Efficiency measurements of the VIRTIS-M grating

VIRTIS is an acronym for Visible and Infrared Thermal Imaging Spectrometer. It is an imaging spectrometer for the Rosetta mission, a mission to the comet 46P/Wirtanen. It is made of two channels: a high resolution one, named VIRTIS-H (resolving power up to 2000, spectral range 1–5 μm) and the classical imaging spectrometer VIRTIS-M. The M in particular stands for “mapping” and is devoted to medium spectral resolution imaging. The spectral range of VIRTIS-M between 0.25 and 5 μm is divided into two channels with a boundary at 1 μm and a spectral resolution better than 3 nm. The peculiarity of the VIRTIS-M design is a custom grating produced by ZEISS, which consists of two different regions of groove spacing and shape to cover visible and infrared spectral regions with a single device. The grating efficiency has been designed in such a way to make the overall response of the spectrometer-target characteristic flat. The complex geometry of the grating makes the testing of the device alone, difficult. Another difficulty is that measurements should be performed at a low temperature (135 K). Therefore, we decided to use the development model of VIRTIS-M to perform the measurement of the grating efficiency first at ambient temperature. The measurement is done by comparing the light power of the order of interest of the grating to the light power of the zero-order when the grating is replaced by a mirror of the same geometry (no grooves). In this way we obtain a “relative” value. The efficiency is then compared to that of the mathematical model provided by ZEISS. The measurements compare well with the theoretical calculations.