We aim for
a cryogenic spectrograph (dual pupil design) featuring a 15cm pupil, 2 parabolic off-axis collimators (with 1.2m focal length), a R2 diffraction grating (with 23.2gr/mm and 154x306mm ruled surface), a prism-train cross-disperser (providing minimum inter-order separation of 0.55mm at detector level) and a f/2 fully dioptric camera (30cm focal length). Orders #85 (0.98μm) to #32 (2.4μm) can be recorded simultaneously on a hawaii-2 detector with 18µm pixels. The optics must provide spot diagrams (80% encircled energy) smaller than 1 ccd pxl throughout the full spectral domain (0.98-2.4µm) and over the ±5deg field.
This design is significantly different than that of GIANO; in particular, it includes an efficient (>80% throughput) fully dioptric camera (whereas GIANO uses mirrors).
A first (preliminary) optical design has been obtained, demonstrating that our goal is indeed reachable; in particular, adequate optical quality is achieved throughout the whole wavelength domain. Several options (eg concerning the actual location of the cross-dispersor) are possible and will be studied in more details in the next design stage. One possible solution is shown on the right.
A preliminary thermal background budget of the instrument indicates that the main contributors are (i) the polarimeter itself (and in particular the last lens of the focal reducer) and (ii) the image slicer. To solve this issue, we will incorporate the polarimeter output optics into a small dewar cooled to ~230 K and featuring the output polarimeter lens on one side, the fiber-head holder on the other side and a polished baffle linking both. We will also put the whole image slicer into a similar device. With such a setup, we decrease the thermal background of the polarimeter and image slicer by an order of magnitude and ensure that both are much smaller than the telescope thermal emission (the dominant contribution in the K band, having K~13.5).
The thermal contribution of the cryogenics spectrograph itself remains small provided we implement a cold slit at the entrance.