INT - The Mounting
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Isaac Newton Telescope

The Mounting

The telescope has a polar disc/fork type equatorial mounting supported by five axial and three radial hydrostatic oil bearing pads. The tube, a conventional open Serrurièr truss structure, supports the prime-focus assembly or secondary mirror for Cassegrain and Coudé operation.

Declination coverage was improved when moved to La Palma by cutting a sector out of the polar disc, and by a redesign of the dome aperture (a proposal to remount the telescope on an altazimuth mount was found to have some attractive features, but was rejected because of extra costs.) Drive limits are currently set at -30 degrees Declination, 70 degrees zenith distance and ±6h Hour Angle (operation below the pole is also possible).

DC servo motors with integral tachogenerators are used for the slow-motion drives on both arms. The RA axis is driven by a worn/worm-wheel assembly and the Declination axis by a recirculating ball screw and nut. Preload and quick motion drive are provided by a pair of motors driving through spur gearing. 

The telescope's position is determined by Moiré fringe grating encoders mounted on each axis. There are three reading heads per axis. One bit corresponds to 0.3 arc sec in RA and 1 arc sec in Declination, although systematic errors are larger. When the telescope is being driven by the slow-motion motors, optical incremental encoders are used, with resolutions of 0.01875 and 0.01 arc sec in RA and Declination, respectively, mounted on the drive shafts.

The pointing model for the INT comprises the standard errors of an equatorially mounted telescope (offsets in Hour Angle and Declination, collimation error, misalignment of the polar axis and non-perpendicularity of axes), together with an empirical (Fourièr series and polynomial) model of flexure in the telescope structure and large-scale errors in the encoder readings. The r.m.s. errors in the absolute pointing of the INT are always less than 5 arc sec; values between 2.8 and 4.5 arc sec were obtained during tests at Prime and Cassegrain foci during 1985. Short-term tracking errors are less than 0.25 arc sec; longer-term drifts are removed by the autoguiders.

Telescope limits:

  • Zenith distance < 70°
  • - 6 h < hour angle < + 6 h (above pole)
  • Declination > -30° 09' 30"
Operation below the pole is possible, but only gains a small extra area of sky; consider it in exceptional circumstances.

The lower windshield causes vignetting for zenith distances > 57° and is raised for such observations.

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Last modified: 11 January 2013