Very nice Podcast and good audio quality :-)