Complex spectral line profiles resulting from cryogenic deformation of the SINFONI/SPIFFI diffraction gratings

Abstract

Abstract. The integral field spectrograph, spectrometer for infrared faint field imaging (SPIFFI), has complex line profile shapes that vary with wavelength and pixel scale, the origins of which have been sought since the instrument construction. SPIFFI is currently operational as part of SINFONI at the Very Large Telescope (VLT) and will be upgraded and incorporated into the VLT instrument enhanced resolution imager and spectrograph (ERIS). We conducted an investigation of the line profiles based on the measurements we could take with the instrument calibration unit, as well as laboratory measurements of spare SPIFFI optical components. Cryogenic measurements of a spare SPIFFI diffraction grating showed significant periodic deformation. These measurements match the cryogenic deformation expected from bimetallic bending stress based on a finite element analysis of the lightweighted grating blank. The periodic deformation of the grating surface gives rise to satellite peaks in the diffraction pattern of the grating. An optical simulation including the cryogenic grating deformation reproduces the behavior of the SPIFFI line profiles with both wavelength and pixel scale as measured with the instrument calibration unit. The conclusion is that cryogenic deformation of the diffraction gratings is responsible for the nonideal line profiles, and that the diffraction gratings should be replaced during the upgrade for optimal instrument performance. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

First on-sky results of ERIS at VLT

“…It was found that the interplay of the lightweighting structure with the stress induced by the NiP coating at cryogenic temperatures caused deformations of the grating surface and led to degraded instrumental line profiles. 12 The new gratings of SPIFFIER are based on Zerodur blanks without any lightweighting and, therefore, substantially improve the shapes of the spectral lines. Since line profiles are undersampled on the detector (the widths are less than two pixels), an approach similar to 13 was used to obtain hypersampled spectral line profiles for the detailed line-shape analysis.…”

ERIS: revitalising an adaptive optics instrument for the VLT

“…When they are cooled, this leads to an 'eggbox' structure that mimics a grating. 36 The gratings will be replaced when SPIFFI is refurbished as SPIFFIER and integrated into ERIS.…”

“…9 shows, the rationale for the latter step is that the light-weighting of the current gratings leads to an 'eggbox' deformation when cooled, which acts as an an additional grating and leads to a complex spectral profile. 36 With new collimator mirrors and gratings, as well as filters (those installed in 2016 have a throughput exceeding 98% across the full band 35 ), the sensitivity is expected to increase by a significant factor. In addition, the entrance window will be replaced by a smaller one matched to the nominal field (since the sky spider has been removed), and the pre-optics collimator will be replaced to accommodate the new AO optical interface.…”