The Telescopio Nazionale Galileo (TNG) is an altazimuth telescope with a Ritchey-Chretien optical configuration and a flat tertiary mirror feeding two opposite Nasmyth foci. The following table shows the main TNG optical parameters.
|Focal length||38.5m (f/11)|
|M2 baffle diam.||1.165m|
|Vignetting-free field||25arcmin diameter|
The main feature of the TNG is the presence of an Active Optics (AO) system to perform real-time, low frequency correction of the optical components in order to ensure the best optical performances in all conditions and to compensate for the deformations of the primary mirror (M1), which is too thin to be completely rigid. The AO system consists of two Shack-Hartman wavefront sensors (one for each focus) to sense wavefront deformations using an off-axis star. This information, properly treated, is used to correct the optical surface of the primary mirror (M1) and the positions of the secondary and tertiary mirrors (M2 and M3). The M1 surface is modified through 78 mechanical actuators pushing axially on the M1 back face; M2 is mounted on an exapod system (six expandable arms) used to keep the mirror in the correct position and tilt with respect to M1; a three piezoelectric actuators system can tilt M3 around two perpendicular axis up to a frequency of 5 Hz. Measurements of the optical performances achieved during tests done at the Zeiss laboratories show that the 80% of the Encircled Energy is within 0.11" at a wavelength of 632nm.
Because of the altazimuth mount TNG cannot track objects within a "dead" zone around the zenith which has a radius of about one degree. For security reasons, observations are only performed at elevations larger than 14 degrees and when the sun is below the horizon. However, this last constrain could be somewhat relaxed under exceptional circumnstances, e.g. observations of outstandingly important objects.
The interface between the telescope fork and the instruments at both Nasmyth foci is provided by two rotator/adapters (R/A). Their main function is to compensate for the field rotation by a mechanical counter rotation. Each R/A also includes probes and sensors for the AO wavefront sensing and for the guide star tracking. All the available instruments are permanently mounted at the telescope. The first focal station, Nasmyth A, supports the infrared camera-spectrometer (NICS). The second focal station, Nasmyth B, supports the optical multimode instrument (DOLORES) and the High Accuracy Radial velocity Planet Searcher (HARPS-N).
Nasmyth A Interface
The instrument located at the Nasmyth A focus is connected to the R/A via an opto-mechanical interface which provides the distribution of light between the instruments and the mechanical support.