VSTceN Portal

The VST optics has been designed at INAF-OAC by Technology Working Group (TWG) in collaboration with ESO. The peculiarity of this telescope optics is that it is provided with two switchable correctors, for observations at different observing angles:

a two-lens corrector and ADC (Atmospherical Dispersion Corrector) plus one different lens corrector.

The VST optics were procured through a contract with Carl Zeiss Jena GmbH, with the Russian firm LZOS JSC as subcontractor. The primary (M1) and secondary (M2) mirrors are made from Sitall, an optical glass produced by LZOS JSC and similar to Zerodur.

The lenses for the WF correctors are of fused silica produced by Schott.
The prismatic lenses for the atmospheric dispersion corrector (ADC) made from special glasses, also by Schott.

In 2002 the primary mirror was destroyed in an accident during shipment from Europe to Chile. This enforced a series of emergency actions to resolve the problem with minimum impact on the final VST schedule. The new contract with the LZOS JSC envisages the supply of the replica mirror at the end of August of 2005. It is expected that the new M1 shall maintain the outstanding quality figures measured on the original mirror. M2 was also slightly damaged during transportation to Chile, and has been returned to LZOS JSC for repair.

The VST is optimized for seeing-limited spatial resolution over a field of view of 1° x1° matched to a 16k x16k pixels CCD mosaic camera. It operates two different correctors: the first, consisting of two lenses, works in the range from 320 nm to 1014 nm for observations at small zenithal distances; the second, composed of a rotating ADC coupled to one lens only, operates in the range from 365 nm 1014 nm down to an elevation of 30°. A mechanical switching unit allows the appropriate corrector to be selected.
An active-optics system controls the shape of the primary mirror and the position of the secondary. M1 is supported by 84 axial actuators distributed on four rings of 12, 18, 24, and 30 elements (with a force resolution of 1 N), and by 24 radial actuators (5 N resolution).
The M2 position is controlled by a double-stage system, a classical hexapod and a piezo-based fine positioning device, capable of a tilt resolution of 10-3 arcsec and a linear resolution of 1.2 nm. They allow the control system to preset and track the position of M2 in advance of and during the scientific exposure. The major aberrations corrected by the active-optics system include defocus, coma, spherical, astigmatism, quad-astigmatism, and tri-coma. The wave-front analysis is obtained by a Shack-Hartmann unit included in the probe, sensitive enough to work on guide stars of 14 mag for integration times of 30s. Given the number of active systems on board of VST and the compactness of the telescope, distributed intelligence solutions have been implemented through the design and realization of embedded controllers connected to the high level control system through a CAN BUS solution.

M1 characteristics

diameter	2650 mm
hole	600 mm
thickness	140 mm
curvature radius	9509 mm (f/3.6)
conical constant	-1.139899
basic quality	< 80 nm
intrinsic quality	< 20 nm
EE(80%)	0.15" (meas. 0.12")

M2 characteristics

diameter	938 mm
hole	130 mm
thickness	130 mm
curvature radius	4374 mm (f/4.7)
conical constant	-5.421864
basic quality	< 80 nm
intrinsic quality	< 20 nm

M1 + M2 (f/5.5)

quality	basic < 114 nm	measured 31 nm
	intrinsic < 29 nm	measured 17.1 nm
EE(80%)	basic < 0.30"	measured 0.16"
	intrinsic < 0.15"	measured 0.12"