Why are they building the E-ELT?

[pipnina]

I watched a video recently about how the VLT works. They explained that all four 8.2m scopes were linked by an excavated ditch and used interferometry to get the combined resolution of a 130m~ telescope. But if they already have a telescope that has way more resolution than the E-ELT with its single 39m mirror, why are they still building it? Are they going to link that to the VLT for interferometry as well?

Something else I don't understand about the E-ELT is how it's going to be practical once it's built. The VLT already needs its mirrors removed cleaned and resurfaced every couple of months due to dust buildup. How are they going to do that with a 39m mirror?

[JamesF]

Perhaps there's just not enough time available for all the people who want to use it.  Also, the VLT is used far more as separate telescopes than combined so the E-ELT should actually out-perform the VLT most of the time.

As regards cleaning etc., I don't know if the requirements are going to be the same given that technology has moved on since the VLT was designed (it will be twenty five years since first light on the VLT when the E-ELT comes into use), but even so the E-ELT has a segmented mirror which may mean that individual segments can be removed and replaced on a regular basis.

James

[jnb]

The EELT will not only have the resolving power of a true 39m telescope but it will have the light gathering power of a 39m telescope, something which the VLT can not achieve. The EELT does not have a single 39m mirror it is made up of multiple mirrors (in a single common mount so they behaves a one mirror but multiple mirrors) each of which could be independently removed and cleaned if necessary. Additionally while interferometry allows high resolution it does not necessarily allow imaging at that resolution, there are ways to do that but as it depends on fourier analysis of the signals which in turn is frequency specific if parts are moving differentially (or are outside the plane of the interferometer it becomes problematic). So for example using a two mirror setup you could quite easily obtain extremely high resolution and directly measure the size of a star. If that star had an orbiting planet although it might be separated by more than that resolution it would not necessarily be possible to image it. I suspect it would also not be possible in such a setup to run spectroscopy on one part of that image whereas with the EELT it would be possible do a spectrographic analysis of a planet independently of the analysis of the star.

The final argument is cost the EELT is a billion euro project, the VLT at todays prices would be a half bn euro project (the James Webb is a 10bn $ project!). So it is not a major increase in cost for a significant increase in functionality.

[pipnina]

Perhaps there's just not enough time available for all the people who want to use it.  Also, the VLT is used far more as separate telescopes than combined so the E-ELT should actually out-perform the VLT most of the time.

As regards cleaning etc., I don't know if the requirements are going to be the same given that technology has moved on since the VLT was designed (it will be twenty five years since first light on the VLT when the E-ELT comes into use), but even so the E-ELT has a segmented mirror which may mean that individual segments can be removed and replaced on a regular basis.

James

Some good points. I hadn't considered that most telescopes have an overabundance of people wanting to use them. I've even watched a video recently about electoral theory when it comes to assigning telescope time recently.

The EELT will not only have the resolving power of a true 39m telescope but it will have the light gathering power of a 39m telescope, something which the VLT can not achieve. The EELT does not have a single 39m mirror it is made up of multiple mirrors each of which could be indpependently removed and cleaned if necessary. Additionally while interferometry allows high resolution it does not necessarily allow imaging at that resolution, there are ways to do that but as it depends on fourier analysis of the signals which in turn is highl;y frequency specific if parts are moving differentially (or are outside the plane of the interferometer it becomes problematic). So for example using a two mirror setup you could quite easily obtain extremely high resolution and directly measure the size of a star. If that star had an orbiting planet although it might be separated by more than that resolution it would not necessarily be possible to image it. I suspect it would also not be possible in such a setup to run spectroscopy on one part of that image whereas with the EELT it would be possible do a spectrographic analysis of a planet independently of the analysis of the star.

The final argument is cost the EELT is a billion euro project, the VLT at todays prices would be a half bn euro project (the James Webb is a 10bn $ project!). So it is not a major increase in cost for a significant increase in functionality.

So interferometry can't be used for imaging or spectroscopy with the same effectiveness as a single mirror? That's a shame. Huge telescopes are rediculously hard to make. (I'm fairly certain the E-ELT was originally going to be called the E-OLT (overwhelmingly large XD) and have a 100m mirror but they decided it would be too big.

