Telescope Optical Unit (TOU)

There is no general difference in the TOU design for normal and fast telescopes, but the latter will include filters in form of special coatings on an optical surface of the optical train.


The TOU optical layout shown together with detector position (FPA at the right side).

The optical configuration consists of 6 lenses, plus one window, placed at the entrance of the telescope, providing protection against radiation and thermal shocks. The first surface of the first lens contains even aspherical terms (K, a4, a6), while the second surface is flat in order to facilitate the interferometric surface measure during the aspheric manufacturing.


Exploited view of a TOU

All the other lenses are standard spherical surfaces. The first surface of the third lens is the optical system stop and guarantees a real entrance pupil diameter of 120 mm. This configuration provides a corrected field-of-view up to 18.9° accepting slightly degraded image quality, as well as a ~14% vignetting, in this small region at the edge of the field.

The TOU main structure consists of a machined tube with all the interface planes, threads and holes necessary to mount the other components.


TOU Structure with highlighted subcomponents 

The heat dissipated by the CCD needs to be transported through the TOU structure, which therefore must be made of material with high thermal conductivity. In addition, the large temperature difference between integration and operation requires a design able to accommodate the dimensional changes of the assembled components without leading to unacceptable mechanical stresses. The TOU is quasi-static mounted to the Camera Support Structure with three bipods.

TOU parameters

Spectral range 500 – 1000 nm
Entrance Pupil Diameter 120 mm
Working f/# 2.06 @ 700 nm
Field of View ~ 1037 deg2
Image quality 90% enclosed energy within 2.532.5 pixel² for 95% of nominal camera FoV
Maximum Field Distortion 3.84%
Plate scale 15 arcsec/pixel
Working Temperature –80°C


TOU Team

The Telescope Optical Units are responsability of Italy, with the contribution of  Switzerland, and Sweden. More on the team composition.