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It's so cold!

13/07/2005

One hundred and ninety six degrees below zero. We're not talking about a bitter winter nor about bad weather. This is the temperature needed to test the Configurable Slit Unit prototype for EMIR, a second generation instrument for the Gran Telescopio CANARIAS (GTC).

EMIR is the most ambitious instrument of its kind currently under construction. It will be a wide field multi-object spectrograph and its mechanisms, as in almost all instruments for near infrared (less than 2.5 microns in wavelength), will be housed in a cryostat to keep them at a temperature of –196 ºC (77 ºK).

EMIR's cryostat will house a 'robotic' multislit mask system. This will be able to reconfigure the slits without warming and opening the system, swapping masks and closing and re-cooling every time the pattern needs to be changed. To do all this manually would take up to a week in an instrument as large as EMIR.

The Configurable Slit Unit or CSU is made up of sliding bars that can move into and out of the field of view. It will block some of the light beam to leave only the part which is of interest. The specifications demand that the slit pattern can be changed in just five minutes.

The Dutch company “Janssen Precision Engineering” (JPE), which has been contracted to design and build the CSU demonstration prototype, has reached the first milestone: the construction and testing at cryogenic temperatures of a model of the linear actuator that will manipulate the individual bars inside the CSU.

The tests, which were carried out at the IAC in conjunction with a team from EMIR, were successful. Using a specially adapted cryostat refrigerated with liquid nitrogen, the mechanism's performance in critical positions - those which could give rise to the most problems - was tested for a week. This allowed it to be optimised and its compliance with specifications to be checked.

The next step will be to complete the position control system, which will place each bar in position to within 10 microns in a trajectory of 250 mm. The final prototype is due to be delivered in December. It will have 6 bars with their 6 actuators and control systems - a mini CSU which, by demonstrating how it will work, could be said to be the predecessor of the final, 100 bar unit.

Text: Natalia R. Zelman
Pablo Redondo (EMIR team)

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