equatorial mounting
A telescope mounting with one
axis, called the polar axis, that is parallel to Earth's
rotation axis, and the other, called the declination axis,
at right angles to it. The telescope moves
north-south about the declination axis and
east-west about the polar axis. To point point at a target
requires moving the telescope about both axes. To track
a target, however, requires movement about the polar axis only, at the same
rate that Earth spins. This is the chief advantage of the equatorial mounting:
the N-S position doesn't change, and a single drive can regulate the E-W
tracking. The main drawback is that the polar axis is difficult to orientate
with respect to the ground, and it is different for every observatory. Positioning
the mounting at odd angles creates difficulties that increase rapidly as
the size and mass of the telescope increase. For this reason, computer-controlled
altazimuth mountings are now
used exclusively for new large instruments. Several designs of equatorial
mounting have been used extensively over the years:
German mounting
In this approach, the declination axis is at the end of the polar axis,
which is on top of a pier to raise the telescope to a convenient height.
This arrangement can point to any part of the sky, but it results in a lot
of mechanical stress because the weight of the telescope has to be held
at the end of the axis. A counterweight is used to balance the telescope
on the declination axis, but this doubles the weight that the polar axis
must support and limits the German mounting to relatively small, light telescopes.
Fork mounting
In this scheme, the polar axis branches into a fork, while the declination
axis holding the telescope is anchored on both ends by the two sides of
the fork. This provides much stronger declination support for the telescope,
but there is still no support for the weight at the end of the polar axis.
In addition, there is limited room for the telescope at the bottom of the
fork. Either the telescope can have only a small tube if it is to view all
parts of the sky, or it cannot view along the polar axis if it is used with
larger tubes. The fork was used for telescopes up to mirror diameters of
2.5 m.
English mounting
In this design, the polar axis is supported at the top and the bottom on
vertical piers. This relieves the stress on the polar axis but transfers
it to the declination axis. The English mounting isn't restricted to just
part of the sky, as in the case with the fork, but it is convenient for
only half of the sky at one time. For one half of the sky the telescope
is slung beneath the polar axis, making it easy to reach the focus for observing.
For the other side of the sky, however, the telescope is on top of the polar
axis, making it difficult to reach the focus, except by standing on a tall
ladder. The difficulty can be solved by reversing the side of the polar
axis on which the telescope is attached, but this only swaps the sides of
the sky that the telescope can view easily.
Horseshoe
mounting
A solution to the problem of supporting telescopes larger than 2.5 m; it
was introduced with the Hale Telescope in the late 1940s and used for all
the 3- to 5-m telescopes built from 1950 to 1970. The Canada-France-Hawaii
Telescope, finished in the early 1970s, was one of the last telescopes built
to this plan. The fork-like polar axis is mounted on the top end to a horseshoe-shaped
support. The fork provides support for both ends of the declination axis,
and the horseshoe mount provides support at the top end of the polar axis.
Because the horseshoe is open, the telescope can be tilted down all the
way to see objects that lie directly along the polar axis in the sky. For
every mount, the pier(s) on which the polar axis rests are separate from
the floor and the remainder of the observatory building. The piers extend
down through the building and into the ground, down to the level of solid
rock. In this way, the telescope is isolated from any vibrations in the
observatory building.
Related category
• TELESCOPE
EQUIPMENT AND TECHNIQUES
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