The Poncet Platform

Part of an article in Sky and Telescope, March 1980 by Roger W Sinnot about a platform built by Jack Ells.

Bexley, England. Since 1963, members of the Crayford Manor House Astronomical Society have used the observatory hut built by J. W. Ells of Bexleyheath according to a design proposed by E. G. Hill. One of its compartments houses a 32-cm (12 ½ inch) f/6.3 reflector that has its altitude bearing near the Newtonian eye-piece. The observer peers into the eyepiece from a separate, heated compartment, and the whole building can turn to follow an object in azimuth. Although motorizing the altazimuth axes was once attempted, to simulate equatorial motion, the results were unsatisfactory.

In England, J. W. Ells' equatorial platform uses eight inclined planes, each adjusted to face the north celestial pole.Faint grease marks on their surfaces (above) show the travel directions of individual rollers.The planes carry the 90-kilogram secondary frame (below), 730-kg rotating observatory, and as many as three observers.

Mr. Ells was inspired by the 1977 Poncet article to rebuild the observatory's base structure completely, as shown in the accompanying photographs. In the Journal of the British Astronomical Association he writes:

"For relatively lightweight instruments M. Poncet's concept can be used directly with a single [inclined] plane. The observatory under consideration, however, is large and heavy and a slight extension of the concept was needed. This is as follows: 'A body supported on a number of parallel planes, each of which is perpen-dicular to the polar axis, and constrained to move in a circle will rotate about the polar axis."'

The completed observatory is shown here with maximum tilt toward the west. Only 35 seconds are needed to slew it back to the startingposition. An azimuth stop prevents rotation of the hut more than 390°, to avoid excessive coiling of the power cable (visible below) around thecentral bearing.

The photographs show how he applied this principle to the foundation of the existing observatory. The new base frame includes eight fundamental planes positioned around the bottom of a shallow concrete pit. At the center is a stubby polar axis, which constrains the upper or secondary frame to rotate on eight rollers, each in contact with one of the planes. The original azimuth track, central bearing, and observatory are secured to the top of the secondary frame. He continues:

". .. Due to the variable weight and positions of person(s) in the observing cabin ... lack of balance would cause ro¬tation about the polar axis in exactly the same way as for a conventional equatorial. For this reason a second attachment is required between the base frame and secondary frames — this attachment is also used to provide the drive to the secondary frame."

(See photograph below.) It was decided to make the supporting planes large enough for 40 minutes of stellar tracking, which at latitude 51 ½ deg north gives a tilt of at most 3°.1 on either side of the horizontal mid-position of the observatory floor, not enough to "cause the observer to stumble or cause objects to slide off the shelves or desk."

When the secondary frame was first fitted to the base frame a 90-kilogram (200-pound) test load was applied at the extreme edge of the secondary ring, causing an elastic bending of the frame that would have shifted star images about 30 minutes of arc. Extra stiffening reduced this amount to one minute of arc, which is deemed acceptable. The observer or ob¬servers in the cabin must remain reasonably still, otherwise an object in view will dance about. This restriction has not proved to be an inconvenience.It took a year to achieve all this, and Mr. Ells gives full construction details in his JBAA article (December, 1978, page 66). The new mounting is being used successfully for astrophotography by members of the Crayford Manor House society.

The drive mechanism
consists of a gently curved
threaded rod, firmly secured
to the secondary frame, and
a large phosphor-bronze
drive nut (lower left) that can
be turned in either direction
by the slewing and tracking

CMHASD - JackEllisPoncet