The Cassegrain module is mechanically attached under the Telescope Bernard Lyot, more precisely to its focal plateform called the Cassegrain plateform. It is the first module of the SPIP instrument to recover the light from the stars collected by the telescope.
This module not only corrects and stabilizes the resulting image but also performs the polarimetric analysis of the light and injects the calibration sources, before transmitting the light to the cryogenic spectrograph. In particular, it consists of an atmospheric corrector, an image stabilizer, a guiding camera, a motorized wheel and several optical components for calibration as well as a polarimeter.
At the exit of the Cassegrain module, a pair of 45-meter-long Fluoride optical fibers (the only ones capable of transmitting infrared light efficiently up to 2.5 µm wavelength) transmits light to the very high resolution spectrograph (~70,000). A calibration module containing the different calibration sources is also connected by a fiber to the Cassegrain module. This calibration light follows the same path as the light coming from the telescope, allowing to calibrate the spectra obtained in order to achieve a very high precision of measurement of the stars radial velocity (< 1-2 m/s).
The Cassegrain module of SPIP is very similar to its SPIRou companion at CFHT (3,6 m of diameter, aperture of F/8, focale of 28,8 m) but its optical and mechanical dimensions have been adapted to match the configuration of the Telescope Bernard Lyot (2 m of diameter, aperture of F/25, focale of 50 m).
Sectional view of the SPIP Cassegrain module and description of its components