2. Observing capabilities

The new 3.5 m Italian National Telescope Galileo (TNG) [2]. , which will be completed in 1998 on La Palma (Canary Islands), includes a near-infrared imager/spectrometer in its current Instrument Plan for first light operations [4][5]. NICS operates in the wavelength range 0.95µm - 2.50µm, that is conveniently covered by the currently available HgCdTe (MCT) large-format focal-plane array detectors. NICS will be the only infrared instrument for the first light of the TNG; as a consequence, we decided to incorporate a sufficient degree of operation flexibility by providing the instrument with six different observing modes, three aimed at imaging and three at spectroscopy:

  1. Wide field imaging with a plate scale of 0.25"/pixel and a total field, as projected on the sky, of more than 4'x4'. Both wide- and narrow-band filters will be available for photometry and in-line imaging. As well, thanks to a double Wollaston prism it will be possible to perform polarimetric measurements.
  2. Small field imaging with a plate scale of 0.13"/pixel (~ 2'x2'field of view) and very good image quality. The photometric capabilities are the same as for the wide field mode. The Wollaston polarizers are not available in this mode.
  3. Imaging with fast read-out rate. While the minimum integration time will be ~ 0.6 secs when using all the sensitive array detector area, it will be possible to read a sub-area of 16x16 pixels at a rate of 1 kHz, or more, for fast tracking purposes, tip-tilt correction, lunar occultation observations, and other observations based on fast photometry.

  1. Low dispersion long-slit (4' slit) spectroscopy with a resolving power between 300 and 1300. Thanks to another double Wollaston prism it will be possible to perform spectro-polarimetric measurements.
  2. High resolution (R~ 9500) echelle spectroscopy by means of a silicon grism and an optimized camera.
  3. High resolution long-slit spectroscopy, as at point II, at the wavelengths where narrow-band filters are available.

The observing modes are summarized in Table 1.

Table 1: Observing modes

Table 1 Observing Modes

Moreover, high spatial resolution imaging, at the diffraction limit of the telescope, is possible by means of the external adaptive optics module (for first light, only the tip-tilt correction will be implemented) [14]. By using the reimaging optics of the adaptive module, which has an f/33 output beam, two plate scales (about 0.08" and 0.04" per pixel, respectively) will be available with the optics of the wide-and small-field cameras.

A preliminary list of filters is reported in Table 2: they include the ordinary broad band filters, J, H, and K, the K'short (that minimizes the K-band thermal background), newly defined I and J filters, narrow-band filters for in-line imaging of the most intense lines emitted by many astronomical objects, and the corresponding narrow-band filter to sample the near continuum.

Table 2: Filters

Table 2 Filters

Five low-resolution grisms are currently available, their properties are listed in Table 3 [9]. The high resolution observing mode will use a chemically etched silicon grism that should operate along with low-angle Si grisms and/or resin replica grisms, used as cross-dispersors, to produce echelle spectra from 1.15 µm to 2.48 µm with a resolution of ~ 104 [9]. It will be possible to use the narrow band filters as order sorters; this will allow to perform high resolution long-slit spectroscopy in the wavelength ranges covered by the available narrow band filters. However, the high resolution mode is still under study and its implementation will be delayed to follow technological developments of silicon grisms.

Table 3: Grisms

Table 3 Grisms

The polarimetric modes, both imaging and spectroscopic, take advantage of two wedged double Wollaston prisms that will allow to measure the polarized flux at angles 0, 45, 90 and 135 degrees in one shot. Therefore, it will be possible to determine the first three elements of the Stokes vectors with no concern about variability of the atmospheric transmission [13].

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