General Description

Telescope

Telescopio Nazionale Galileo (TNG) is an alt-azimuth telescope with a Ritchey-Chrétien optical configuration and a flat tertiary mirror feeding two opposite Nasmyth foci. The following table shows the main TNG optical parameters.

One of the main features 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-Hartmann wavefront sensors (one for each focus) to sense wavefront deformations using an on-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. Measurements of the optical performances achieved during tests done at the Zeiss laboratories show that 80% of the Encircled Energy is within 0.11" at a wavelength of 632 nm. M2 is mounted on a hexapod system (six expandable arms), used to keep the mirror in the correct position and tilt with respect to M1. The hexapod control system allows for image fine-focus and coma correction.

Because of the alt-azimuth 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 above the ho

The first focal station, Nasmyth A, supports the infrared camera-spectrometer (NICS) and the extremely fast photometer (SiFAP2). The second focal station, Nasmyth B, supports the optical multimode instrument (DOLORES), the High Accuracy Radial velocity Planet Searcher (HARPS-N), and the near IR high resolution echelle spectrograph (GIANO).

Guest Instruments Interface

The rotator/adapter in Nasmyth A provides a mechanical connection point to allow for guest instruments to be installed temporarily. Mechanical dimensions and characteristics are available upon request.

rizon and when the sun is below the horizon. However, this last constraint could be somewhat relaxed under exceptional circumstances, e.g. observations of outstandingly important objects.

Tracking Performance

The telescope has a global pointing accuracy of 10 arcsec across the sky, and a tracking performance of 0.1 arcsec stdev in Az and Alt axis for sidereal targets. It can track non-sidereal targets with constant differential speeds up to ~200 arcsec/h, depending on the sky coordinates. For such objects, the autoguiding time is limited to the time the sidereal star used for guiding remains in the field of view of the autoguider camera (180 arcsec FoV).

Rotator-Adapters

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) and the extremely fast photometer (SiFAP2). The second focal station, Nasmyth B, supports the optical multimode instrument (DOLORES), the High Accuracy Radial velocity Planet Searcher (HARPS-N), and the near IR high resolution echelle spectrograph (GIANO).

Guest Instruments Interface

The rotator/adapter in Nasmyth A provides a mechanical connection point to allow for guest instruments to be installed temporarily. Mechanical dimensions and characteristics are available upon request.