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

The integral field spectrograph, spectrometer for infrared faint field imaging (SPIFFI), has complex lineprofile 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. Cryogenicmeasurements of a spare SPIFFI diffraction gratingshowed significant periodic deformation. These measurements match the cryogenicdeformation expected from bimetallic bending stress based on a finite element analysis of the lightweighted gratingblank. The periodic deformation of the gratingsurface gives rise to satellite peaks in the diffraction pattern of the grating. An optical simulation including the cryogenic gratingdeformation 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 cryogenicdeformation of the diffraction gratingsis responsible for the nonideal line profiles, and that the diffraction gratingsshould be replaced during the upgrade for optimal instrument performance.