The VLTI Auxiliary Telescopes

Main Technical Characteristics

The Auxiliary Telescopes have following characteristics:

  • they are relocatable
  • they are totally self-sufficient with only electrical connections to the site
  • they carry their own dome and do not need additional environmental protection
  • they must fulfill the very stringent mechanical stability requirementsimposed by interferometry at the level of a few tens of nanometers.

The main design and performance parameters are given below:


Optical Configuration RitcheyChretien with Coudé train and Coudé Relay Optics sendinga 18 mm collimated beam horizontally underground towards the Delay LineTunnel.
Primary mirror Φ  1.8 m, F/1.5 (k=-1.0006)
Secondary Mirror Φ 138 mm, F/1.6 (k=-1.281)
Coudé Mirrors 8 mirrors (flat, cylindrical and spherical) with intermediate pupil image to locate an Adaptive Optics deformable mirror.
Coudé focus F/36.2. Located underground. 
Equipped with field acquisition camera (CCD) and fast tip-tilt sensor (APD Quad-Cell)
Coudé Relay Mirrors 1 dichroïc + 2 mirrors including steerable field mirror for lateral pupil adjustment.
Output beam Φ  18 mm collimated, 1.25 m underground
Optical Quality < 110 nm FWE rms in the output pupil.
Optical Path stability < 15 nm rms over any 0.01 sec (allowing interferometry down to visible spectral range)
Magnification Factor 100 +/- 1
Polarization < 30° linear retardation: (Φps)Tel.2-(Φps)Tel.1
Spectral range 0.4 to 25 µm
Mount type Alt-Az. Altitude axis 5 meters above ground to limit the impact of "ground seeing".
Telescope drives (Alt & Az) Direct drives with tachometers and high accuracy incremental optical encoders
Pointing accuracy < 2.0 arcsec rms absolute (over all sky) 
< 0.2 arcsec rms relative (over 10 arcmin)
Tracking accuracy < 0.2 arcsec rms over 10 sec 
< 0.5 arcsec rms over 10 min
Sky coverage Zenith distance: 0.5° to 60°. Azimuth: >360°
Atmospheric compensation Tip-tilt in a first phase: diffraction limited at 2.2 µm (< 0.025" rms, for mV<16) 
Full Adaptive Optics in a later phase

 Transporter/Enclosure/Service Modules

Total mass of the system (including telescope) < 30Tons
Time to relocate between observing stations (worst case) < 3 hours
Positioning accuracy of the Telescope after relocation +/- 0.1 mm displacement, +/- 10 arcsec angular
Enclosure type Segmented half sphere retractable down to Altitude axis for good ventilation (dome seeing). 
Air conditioned during daytime.
Heat dissipated close to the light path < 25W
Primary mirror thermal control accuracy +/- 0.5°C


Operational Wind Speed (full performance) 10 m/sec
Survival Wind Speed 47 m/sec (no damage) 
54 m/sec (minor damages allowed)
Operational Temperature range (full performance) 0 to 15°C
Micro-seismic activity (full performance) 0.2 µg/ÖHz in [2-100]Hz
Survival seismic activity MR=7.75 at 100 km / Peak acceleration 0.24g (no damage) 
MR=8.5 at 150 km / Peak acceleration 0.34g (damages allowed)