SALT - Southern African Large Telescope

SALT Key Design & Performance Characteristics

  • Telescope location : Sutherland, Northern Cape, South Africa
  • Telescope elevation: 1798m above mean sea level
  • Site atmospheric zenithal V-band seeing: FWHM = 0.92" (median)
  • Telescope design:
    • fixed altitude
    • prime focus tracking
    • moving pupil
    • based on the Hobby Eberly Telescope
    • spherical primary mirror
    • prime focus 4-mirror spherical aberration corrector
    • Telescope tilt angle: 37° from vertical (Zenith Distance) 
    • Telescope Field-of-Regard: annulus on the sky, with width 12°
    • Telescope altitude range: 47° to 59°
    • Telescope azimuth range: 0° to 360°
    • Declination range: -75° 22' to 10° 37'
    • Primary mirror size: 10m x 11m hexagonal array
    • Primary mirror shape: spherical
    • Entrance pupil diameter: 11m
    • Telescope optical image quality: EE(50) ≤ 0.6" (without atmospheric seeing)
    • Telescope delivered image quality at Z.D. of 37°:   EE50 = 1.2 arcsec (median; V-band)
    • Operational wavelengths: 320nm to 1700nm
    • Science Field-of-View: circle of diameter 8'
    • Guidance Field-of-View: circle of diameter 10' (can overlap science field of view)
    • Telescope operation: queue scheduled 100% of time
    • Normal operational temperature range: 0°C to 20°C
    • Normal operational wind speed limit: 60 km/s
    • Marginal operational temperature range: -10°C to 25°C (20% performance degradation)
    • Marginal operational wind speed limit: 76 km/s  (20% performance degradation)
    • Average nightly operational efficiency:
      • Science observations: ≥ 65%
        • shutter open time + detector readout time: ≥50%
        • overheads: <15%
      • Bad weather: 25%
      • Engineering, mirror alignments, calibrations: ≤7%
      • Faults: ≤3%

    SALT's Viewing Window

    The altitude restrictions on SALT (47° to 59°) place the following observing constraints in terms of instantaneous sky access (Hour Angle & Declination).

    The SALT Viewing Annulus

    SALT's Tracker Access Limits

    Once an object which is inside the viewing window is selected to be observed, SALT acquires it by slewing the entire telescope structure in azimuth to the desired position. This is done by inflating air bearings and rotating the telescope on its flat and smooth circular concrete pier. The air bearings are then deflated and the telescope come to rest on 4 "feet" on pier.  

    The objects are then accurately positioned in the science instruments and tracked by moving the Prime Focus Payload (PFP), on which the instruments are mounted, at the top of the telescope. The PFP can be postioned anywhere inside a square shaped area which is accessed by the X and Y drives of the tracker. This accessible region is shown in the following 2 diagrams.

     

    SALT Tracker X-Y Range
    SALT Tracker range in Altitude and Azimuth

    SALT's Track Times

    The total observing time for any object visible to SALT is determined by the width of the viewing annulus - in Altitude - at a given Declination. In addition, the total maximum track time for an object is determined by the tracker limits above.

    The first plot below shows the total track time for objects as a function of Declination. The different curves show:

    1.  the specified 12° of altitude requirement

    2.  the as-designed predicted track times

    3.  the as-built actual track times  

    For some Declinations (in the South and North), it may be possible to reacquire an object by stepping the telescope in azimuth one or two times, thereby extending the total observing time, as shown in the last graph.  

    SALT Track Times for azimuth moves