Linux at the Arcetri Observatory Infrared Group

The Infrared group at Arcetri Observatory is finishing the debugging phase of Near Infrared Camera Spectrometer (NICS), an advanced instrument for imaging, spectroscopy and polarimetry in the near infrared wavelength range (1-2.6 \mu m). It will be one of the scientific instruments hosted at the Galileo National Telescope (TNG). NICS is an optical/infrared four-meter class telescope with active optics control, built by the Italian astronomical community in Canary island of La Palma (Spain).

Figure 1. Galileo National Telescope

The Arcetri Infrared Group began in the late 1970s and is composed of scientists and technical personnel who design and build infrared instrumentation. The group has acquired experience in the fields of optical design, cryogenics, mechanics, electronic data acquisition, control software, user interface, data storage and archiving. Our group has operated in the framework of a CNR-OAA cooperation.

Figure 2. A View of the Instrument during Laboratory Testing

The NICS is composed of many small subsystems interacting together, to achieve very flexible observing capabilities. The optical, mechanical and detector parts are enclosed in an aluminum cryostat built by Infrared Labs, Tucson, Arizona. This enclosure maintains component temperature of 80 degrees Kelvin and a pressure well below 10<+>-3<+> mbar (see Figure 2). The cooling is achieved by a CTI cryocooler, a refrigerating machine working with high-pressure gaseous helium, while the low pressures required are attained by a three-stage pump and a chemical adsorption pump placed inside the cryostat. The mechanical assembly is complex and comprises seven stepper motors driving the mobile components inside the dewar and their reduction gears. The motors are mounted on the external surface of the cryostat and are driven by a commercial controller that receives commands from a RS-232 serial line. The flexibility of NICS was attained by means of a multiconfiguration optic. All reconfigurations are motorized and under remote control. The general layout is as follows:

The electronic system executes both acquisition and control operations under the management of an external computer. The head of the acquisition system is a 1024x1024 pixels array of the HAWAII series built by the Rockwell Science Center. The HAWAII (HgCdTe Astronomical Wide Area Array) detector is a ternary DRO (Direct Read Out) sensitive to radiation of wavelengths between 0.90 and 2.6 \mu m, and it is characterized by good values of dark current, efficiency and readout-noise. Particular attention has been paid to planning and building low noise analogic and ADC electronics to not limit array performance. Dealing with very faint objects that are detected by means of a few tens of e<+>-<+>, it is very important to reduce the electronics noise to the lowest possible level. To give an idea of how sensitive NICS is, we claim that it is capable of detecting the IR emission of a burning cigarette placed four times the distance of the Earth to the Moon. The architecture for the array controller grew on the project of the CCD Working Group of TNG, but it has been modified to fulfill the heavier requirements for higher readout speeds required for near-infrared observations. The controller is based on a DSP Motorola 56001 processor that generates the clock pattern to access and read the array multiplexer, and on a set of Transputer processors for commands and data handling. Transputers are old microprocessors that support parallel processing through on-chip hardware and can be connected by means of their fast serial links. We use them as the building blocks for complex parallel processing systems. The Transputer module interfaces with the host computer (a PC station) through a serial optic fiber link with a transfer rate of nearly 20MB/s. Because of the short typical integration times and the large quantity of data acquired (1024X1024X16 bits), some data preprocessing, such as subtraction of the frames in double-sampling mode, is done in the acquisition electronic by the transputers to limit the required bandwidth of the data transfer. Data is then sent to the host computer through the fiber link. Electronics include some other boards for general housekeeping, controlling the motors' movements and collecting the dewar system data such as pressure and temperature.