eso9635-en-au — Organisation Release
ISAAC Undergoes Tests at ESO
14 August 1996
ISAAC (Infrared Spectrograph And Array Camera) will be the first major instrument to be installed at VLT Unit Telescope no. 1; according to the current planning, this will happen by mid-1998. ESO Press Photo eso9635a shows this complex instrument during the present, thorough technical tests in the Infrared Laboratory at the ESO Headquarters in Garching (Germany).
ISAAC is a multi-mode instrument that operates at very low (cryogenic) temperature. The cold structure which supports the various opto-mechanical sub-systems is enclosed in a vacuum vessel and is cooled to 80 K (-193 C) by means of a continuous flow of liquid nitrogen. During normal operation of the instrument, two closed-cycle coolers keep the optical components at 80 K and the two detectors at even lower operating temperatures (30 and 50 K).
The various subsystems of ISAAC have been carefully tested at operating temperature during the last months in a separate test facility. Now the complete instrument has been integrated and the optics has been aligned and adjusted in order to get the best possible images. During this phase the entire system is tested together and all critical areas are very thoroughly measured and analyzed. For instance, the vacuum performance of the system is measured and the leak and outgassing rates are measured by means of a mass-spectrometer (according to specifications, the instrument should maintain the high vacuum for at least 6 months). Moreover, the cryogenic performance of the pre-cooling system (nitrogen) as well as the steady-state system (helium) is tested. For this the instrument is fitted with 15 thermal sensors which are connected to a computerized scanning system.
In addition, the optical quality (especially the alignment of the various optical elements) is tested at cryogenic operating temperature. This measurement is done using an optical interferometer which allows to compare the beam after passing through the instrument with a perfectly flat wave-front.
The tests also concern the mechanical stability of the cold assembly with regard to the external references. For this three small mirrors are glued onto the cold structure. The positions of these mirrors are accurately recorded with a laser interferometer and an electronic autocollimator. Finally, the level of vibration introduced by the cooling system is measured by using an accelerometer and a laser interferometer. This is very important, since an excessive level of vibration would disturb any interferometric observation with the VLT.
eso9635a shows ISAAC during the preceding integration phase. Here the instrument is installed on the special maintenance system which enables it to be lifted onto the optical axis and to be rotated for integration and maintenance.
Inside the vacuum vessel (without the front flange), we can here see the fully assembled cold structure fitted with the pre-cooling tubing in which liquid nitrogen will flow in order to cool the instrument to its operating temperature. Also visible are the two closed-cycle coolers and their thermal links to the cold structure which are used during the operation.