Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.arcetri.astro.it/science/irlab/doc/arnica_sait_93.ps Дата изменения: Thu Jul 28 18:22:23 2005 Дата индексирования: Sat Dec 22 07:44:32 2007 Кодировка: Поисковые слова: optical telescope

ARNICA, THE TIRGO IMAGING CAMERA
FRANCO LISI 1 , CARLO BAFFA 1 , LESLIE HUNT 2
1 Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5 I­50125 Firenze, Italy
2 CAISMI--CNR, Largo E. Fermi, 5 I­50125 Firenze, Italy
ABSTRACT. ARNICA (ARcetri Near Infrared CAmera) is the imaging camera for the near
infrared bands between 1.0 and 2.5 Їm that Arcetri Observatory has designed and built as a
general facility for the TIRGO telescope. The scale is 1 00 per pixel, with sky coverage of more
than 4 0 2 4 0 on the NICMOS 3 (256 2 256 pixels, 40 Їm side) detector array. The optical path is
compact enough to be enclosed in a 25.4 cm diameter dewar; the working temperature of optics
and detector is 76 K. The camera is remotely controlled by a 486 PC, connected to the array
detector control electronics via a fiber--optics link. A C--language package, running under MS--
DOS on the 486 PC, acquires and stores the frames, and controls the timing of the array. We give
an estimate of performance, in terms of sensitivity with an assigned observing time, along with
some details on the procedure of frame acquisition at the telescope.
1. Optics and mechanical design
The design of ARNICA was driven by the interest of Italian astronomers in observations of
diffuse objects; while the main observative mode is imaging of fields as large as 4x4 arcmin
(broad-- and narrow--band photometry, polarimetry), we incorporated into the project the
possibility of using the camera as a long slit spectrometer, based on grisms, with low or
moderate resolving power; a full description of ARNICA is found in Lisi et al. 1993.
The project is the result of a collaboration of the Arcetri Observatory with the De­
partment of Astronomy of the University of Florence and the National Research Council
of Italy (CNR--CAISMI). The final design was completed at the end of 1990; the first func­
tional and observational tests have been performed at the TIRGO telescope in November
and December, 1992.
The optical scheme of ARNICA is sketched in Fig. 1. A set of four lenses accepts the
image of the f/20, 1.5 m Cassegrain TIRGO telescope and transfers it onto the detector,
while modifying the plate scale and creating an image of the entrance pupil. After the
calcium fluoride entrance window (about 70 mm diameter), the cold stop, mounted onto
a three--position slide, defines the field of view at the Cassegrain focus of the telescope.
In addition to the field stop, the slide, which is manually operable and in close thermal
contact with the radiation shield, holds the slit for spectroscopy and a shutter which is
used to completely blind the detector. The first two lenses, a detached doublet made of
fused silica and zinc sulfide for achromatization, create an image of the telescope entrance
pupil about 5 mm in diameter. The second lens of the doublet has an aspheric surface,
while the other optical surfaces are plane or spherical. The cold pupil stop is placed at the
pupil plane where the beam is nearly collimated; just before this plane there is the filter
which defines the wavelength range. The last two lenses, a detached doublet made of zinc
sulfide plus fused silica, relays the image of the field onto the detector, and modifies the
plate scale from 6.8 arcsec/mm to 25.0 arcsec/mm. The total size of the optical train is
such that it can be mounted on a cold plate with a diameter of 25 cm.
The lenses are coated with an anti--reflection coating which gives a reflectivity less
than 2% on each optical surface in the whole wavelength range 0.9--2.5 Їm. This allows
for an optical efficiency of more than 80% from window and lenses alone, not considering
the transmission of filters.

Fig.1. Optical system of ARNICA
The filter wheel can hold a total of eight 1--inch filters in the current implementation;
besides the standard set of astronomical filters for the J; H;K bands, presently mounted
are four narrow--band filters for collecting images in selected spectral lines (FeII, HeI, H 2
and Brfl).
In order to simplify the mechanics, a standard Infrared Laboratories HD--3(10) dewar
was selected. Cooling is provided by means of liquid nitrogen contained in two vessels. The
first vessel cools the radiation shield, while the second vessel is in close thermal contact with
the optics and the detector support; it can be pumped if needed. The working temperature
could be as low as 50 K, but the temperature of boiling nitrogen proved to be sufficient to
ensure low dark current.
A space frame mechanical interface links ARNICA to the TIRGO telescope, allowing
an easy alignment with the optical axis of the telescope.
2. System performance
We summarize the main parameters that characterize the performance of the camera at
TIRGO; for more details, see Lisi et al. (1993).
Assuming a source (extended or seeing--broadened point--like) with an intrinsic flux
level per pixel much lower than the sky flux, the typical observing procedure consists of
a group of exposures on the source itself, interleaved by exposures on blank sky. The sky
exposures are conveniently shifted relative to the source position, and typically are taken
on different non--overlapping positions. In case higher sensitivity is required, more source--
sky exposures are repeated in order to obtain the appropriate total integration time on the
source. The sky frames are combined with a stack median filter and a clipping algorithm
to remove most field stars; this provides a reference frame for flat fielding. The process of
read--out using double--sampling obviates the need for explicit subtraction of a bias frame.
Also, at present we do not subtract dark current frames; the dark current level is very low
and the associated fluctuations are negligible.

In Table 1 we list the parameters representing the performance of the camera ARNICA
at TIRGO. We note that most of them are preliminary, in the sense that their values may
be modified in the course of the completion of the data reduction process following the
next test runs. Note also the variations of the background flux, which is observed on time
scale of a few hours in the same night.
TABLE 1. Typical performance of ARNICA at TIRGO
J H K
Background 0:7 0 1 2 10 3 6 0 10 2 10 3 2:5 0 6 2 10 3
(e s 01 arcsec 02 )
Efficiency 0.08 0.20 0.20
(e/photons)
Background 15--15.5 12--13 12.5--13.5
(mag arcsec 02 )
Limiting magnitude 20.5--21 19.3--19.6 19.1--19.6
(arcsec 02 , 3oe, 60 s)
Limiting magnitude 18.8--19.3 17.6--17.9 17.4--17.9
(5 00 apert., 3oe, 60 s)
While most of the entries of Table 1 are self--explanatory, we comment briefly on the
definitions of limiting magnitude. The limiting magnitude per square arcsecond assumes
that all the flux from the source falls on a single pixel, as in the case of extended sources or
the sky. The atmospheric turbulence has the effect of broadening the image of a point--like
source, spreading the flux over a number of pixels. In this situation, the limiting magnitude
is rescaled to take into account the dimension of the aperture where most of the flux is
collected. We chose an aperture of 5 00 as representative of the dimension of the seeing disk
at TIRGO; obviously, under this assumption, the sensitivity is strictly dependent on the
seeing conditions at the moment of the observation.
References
Lisi, F., Baffa, C., Hunt, L.: 1993, in SPIES's International Symposium on optical Engi­
neering and Photonics in Aerospace and remote Sensing (ARNICA: the Arcetri Ob­
servatory NICMOS 3 imaging camera), Orlando (USA), p. .