New far infrared and millimetric telescopes for differential measurements with a large chopping angle in the sky.

The optical design of two Cassegrain-type telescopes of 1200- and 2500-mm primary mirrors has been studied by wobbling subreflectors to obtain differential measurements in wavelengths ranging from 300 microm to 2 mm of the spectrum. Wide chopping angles in the sky have been reached in spite of the large apertures and high secondary magnifications. Diffraction-limited performances are maintained by tilting the secondary mirror around the primary focus. This work is included in the TIR (Telescopio InfraRosso) project.