William Herschel Telescope

The William Herschel Telescope is of classical Cassegrain optical configuration. The paraboloidal primary mirror is made of a glass-ceramic material (Cervit). 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. 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 arc sec in diameter.

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 arc min diameter extending to 60 arc min 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 arc min at the direct Cassegrain focus and 5 arc min at the Nasmyth and folded Cassegrain foci.

The William Herschel Telescope has an altazimuth mount. Maximum rotations are 0-95° from the horizontal and ±270° about East. While observing, however, the accesible altitude is limited to 89.8°-12.0°. The reasons for these limitations are the existence of a zenith blind spot for zenital distances lower than 0.2° imposed by the speed limit in azimuth and that the telescope is partially obscured at elevations below 12° by the dome rail.

Coordinates of the WHT
Latitude:	28° 45' 38.3" N (+28.761°)
Longitude:	17° 52' 53.9" W (-17.882°)
Ground floor height:	2332 m

A more detailed description of the optics and the mounting of the WHT is available in the Observers' Guide. The WHT's Telescope Control System runs on a DEC Alpha machine (see the WHT TCS manual).

For other facts on the WHT see the public information web pages.

Observers can apply for time with these instruments on the 4.2-m William Herschel Telescope. A historical record of instruments in use at the WHT is kept at the past, present and future instruments web pages.

Visiting instruments are mounted at the Cassegrain or Nasmyth f/11 foci. The enclosure at the spare Nasmyth focus includes an optical bench, and can be used for experimental work, e.g. for instruments requiring mechanical stability. Information on the Nasmyth derotators is available.

Development work at the WHT is currently focussed on the adaptive-optics suite, and on provision of a Rayleigh laser guide star to allow adaptive-optics observations almost anywhere on the northern sky.

For applying for observing time and preparing observations, please visit the ING Astronomy web pages. A useful command glossary and a summary of CCD orientations are available.

The WHT's quality control programme focuses on maximising both the S:N ratio of the acquired data and the time available for science observations. The operation of the WHT is summarised in these half-yearly reports.