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June 2, 2023



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Points of view: Jordi Cepa. "The challenge of Osiris made real"


When the Gran Telescopio Canarias (GTC) sees first light next year, it will be the most advanced optical telescope in the world. The GTC will rely on OSIRIS, an instrument built in Spain, to allow it to work to its full potential during the first few years.

Amongst other things, Jordi Cepa is Professor of the Astrophysics Department at the University of La Laguna and a Researcher at the Instituto de Astrofísica de Canarias. However, he is also Principal Researcher for OSIRIS. In this edition of the bulletin we are going to tell you about the secret that makes OSIRIS such a powerful instrument: tunable filters.


In the coming year, we will start to test the alignment of the GTC's 36 primary mirror segments. The GTC's primary mirror will be 10.4 metres in diameter. When it takes its place in the array of telescopes at the Observatorio del Roque de los Muchachos, the GTC will work with almost ten times more light than any of the others. Only the giant Keck telescopes in Hawaii will come anywhere close.

A telescope's capacity for grasping light is enormously important to its ability to observe the weakest and farthest-away objects. However, the high-precision instruments mounted on the telescope are the tools that will deliver the kind of discoveries that will push back the boundaries of astronomy.

OSIRIS is a Spanish-built instrument that will be in operation at the GTC from first light. It will be the only Spanish-built instrument working during the GTC's first few years. OSIRIS is the product of collaboration between the Instituto de Astrofísica de Canarias (IAC) - which controls the project and is the majority partner - and the Instituto de Astronomía de la Universidad Nacional Autónoma de México (IA-UNAM). OSIRIS has received financal backing from GRANTECAN, the public company responsible for building the GTC; the Spanish Government Science and Technology Ministry's National Plan for Astronomy and Astrophysics; and the Instituto de Astrofísica de Canarias.

With the construction of the GTC and of OSIRIS, Spain will join the league of big players in the field of astronomy, competing with the United States and the European Southern Observatory (ESO). Joining this exclusive club is not easy: the construction of an instrument for a 10 metre class telescope is feat of extraordinary complexity. The reason? Well, the size and weight of the instrument's components are in proportion with the size of the telescope, but the precision with which they are engineered has to be even greater. That is why it is much more difficult, and costly, to build an instrument for a 10 metre than for a 4 metre telescope, and it calls for very significant investments in terms of both effort and technology.

The construction of such an instrument also demands the total commitment of a team of astronomers and engineers for a period of several years - typically five or six in the case of OSIRIS, although the development of some instruments in other countries has been an adventure lasting eight or more years.

Spanish and overseas companies have been involved in the construction of the marvel of technology and precision that is OSIRIS, and they have all been working under the direction and supervision of engineers from the Instituto de Astrofísica de Canarias.

The power of tunable filters

OSIRIS' power comes from its tunable filters, which make it a unique and cutting-edge world-class instrument, an example of the innovation and technological development demanded in instruments designed for large telescopes.

The concept of a filter will be familiar to all, whether it be a solar filter used to observe the sun through a telescope, the filter in a camera or even a pair of sunglasses. The filters used in astronomy are not very different: they select a part of the light arriving at an instrument so that each colour can be studied separately. The only difference between these and normal filters is the quality and precision that they bring to the job.

"Wide band" filters through just a single colour of the spectrum, in the same way as a colour filter. "Narrow band" filters let through only one of the tonalities of a colour. In more technical terms, they let through a smaller spectral interval. With narrow band filters we can look with total precision and spatial resolution at the gas emisssions of objects like the galaxies of Andromeda, planetary nebulae and ionised hydrogen regions like Orion. This would not be possible using other techniques like spectroscopy.

The gas from these objects is emitted in many different zones of the spectrum - these are what we call "emission lines". Each one of the chemical elements emits a number of these lines in different zones of the spectrum. Emission lines give us valuable information about the temperature, chemical composition and other features of young and massive gas ionising stars and other objects, so that it is vital for us to be able to observe as many of them as possible. Just to make things more complicated, galaxies exhibit "redshift", a phenomenon in which the emission lines are progressively displaced towards the red areas of the spectrum in proportion to our distance from them.

"Conventional" filters only let through a colour, tonality or a fixed spectral range. If we want to look at a different spectral range in order to study a different emission line, or the same line in another galaxy with redshift, then we need a different kind of filter.

To conclude then, in order to study phenomena like the evolution of galaxies, we would need a huge number of narrow band conventional filters to cover the whole range of the visible spectrum. This would allow us to look at any group of emission lines in any galaxy and to compensate for redshift. Of course, working with this number of conventional filters is not possible: no observatory has them in sufficient quantities due to the prohibitive cost. In fact, few observatories have enough filters even to allow accurate observation of nearby galaxies.

Tunable filters provide a solution to this problem. As their name indicates, they can be tuned at will to alter the wavelength, bandwidth or spectral range that they let through. They can do this because the laminates they are made of are extraordinarily flat and parallel, are very close together, and have a reflective layer within them. Three piece electric devices embedded between the laminates give precise and rapid control of their parallel alignment and of the distance separating them. To give you an idea of how accurate they are: if the tunable filter had the same diameter as Spain, its surface would have no bumps higher than 1 cm, whilst the distance between the layers of laminate could be adjusted to within a margin of error of just 2 mm. The part of the spectrum and the spectral range (bandwidth) that pass through the filter are determined by the distance separating the laminates, which can be altered by the piece electrics in just a tenth of a second.

Tunable filters are extraordinarily versatile: each one of OSIRIS' tunable filters has the same power as 19,000 conventional filters. If this number of filters were placed on top of one another they would reach a height of 152m, the same height as the Torre Picasso in Madrid or the Torre Mapfre in Barcelona, and 41 metres higher than St Paul's cathedral in London.


Narrow band conventional filters only let through a single fixed and unchanging colour in the visible spectrum.

Narrow band conventional filters let through just one fixed and unchanging tonality of a single colour in the visible spectrum.

One of OSIRIS' tunable filters. It is made of two layers of ceramic laminates with an internal reflective coating. Three piece electric devices (for greater clarity only two of them are shown) control the parallel alignment and distance separating the laminates with great precision. They are so accurate that, if the filter had the same diameter as Spain, the distance between the laminates could be adjusted to within a margin of error of just 2mm. By altering the distance between the laminates the part of the spectrum and bandwidth that the filter lets through can be altered at will (tuned) in just a tenth of a second. The extreme versatility of OSIRIS' tunable filter makes it as powerful as 19,000 conventional narrowband filters. In addition, because it can isolate narrower spectral ranges than conventional filters it allows us to observe much weaker objects.


Text: Jordi Cepa, Principal Researcher of OSIRIS and the OTELO project, Professor of the Department of Astrophyiscs at the University of La Laguna and Researcher at the Instituto de Astrofísica de Canarias. In addition, Jordi Cepa is currently Director of the Department of Astrophysics and Education Coordinator at the Instituto de Astrofísica de Canarias. The development of OSIRIS has been made possible by the support of astronomers from a number of Spanish and overseas institutions, as well as by the dedication of an extraordinary team of engineers from the IAC and IA-UNAM.

For more information:

Natalia R. Zelman

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