[ronin]

The real reason is so they can think up a more extreme name for them.

VLT = Very Large Telescope,

ELT = Extremely Large Telescope,

What is the next one ???

BBT perhaps.

That is the real purpose! :grin:

Also Pipnina, could you add a "u" into Eqipment, it is slowly driving me nuts. :eek:

I always read what a person has and I always have to look twice. :D

Part of the purpose of the E-ELT is to image (hopefully) exo-planets and exo-planet formation directly.

They need the spectroscopy aspect to (again hopefully) determine the spectra from the exoplanet's atmosphere to determine the composition - water+- oxygen etc = LGM.

Using all scopes that comprise the VLT array mean all scopes are used for a single aspect, so in effect someone is using lots of scopes at once.

Getting all of them set up and synchronised is also highly computer power dependant and that could also be a factor.

[JamesF]

The real reason is so they can think up a more extreme name for them.

VLT = Very Large Telescope,

ELT = Extremely Large Telescope,

What is the next one ???

BBT perhaps.

I think when it was planned to be even bigger, the ELT was called the OLT, or "Obscenely Large Telescope".

James

[Earl]

i seem to remember reading that most scopes are booked up years in advance, not enough to go around at all.

[hgjevans]

I don't know the particulars, or course, but I would expect it will be down to the limitations of an array of small scopes over against the versatility and sensitivity of a single large scope. 

The resolution of an array of sensors depends on its configuration and the direction you are trying to look. The 'aperture' is only as big as the profile of the array as viewed from the look-direction, so there will only be limited parts of the sky in which the VLT can look with the resolution of a 130m aperture, and it will only have 130m-resolution in one orientation - perpendicular to that it will be something rather less. OTOH, a single 39m scope will always be able to point at any part of the sky, and gather the light of a 39m aperture - which, of course will be vastly greater than the combined total of the four 8m VLT scopes. 

So all in all, I would think it's probably just a case of horses for courses - the VLT array and the ELT will be very different tools for very different jobs.

[pipnina]

The real reason is so they can think up a more extreme name for them.

VLT = Very Large Telescope,

ELT = Extremely Large Telescope,

What is the next one ???

BBT perhaps.

That is the real purpose! :grin:

Also Pipnina, could you add a "u" into Eqipment, it is slowly driving me nuts. :eek:

I always read what a person has and I always have to look twice. :D

Part of the purpose of the E-ELT is to image (hopefully) exo-planets and exo-planet formation directly.

They need the spectroscopy aspect to (again hopefully) determine the spectra from the exoplanet's atmosphere to determine the composition - water+- oxygen etc = LGM.

Using all scopes that comprise the VLT array mean all scopes are used for a single aspect, so in effect someone is using lots of scopes at once.

Getting all of them set up and synchronised is also highly computer power dependant and that could also be a factor.

Fixed it! I don't read my equipment list, so I'd never have noticed if you hadn't pointed it out!

[Nigeyboy]

Anyone seen THIS The OWL Telescope concept - Overwhelmingly Large Telescope with a primary of 100m!!! The secondary, 26.6m!!!!! Now that's just crazy!!

[Coco]

Fascinating stuff..

[DaveS]

The ultimate roll-off-shed obsy! Maybe they could look in our obsy section for ideas

[dph1nm]

Anyone seen THIS The OWL Telescope concept - Overwhelmingly Large Telescope with a primary of 100m!!! The secondary, 26.6m!!!!! Now that's just crazy!!

That is what has become the E-ELT. Basically OWL was just too expensive!

NigelM

[dph1nm]

i seem to remember reading that most scopes are booked up years in advance, not enough to go around at all.

Time is competitively allocated, usually on 6 months time scales, although longer projects are sometime guaranteed time (this has become more prevalent in recent years). But, yes, most of the larger telescopes are oversubscribed by factor of  2 or more, and I imagine E-ELT will be no different.

NigelM

[Astrobits]

The Wikipedia entry is interesting:

http://en.wikipedia.org/wiki/Very_Large_Telescope

In the interferometric mode the light bounces off 32 mirrors and 95% is lost at 1 micron wavelength. The limiting magnitudes for observations are surprising considering how deep we go with relatively small telescopes.

Nigel