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May 28, 2023



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EMIR, seeing through slits (II)


Our last issue (EMIR, seeing through slits I) highlighted the engineering challenges of EMIR (Espectrógrafo Multiobjeto Infrarrojo or Multi-object Infrared Spectrograph), a second generation instrument for the Gran Telescopio CANARIAS (GTC).

The instrument has been designed to be kept at cryogenic temperatures, below –200 ºC (73 ºK), with a robotic multislit mask that will be mounted inside the cryostat itself. This will change the “shape” of the masks as and when needed, minimising the amount that the instrument needs to be handled.


Developing a new mask system for EMIR is no small task. This technique has already been used in the visible range, when the masks are kept at room temperature, but it has never before been considered for use with an infrared instrument. This meant that, in March 2003, EMIR reached a critical point: a Review Committee was asked to approve the preliminary design and a key factor in their decision was whether it would be possible to use this kind of mask system with this type of instrument. The stark reality was that without multislit masks EMIR would be a far less attractive proposition for the scientists.

A prototype of this new type of robot mask, which can be remotely reconfigured, is being developed for the “James Webb Space Telescope” (JWST). A similar prototype, designed to be used with EMIR, was considered by the review panel. It convinced even the most dubious that it was possible to mount it on EMIR and to make it work.

The system, dubbed Configurable Slit Unit (CSU) consists of sliding bars that move into and out of the field of view, blocking out some of the light so that only light of interest gets through. This will create the slit mode.

Although the final number has yet to be decided, the current design uses 104 bars, split into two lots of 52, which are mounted at either side of the field of view. Each bar on one side is aligned with another opposite, and they can move together closely into the field of view leaving just a small gap, called an “individual slit”.

The multislit mode will be created by moving each pair of bars into the field of view to produce 52 slits in the focal plane. When the instrument is not working in the infrared, and Image mode is needed, the bars will withdraw completely from the 30x30 cm (6’x6’ arc minutes squared of sky) field of view. The system will also be able to operate in wide slit mode: the bars will all move into the same position, leaving a single 30cm slit the length of the field of view.

The specifications require the entire mask to reconfigurable (to change the multislit pattern) in just five minutes, developing all the process at a temperature of –200 ºC (73 ºK).


Looking at EMIR it is easy to see why it is the most ambitious instrument of its kind currently under construction: it will work in the near infrared, will be a wide field multi object instrument and it will be mounted on a 10.4 m telescope.

In addition, the size of the focal plane - 30x30 cm - will give a wide field of view, which will increase the instrument's scientific appeal because of the number of objects that it will be able to observe at the same time and the amount of time that this will save.

Many scientists are eagerly watching to see whether the instrument will work, in the hope that these it will be possible to use these techniques on instruments being developed for other telescopes. This means that there is a twofold challenge for the EMIR team: building a completely novel instrument and blazing the trail for a new technique that holds out enormous promise.

The words at the beginning of these EMIR bulletins say it all: "Throw caution to the wind, mortgage the Prado if need be but "go for it!...In my opinion, there's just no choice. You've got to make EMIR happen”.

Or the words said about EMIR by Roser Pelló, from the Observatoire de Midi Pyrénées Astrophysics Laboratory in France, at the second “International Workshop on Science with the GTC”, held in México in February 2004: “The benefits it will bring relative to the work we are currently doing with telescopes and the instruments currently available are immense. It will easily increase efficiency by a factor of 60 to 100”.

These are the words of people passionate about astronomy who, like us, will be witness to the progress of EMIR. If you come up close and look you might just catch a glimpse...through slits.

Natalia R. Zelman

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