S.Gennari (1) and L.Vanzi (2)

(1) Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5, 50125 Firenze, Italy

(2) Università degli Studi di Firenze - Dipartimento di Astronomia, Largo E.Fermi 5, 50125 Firenze, Italy


We present a new cooled grating near infra-red spectrometer designed to acquire spectra at the TIRGO telescope in the 0.9-2.5 micron region with a resolving power of 1000, equiped with a Rockwell NICMOS 3 detector.

keywords: Infrared Spectroscopy :: NICMOS 3 :: TIRGO


The LonGSp (Longslit Gornergrat Spectrometer) project is part of the work that the Osservatorio Astrofisico di Arcetri is doing in order to build a new generation of instruments for the IR telescope of Gornergrat (TIRGO).

The spectrometer has a spectral resolution in the range $300/div4000$, an angular resolution of 1.73 arcsec/pix on a field of 70 arcsec and it operates in the near infrared ($0.9/div2.5/mu m$).

The detector is a Rockwell $256/times 256$ NICMOS3 engineering grade array of which is used a subsection of $70/times40$ pixel is used.

Optical Design

The optics are designed in order to match the F/20 focal ratio of the TIRGO telescope.

The optical design of the spectrometer is presented in fig.1: The lens B is a field lens. The mirros D and E are a cassegrain system (f/20) that collimates the optical beam coming from the telescope. The collimated beam is reflected on the grating G by the plane mirrow F. Finally the camera of the spectrometer (mirrors H and I), focalize the radiation on the detector L.

All the mirrors are gold coated. The grating has 150 grooves per mm and a blaze of 8.6 degree (2 micron at the first order) and is used in the Littrow configuration, the back side is a plane mirror that can be used to obtain images of faint sources.

The grating motion is assured by an external motor via a ferrofluidic feedthrough, two internal stepper motors allow the selection of slits and filters in the positions A and C.


LonGSp has two indipendent cooling reservoirs. The first one provides cooling for optical and mechanical parts, the second one is used to cool the detector, through a cold strap.

A little amount of active charcoal is present in order to maintain the value of the pressure required (˜10e-4 mb) for a sufficiently long time (more than 20 days):.

The cooling and heating times are shorter than 7 hours and the rate of evaporation of the nitrogen from the main reservoir allows about 19 hours of operation in working conditions.


The control system is based on electronic developed at Infrared Laboratories Industries &040;Tucson, AZ) that controls the data acquisition and provides the necessary clocks. A 80486-based PC controls the electronics via a fiber-optics link. Communication with other components of control electronic (motors controls, temperature controls, etc) is assured by serial ports (RS-232) encoded on the same fiber-optics link.


S.Gennari, L.Vanzi (1993) - Mem.S.A.It. (in press)

S.Gennari (1993) - Technical Report of Astrophysical Observatory of Arcetri 6/1993

Last update 4 April 96

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