logosmallarcetri Arcetri Technical Reports 3/1995

The `Final' Optical Design of NICS, the Galileo IR Instrument.

E.Oliva and S.Gennari

Osservatorio Astrofisico di Arcetri

Abstract

We present the `final' optical design of NICS, the IR imager-spectrometer for the Galileo 3.5m telescope. It differs significantly, and in particular it is simpler, than that described in the TNG Instrument plan (Gennari and Oliva 1994), the main changes are as follows:

  • Materials: we use BaF-IRG2  and/or SrF-IRG3 which are by far the best Crown--Flint combinations for near IR instruments (Delabre 1994, Oliva and Gennari 1995). 
  • Scales: the `adaptive optic field' (<0.05"/pix) does not require an extra objective because the small--field (F/8.4) camera coupled with the F/35 beam from the tip--tilt (and future adaptive optics) module yields 0.04"/pix.
  • Collimator: it is now fixed and the pupil diameter is 22 mm. A BaF-IRG2 doublet gives the best performances but can be substituted with SrF  (much less fragile than BaF) -IRG3  doublet with a marginal loss of image quality. Both options are considered here.
  • High spectral resolution (Si--echelle) channel: the feasibility study was completed, we developed an optical design which can handle an extreme R4 (76^o) Si--grism. The spectral resolving power is R nearly 10000 (with a 1" slit), we propose to include this mode in the instrument baseline.