Calibrating and characterizing the performance of an astronomical instrument is the prime step required to allow high-quality, science-grade data to be obtained from it. To do so, detailed knowledge of all its comprising hardware, from a mechanical, thermal and optical point of view is of paramount importance and similarly, its end-to-end characteristics are too. In the case of a complex instrument such as the Mid-indrared E-ELT Imager and Spectrograph (METIS Brandl 2018), built by a large European consortium of research institutions, a dedicated system that allows calibration, characterization and long-term monitoring of its end-to-end performance is mandatory. In addition, given that METIS will be a first-light instrument of a state-of-the-art telescope such as the E-ELT, the presence of a fully-equipped troubleshooting system does undoubtedly strengthen the success of such a project.
In terms of performance, METIS will be complementing JWST and ALMA observations with the highest angular and spectral resolution data in the thermal infrared (3-19 µm). Particularly, it will allow astronomers to benefit from the following observing modes: imaging at L,M,N and Q bands with associated low-resolution longslit spectroscopy, high resolution IFU spectroscopy (R ~ 100000 at LM bands) and integrated high contrast imaging capabilities in all of these modes. With such a large variety of functions foreseen, having a dedicated and independent support to on-sky calibrations is extremely useful, as it will complement the information provided during observation and speed up the data-reduction process. These a-priori information, such as detectors response and line kernel function (amongst many other) are best acquired during AIV/T, when thorough measurements of the whole instrument's performance can be carried out. In addition, elements like the image quality that can be achieved at the science focal planes, need to be validated before the final integration at the E-ELT, where internal optical effects (stray-light, ghosts, etc.) might become difficult to disentangle otherwise.
Warm Calibration Unit
The Warm Calibration Unit (N. Baccichet 2018) (WCU) is being built by the University of Cologne to specifically supply METIS with the functionalities required to perform internal calibrations, characterization tests, and serve as the primary tool for internal alignment checks. Due to its multifunctional nature, it is built detached from the main cryostat and will operate independently from the remaining subsystems, which are illuminated via a periscopic arm situated in front of the main entrance window. Light sources and masks are distributed using a set of fold mirrors between two object planes, one mainly used for calibration and characterization and a second one for alignment; in addition, a modified Offner relay allows the pupil to be accessed so that further functionalities can be accommodated.