Instrument Host Overview
  ========================
    The information in this document was taken from the website
    www.ing.iac.es/ on June 19, 2001.
 
    The Isaac Newton Group of Telescopes (ING) operates the 4.2m
    William Herschel Telescope, the 2.5m Isaac Newton Telescope and
    the 1.0m Jacobus Kapteyn Telescope on behalf of the Particle
    Physics and Astronomy Research Council (PPARC) of the United
    Kingdom and the Netherlands Organization for Scientific Research
    (NWO) of the Netherlands.  The ING is located at the Roque de Los
    Muchachos Observatory, La Palma, Spain.
 
    The William Herschel Telescope is of classical Cassegrain optical
    configuration.  The paraboloidal primary mirror is made of a
    glass-ceramic material (Cervit) having near-zero coefficient of
    expansion over the operating temperature range.  It has a clear
    aperture of 4.2 m and a focal length of 10.5 m (f/2.5).  Its
    diameter-to-thickness ratio of 8 makes it thinner than for most
    large telescopes at the time it was built, but it is not
    classifiable as a thin mirror and raises no special problems for
    its support system.  The precise diameter of 4.2 m was determined
    by the availability of the mirror blank, made by Owens-Illinois.
    The mirror was figured at Grubb Parsons.  It was believed that
    this was the most accurate large mirror yet made, concentrating
    85% of the light of a distant star into an area only 0.3
    arcsecond in diameter.  It is necessary for the mirror to be very
    smooth and accurate on scales of 20 cm or less, since this is
    about the size of the atmospheric cells above La Palma site.
    Portions on the mirror separated by larger distances than this
    may be tilted relative to one another so long as they direct the
    light within the 0.3 arcsecond tolerance.  Thus, the mirror is
    accurate to within 1/50 of the wavelength of light at a scale of
    2 cm, about 1/15 wavelength at 8 cm and about 1/2 wavelength at 1
    m or more.
 
    The primary mirror weighs 16.5 tons and the diameter of the
    central obstruction is 1.21 m.  The mirror surface is aluminum,
    and when fresh reflects about 85% of the light falling on it.
    The primary mirror suffers particularly from exposure to the
    elements (humidity, pollen, dust, etc.) and it is realuminized
    once every year.  This involves removing the mirror cell from the
    telescope, lowering the mirror to the ground floor, washing off
    the old surface with caustic soda and resurfacing in a large
    vacuum tank.
 
    The focus of the uncorrected primary mirror would show strong
    coma off-axis but the incorporation of a three-element correcting
    lens before the prime focus gives an unvignetted field of 40
    arcminute diameter extending to 60 arcminute diameter at
    nominally 0.6 transmission.  The effective local ratio of the
    primary mirror with corrector is f/2.8.
 
    When not operating at prime focus, a convex hyperboloidal
    secondary mirror, made of Zerodur, 1.0 m in diameter, directs the
    light through a central hole in the primary mirror to the main
    instrumentation mounted at the Cassegrain focus beneath the
    primary mirror cell.  The telescope also incorporates a third
    main mirror, a flat, angled at 45 degrees, which can be
    motor-driven into position at the intersection of the axes, just
    above the primary mirror, so that the light from the secondary is
    diverted sideways either through one of the altitude bearings to
    the Nasmyth platforms where particularly large or massive
    instruments can be placed, or to an intermediate, folded
    Cassegrain position for use of small, subsidiary instruments.  As
    desired during the night, instruments mounted at any of these
    four stations can be selected within minutes by the motion of the
    single Nasmyth flat mirror.  The effective focal length of the
    telescope for the Cassegrain and Nasmyth foci is 46.2 m (f/11).
    The available unvignetted field diameters are 15 arcminute at the
    direct Cassegrain focus and 5 arcminute at the Nasmyth and folded
    Cassegrain foci.
 
 
  Location
  ========
    Coordinates of WHT, from ING Observers' Guide section 1.2
 
    Latitude:             28 45 38.3 N (+28.761 deg)
    Longitude:            17 52 53.9 W (-17.882 deg)
    Ground floor height:  2332 m