Modern NIR spectrographs aiming at the detection and spectral study of faint deep sky objects are potentially sensitive to performances-degrading straylight effects. In particular, the following straylight mechanisms have been identified: - ghost images, inherent to large aperture broadband camera in multi-band spectrographs, have often specific distribution which can (and needs to) be simulated when baffling is not possible; - scattered light generated at the surface of the component is also interesting in the NIR spectrum as it is often at the cross-over between a micro-roughness dominated behaviour and a particulate contamination dominated one; - the use of dispersing element such as diffraction grating brings its own straylight issues related to the control of non-evanescent orders and under incidence different from the one given by the main sequential optical path from telescope to detector; - finally, thermal background from local spectrograph environment or more remote observatory-level sources affects particularly the long wavelength end of the NIR spectrum (H and K bands typically) and in-band or out-of-band rejection is desirable as complementary or even alternative to potentially complex and costly full cryogenic operation. These different points are discussed and illustrated through case studies of several NIR multi-object spectrograph designs like the fibre-fed reflective grating with OH lines suppression FMOS (designed and built), the broadband multi-IFU slicer and slit grating KMOS (under development) and the fibre-fed grism-based SIDE (in conceptual design stage).

Date Published: 9 July 2008
PDF: 12 pages
Proc. SPIE 7014, Ground-based and Airborne Instrumentation for Astronomy II, 701436 (9 July 2008); doi: 10.1117/12.788762

Published in SPIE Proceedings Vol. 7014:
Ground-based and Airborne Instrumentation for Astronomy II
Ian S. McLean; Mark M. Casali, Editor(s)