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:
-
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.
-
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.
-
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.
-
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.
-
High resolution (R~ 9500) echelle spectroscopy by means of
a silicon grism and an optimized camera.
-
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
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
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
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].
