3. General technical description

The optical scheme comprises the single collimator and a set of cameras. All the optical elements are mounted in a cryostat aimed at keeping components at a temperature of about 80 K.

The focal plane masks (field stops or slits) are mounted on a wheel. Another removable field stop, mounted right after the focal plane, makes possible spectro-polarimetric measurements (see below).

The collimator is an achromatic doublet lens, which images the entrance pupil of the TNG and provides an almost parallel beam (F-number about 2000) at the pupil plane, where Lyot stops can be placed.

Right after the pupil plane, two close wheels mount filters and grisms. Three interchangeable optical systems rely the image of the focal plane on the detector plane with the desired magnification. They are three cameras, respectively for wide-field imaging, small-field (high-quality) imaging, and high-resolution echelle spectroscopy. The fourth position is reserved to an optical system which images the entrance pupil on the detector, aimed at an accurate alignment of the telescope to the instrument pupil and not used in normal observations.

The low-resolution spectroscopic mode makes use of the wide-field camera by inserting one of the grisms, located on the second filter wheel, into the parallel beam after the pupil.

The rejection of stray and thermal light is performed as follows. At wavelengths < 1.9µm the thermal emission is negligible and the instrument, as an optical camera, taking full advantage of the M2+M3 TNG baffles. At this wavelength range, the stop holder will move away the Lyot stop; the filter itself provides an oversized pupil stop. At wavelengths > 1.9 µm the thermal emission from the TNG baffles is blocked by the cold stop that is accurately positioned on the pupil plane, where a well-corrected pupil image is available, by the stop holder. The adaptive optics module will modify the size of the pupil and will move its position forward; in this case, a possible option is to to leave the original pupil plane free from obstructions, and put a suitable stop at the correct location by means of a tube mounted on the second filter wheel.

The sensitive element of NICS is the 1024x1024 pixels HgCdTe array detector (Rockwell Science Center), named HAWAII (from HgCdTe Astronomical Wide Area Infrared Array). It has a 18.5 µm pixel pitch and it is sensitive to radiation at wavelengths between ~ 0.90 µm and ~ 2.6µm. Its performance in term of dark current, efficiency, and read noise, as measured on the first few devices, is equal or better than the 256x256 NICMOS 3 [8].

The electronic (read-out) noise is dominated by the detector and by the first amplifier; it is expected to below ~ 20 e-, depending on the actual performance of the detector. The minimum read-out time is limited by the time required by the electronics to send commands to the detector and to perform the analogic-to-digital conversion, that is about 0.3 s to read the entire array. In a common read strategy, during the integration at least two measurements are performed, one at the beginning (to sample the reset bias level), the second at the end of the integration ramp. The electronics can also be programmed to perform multiple (non-destructive) sampling of the integration ramp, still at a maximum rate of 0.3 s, to improve the signal-to-noise of the ramp slope detection, beyond the level of read-out noise. The latter mode will be important for the high resolution spectroscopic observations, when the low background photon noise is not the dominant source of noise.