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:
Telescope
| 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: (Φp-Φs)Tel.2-(Φp-Φs)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 |
Environment
| 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) |