|
Following the availability on the market of IR arrays able to perform ground-based astronomical observations in the atmospheric windows longward of 2.5 micrometers , we started at the Torino Astronomical Observatory a new project aimed at the construction of a thermal IR camera to be installed at the TIRGO telescope. Located in the Swiss alps at 3100 m a.s.l., this Italian facility (1.5 m f/20 IR optimized) provides during the winter months a relatively high number of nights (20%) with first-rate conditions (low temperature and emissivity) for medium-IR observations. In order to fully exploit this potential, we designed an instrument, named Two-Channel Medium IR Camera (TC-MIRC) operating over the entire 1 to 14 micrometers region and optimized for the 2.5 to 14 micrometers thermal bands. TC-MIRC covers such a broad range of wavelengths using two IR array detectors: an InSb device for the 1 to 5 micrometers region and a Si:Ga device for the 8 to 14 micrometers band. The main characteristic of the camera is that both arrays can simultaneously observe the same region. In this way, we can not only approach an almost double observing efficiency (time really spent `on target'), but also use both detectors for correlated observations and testing of unconventional acquisition techniques. It follows that TC-MIRC is a complex instrument presenting several interesting features. Thanks to the presence of two independent optical channels, the user can change the filters, adjust the focus and vary the optical scale on a channel without affecting the acquisition running on the other array. In particular, the possibility of adjusting the optical scale on each detector during the observations allows the user to select the most convenient sampling and field of view on the basis of the actual seeing or diffraction conditions, background level and scientific needs. Moreover, the adopted cryogenic system is entirely based on a mechanical closed-cycle cooler and allows very low-cost operations and easy maintenance on the mountain for long periods of time. In this paper we present the opto/mechanical design and construction, the general structure of the control system and the software architecture. We report the performances reached by the most critical parts of the instrument during the test carried out in the laboratory and at the telescope during the first engineering run in January 1994.
|
