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EMIR, seeing through slits (I)

15/10/2004

"As an engineering challenge, EMIR ranks as the most difficult instrument that I know (…). Nonetheless my recommendation would be to place it above all others. Throw caution to the wind, mortgage the Prado if need be but "go for it!" If you bring it off, it will establish GTC and IAC among the most prominent and scientifically productive observatories in the world. In my opinion, there's just no choice. You've got to make EMIR happen".

These are the words of one of the eminent scientists who took part in the first review of the instruments for the Gran Telescopio CANARIAS' (GTC). EMIR (Espectrógrafo Multiobjeto Infrarrojo or Multi-object Infrared Spectrograph) is a second generation instrument that will be installed at the GTC to study the history of star formation in the Universe.

Until now the main challenge has been to develop a system of multi-slit masks capable of operating at cryogenic temperatures, a technologically complex process on which progress is being made due to the work of the EMIR development team at the Instituto de Astrofísica de Canarias. According to members of the team, EMIR will be useless unless it can be used with multislit masks.

SLITS OF LIGHT

The Academia Real de España (the institution that regulates the Spanish language) defines the word 'rendija' (slit) as a “crack, chink or long narrow opening that runs from one part of a solid object, such as a wall or table etc, to another," and as a "crack through which light and air can pass."

It is this second category that defines the slits we are talking about in this bulletin.

For astrophysicists, slits are synonymous with spectroscopy - studying the physical properties of celestial objects by analysing their spectra (their constituent colours or wavelengths, which together make white light: purple, blue, indigo, green, yellow, orange and red). Observing the spectra of stars and other bodies in the Universe provides important information not just about their chemical composition but also their physical properties (temperature, density, pressure) and speed of rotation.

Spectrographs, or spectroscopes, are instruments that separate both visible and invisible light into their different component colours or light wavelengths. They are like the raindrops that disperse sunlight to produce rainbows.

The combination of the width of the slit through which light enters and the internal optics of the spectrograph define spectral resolution in this type of instrument. The slit also allows one part of an object to be selected for investigation. Light can enter the instrument in a number of ways: in normal, or image, mode - i.e. with no control over the way light enters - or in slit mode. This second technique allows the origin of the light reaching the instrument to be defined by 'covering up' anything that the scientists are not interested in. This is the function of the so-called 'masks', which are usually sheets of metal (most frequently aluminium), into which a hole or slit has been bored so that they only let through light coming from the object being studied.

To maximise the observing efficiency of today's telescopes a range of different techniques are used, and prominent amongst them is multislit spectroscopy. The density of objects in certain parts of the sky is so great that observing them one by one using a single slit would take a prohibitive amount of observing time. This is where multi-slit spectroscopy comes into play, since it allows many objects to be observed at once and so makes best use of the telescope's focal plane. Multislit masks are used for this - sheets of metal with a number of slits which, when mounted in the telescope's focal plane, coincide with the position of the objects that are being observed.

For example, the LIRIS instrument, which was built at the IAC for the 4.2m “William Herschel” telescope, installed at the IAC’s Observatorio del Roque de Los Muchachos on the island of La Palma, can observe 25 objects at the same time by using masks containing the same number of slits, and it offers the option of using 10 different masks simultaneously, but selecting them sequentially. This means that it is possible to look at up to 250 objects without having to make any adjustments to the instrument. If a single wide slit were used instead, just 10 objects would be able to be observed.

Logically, every time either the object under study or the field of observation change, the mask has to be swapped. This is one of the difficulties that EMIR must overcome.

EMIR'S CRYOGENICS

Instruments that work in the infrared have to be mounted inside a cryostat so that they are kept at a temperature of – 200 ºC. This is essential for observing in this range of the electromagnetic spectrum. If it were necessary to change the masks then the whole instrument would have to be warmed up, opened, closed up and re-refrigerated every single time.

For an instrument as large as EMIR this would be very difficult and, even if it were possible, observing time would be lost: it would take around a week for EMIR to complete the room temperature cycle so that it could be opened and the components swapped, and cool down once it has been closed.

In the early days, the possibility of installing the masks in a separate, small cryostat was discussed. This could have been isolated, and heated and cooled independently, so that the masks could have been changed without affecting the instrument itself.

However, another much more interesting proposal also emerged: a robot built into the instrument’s cryostat that would “shape” the masks, avoiding the need to handle the instrument any more than strictly necessary.

There will be more about this mask in our next bulletin. Don’t miss it!

Natalia R. Zelman

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