Put away your binos and spotting 'scopes - you've about to be outgunned by the world's largest telescope with an 'eye' 42m [137ft] across
Attended a great talk at the BAA last week by Dr David Walker [UCL] on the European Extremely Large Telescope [E-ELT] which, when it comes online in 2018, will be 100 times more sensitive than any current worldclass telescope
Here's my summary that's going into our club newsletter...note the cars lower right in this model picture!
42m EUROPEAN EXTREMELY LARGE TELESCOPE [E-ELT] by Dr David Walker
at BAA Meeting - London on 2009 May 27
Dr David Walker of UCL and Zeeko Ltd gave the BAA Meeting at Burlington House - Piccadilly the first presentation of the mirror making techniques for 42m European Extremely Large Telescope [E-ELT] which on completion in 2018 with be 100 times more sensitive than any current world-class telescope. Dr Walker noted that the probable limit had been reached with single monolithic mirrors of 8.4m diameter moulded in a spinning furnace. The primary mirror is only one element of the overall design. The logistics of transport and road system to get the mirror to the mountaintop, the massive aluminising plant to regularly recoat it and the downtime wasted when this happens add enormously to the cost of the project.
The modern approach is more manageable mosaic mirrors made of smaller hexagonal elements that fit together like 'cells' in a giant beehive. The technique is proven with the dual Keck telescopes on Mauna Kea Hawaii. To give some sort of perspective to the ESO project, the Keck mirrors are each 10m [33ft] diameter made from 36 hexagonal segments that work together as a single piece of reflective glass of Zerodur glass-ceramic. The E-ELT will have 984 hexagonal mirrors each 1.45m across forming a single a mirror 42m [137ft] in diameter!
TWO INITIAL ESO DESIGNS eg the 100m Overwhelming Large [OWL] and 50m ELT telescopes were abandoned as too complex where each hexagonal mirror segment was spherical in form [easy to make] but required complex and large secondary optics to remove errors caused by the primary mirror. The breakthrough came with manufacture of aspheric optics that greatly simplified the basic design to just two mirrors in a classic Cassegrain system eg M1+M2. The primary mirror has an amazingly 'fast' f/1 focal ratio with the secondary f/4 focus near M4. Laser light is used to align all 984 mirror segments to form a perfect single image. A smaller third concave mirror M3 and two flat mirrors M4 [perforated] and M5 [with adaptive-optics tweaking the surface 1000 times per second] send the final f/17 beam to the ultra-sensitive CCD detectors. The telescope's field of view is 10-arc minutes eg 1/3rd the diameter of the moon. However the moon will NOT be a target for E-ELT as its light would damage the sensors! A spare of each mirror shape is set aside and can be swapped-out for mirrors in the telescope that need realuminising - telescope downtime is thus minimised.
TEST MIRRORS: Zeeko in UK and a French consortium are making test mirrors for the project. Each segmented mirror is 1.45m across by just 50mm thick and is prone to astigmatic warping but a steel harness behind each mirror tweaks it back to shape!
Dr Walker described the precision grinding machine that initially shapes each mirror by removing glass to form a shallow 6mm* deep concave surface. Each ring of mirrors, outwards from the centre, will have a slightly different shape to comply with the overall design. Zeeko has developed new polishing techniques using inflatable sub-diameter tools to finally shape and figure each mirror segment to fractions of the wavelength of light. From rough grinding to final precise figuring has been reduced to 31 hours per mirror segment - 'speed' is essential if the target for the nearly 1000 mirrors, needed for the complete project, is to be met.
SITE OF E-ELT: Several sites, either side of the Atlantic, are being considered for the new telescope - Roque de los Muchachos (Canary Islands), Ventarrones (Chile), Aklim (Morocco), and Macon (Argentina) - the latter pair have no major observatories currently. Clearly the Canaries and Morocco are the closest to ESO's base in Germany with distinct logistic advantages. Small telescopes [typically amateur 11-inch aperture SCT] are used to test stability of the air etc at the projected observatory sites.
TELESCOPE STRUCTURE: The E-ELT is designed as a Nasmyth telescope. It has a footprint of about 80m diameter and is about 60m high. The altitude and azimuth structures weigh together nearly 5000 tons. This structure supports the five mirror optical design and accommodates two Nasmyth platforms - each about the size of a tennis court and can host several instruments. Various designs were considered for the telescope enclosure - the project settled for a classic dome design.
For E-ELT overview logon to
ESO - The European ELT
Zeeko's polishing techniques at
http://www.zeeko.co.uk/videos/animationb.wmv
*6mm=mirror sagitta /dip =42-sq root [42sq - 0.725sq]MG.
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