From ESOblog (EU): “Up close and personal with the Miniscule Extremely Large Telescope” MELT

From ESOblog (EU)

29 January 2021
HighTech ESO

The optical setup which comprises the MELT. Highlighted are the analogues for the M1 and M4 mirrors.
Credit: ESO.

Justin Tabbett

In a laboratory at ESO headquarters, there is a setup of optics, lenses and mirrors aiding the development of the Extremely Large Telescope — the Miniscule ELT, otherwise known as the MELT. Set to test and validate key elements of the ELT, ESO’s upcoming flagship telescope, the MELT serves as a proving ground to learn valuable lessons for commissioning — setting up and operating — the ELT.

“The MELT has been built to fail and to learn from those failures ahead of the completion of the ELT,” says Carlos Diaz Cano, MELT software engineer and project manager.

The ELT [below] will be the world’s largest optical and near-infrared telescope with a 39-metre main mirror, the largest of a five mirror setup, and is set to push the boundaries of astrophysical research. For such an ambitious project, it is useful to be able to test specific elements of the telescope and how they interact before it is operational. Scaling down the entirety of the ELT to fit in a lab bench barely six square-metres in area would be a near-impossible task. Instead the MELT emulates only specific components of the ELT with the light passed via four different mirrors connected in a “telescope optical path”.

Light reflects in five different mirrors in its journey through the ELT.
Credit: ESO/L. Calçada/ACe Consortium.

MELT’s main mirror and control software

One of the highlight features of the ELT is its main mirror, M1, composed of 798 hexagonal segments, with each segment measuring 1.4 metres across and just 5 cm thick. The M1 analogue on the MELT is composed instead of 61 segments, in total measuring just 153 mm across. This analogue mirror was incorporated from a previous ESO experiment, the Active Phasing Experiment, which was done at ESO’s Paranal Observatory and tested the control of segmented primary mirrors for the ELT. After this experiment returned from Paranal, the engineers at ESO headquarters incorporated adaptive optics and additional optical components. With a few additional changes in hardware and almost completely new software and electronic infrastructure, this system is now known as MELT.

A number of control modules for the MELT controlling several telescope features including motors, light sources, and M5 switches. Credit: ESO.

Sixty one segments are understandably easier to work with than 798, and give us an idea of the challenges we’ll face with the ELT,” says Pascaline Darré, optical engineer for the MELT. “But we have to bear in mind that the ELT will be different because it’s on another scale.”

While there are fewer segments on the MELT’s M1 analogue, there is still a substantial amount of work involved in operating them. Each segment needs to move independently to ensure perfect co-alignment of all the segments, enabling them to behave as a single larger mirror — for both the ELT and the MELT. The movement of these segments is managed by control software. An overarching challenge the MELT team are facing is the development and integration of control systems for the miniaturised telescope.

“The control system is the ‘glue’ that joins all the components together,” says Diaz Cano. “Every bug we fix now is time saved for the ELT in the future.”

The team has been integrating several control system software products developed to perform different functions on the ELT, to allow the different components on the emulated telescope to communicate with each other. In particular, they have been modernising the control system software to match standards for the ELT.

“Aside from using the software for MELT, we plan to develop a set of software libraries which will actually be available for use on the ELT,” explains Diaz Cano.

The MELT’s other mirrors

On the ELT, M2 reflects the light from M1 to M3 allowing the light to continue on its path between mirrors before being captured by scientific instruments. Although MELT simulates the ELT, its optical design is not identical, and it does not have a M3 mirror. Instead, MELT’s M2 analogue is a special lens assembly that emulates the ELT’s M2 and can also intentionally generate optical aberrations (such as astigmatism). These aberrations blur and distort images, allowing engineers to learn how to correct them on the ELT.

The Miniscule ELT (MELT)’s M2 analogue is a special lens assembly that emulates both M2 and M3 of the ELT and can also intentionally generate optical aberrations (such as astigmatism). These aberrations blur and distort images, allowing engineers to learn how to correct them on the ELT. Credit: ESO.

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ESO (EU) is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening star).
elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo.

Glistening against the awesome backdrop of the night sky above ESO_s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 UT4 of the VLT, a major asset of the Adaptive Optics system.

ESO VLT 4 lasers on Yepun.

Part of ESO’s Paranal Observatory, the VLT Survey Telescope (VISTA) observes the brilliantly clear skies above the Atacama Desert of Chile. It is the largest survey telescope in the world in visible light at an elevation of 2,635 metres (8,645 ft) above sea level.

ESO/Cerro LaSilla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres.

ESO ELT 39 meter telescope to be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

ESO/MPIfR APEX high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft).

A novel gamma ray telescope under construction on Mount Hopkins, Arizona. a large project known as the Čerenkov Telescope Array, composed of hundreds of similar telescopes to be situated in the Canary Islands and Chile. The telescope on Mount Hopkins will be fitted with a prototype high-speed camera, assembled at the University of Wisconsin–Madison, and capable of taking pictures at a billion frames per second. Credit: Vladimir Vassiliev .