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

TL;DR

This study identifies cryogenic deformation of diffraction gratings as the cause of complex line profiles in the SPIFFI spectrograph, suggesting grating replacement for improved performance in future upgrades.

Contribution

It provides the first detailed analysis linking cryogenic deformation to line profile complexity, supported by measurements and simulations, informing future instrument improvements.

Findings

Cryogenic deformation causes satellite peaks in diffraction patterns.

Finite element analysis predicts deformation consistent with measurements.

Replacing gratings will enhance instrument performance.

Abstract

The integral field spectrograph, 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 VLT, and will be upgraded and incorporated into the new VLT instrument ERIS. We conducted an investigation of the line profiles based on 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 non-ideal line profiles, and that the diffraction gratings should be replaced during the upgrade for optimal instrument performance.