Between 1983 & 1989
I was occupied with the design and construction of a domed observatory
and the reconstruction of a 10-inch Newtonian reflector by Geo.Calver.
The specific intention was to manufacture a telescope-observatory
system, controllable either at the eyepiece, or from a desktop
computer console. The telescope is principally employed for high
resolution work; i.e. lunar and planetary observation and double
star astrometry. The double star measures are made using either
a bifilar micrometer, a divided-lens double image micrometer,
or a Lyot-Carmichel Spath Blade micrometer.
To employ a filar micrometer effectively, it must be carried on a very stiff
mounting that has a large inertial moment and a very smooth and accurate drive.
The aberrations at the focal plane should be minimal, and the optics must be
capable of yielding high contrast images at very high magnifications.
In his book,
A Plea for Reflectors,
John Browning describes just such a telescope. During the hey day of amateur
double star observation in the latter quarter of the C19th the instrument of
choice was the long focus refractor. Browning championed the Newtonian reflector,
pointing out that there need be no inherent disadvantage in using
one for double star work providing it was of comparable construction.
The silver-on-glass Newtonians that Browning manufactured in his factory at
The Minories employed optics by G.H. With, a retired schoolmaster of the
Bluecoat School, Hereford. With took up his avocation of mirror making in the late
1860's and continued into the late 1870's. In 1887 he offered
his remaining stock of mirrors for sale. Browning ceased making
reflecting telescopes altogether once these had become exhausted.
This left the field to his contemporary, George Calver of Widford, Essex who purchased
the bulk of With's 'special reserves'. In his commercial publication,
Hints on the Use of Silvered Glass Reflectors, he describes
a range of instruments almost identical to Browning's. Unlike
Browning however, Calver produced his own optics that deservedly
merited a reputation ranking equal to With's. The most massive
equatorials manufactured by Calver he termed his No.1 mounting.
Imagine my delight when one came into my possession in December
1981 via the late Cyril Belchem, a former treasurer of the British
Originally housed in the grounds of Eton College, it had fallen into a state of complete
dilapidation, and had been donated to Slough Astronomical Society
in the mid 1960's. The iron castings and the cast steel hour and
declination axes were in sound condition and I considered its
restoration a worthwhile challenge.
Beforehand an observatory was needed, able to house the instrument once completed.
My initial ideas centred upon a 12-foot diameter dome constructed of galvanised
steel sheet on a 1" x 1/4"
square channel framework, surmounting a 4-foot circular wall of
concrete blocks. Following discussions with the late Alan Young
I subsequently modified the design to a 12ft:6in facetted monocoque
dome on a 12-sided galvanised steel fabricated wall. The original
location, in Forest Row, East Sussex, was designated under the
Access to the Countryside Act-1949, yet the building as proposed
was classed as 'Permitted Development' and hence did not require
Planning Permission. It was also 'Partially Exempted' under the
1985 Building Regulations.
The dome is made of 16SWG (0".064) duralamin supplied in half-hard condition,
42 sided, with 3 different facet shapes, each cut, return folded down the
sides and joggled top and bottom, and riveted together on a 2"x2"x1/4"
angle dural base polygon. The shutter is 36-inches wide and of
the up-and-over variety. Both were manufactured by Broadview Engineering
in Burwash, East Sussex. The dome has been designed to withstand
loadings from Beaufort Scale 14 winds.
In the original installation the equatorial rested on a massive 16 ton pier designed to attenuate
traffic vibration. It comprised a sand filled core with a concrete lintel and engineering brick cap
isolated from a 9-inch blinded encasement of
" ballast. The floor base was crowned with a plasticized screed to facilitate rainfall runoff.
The flooring was suspended on 4
"x2" joists and comprised tongued and grooved MDF board, tiled with
'parkiflex' hardwood tiles.
I would still recommend this form of construction, with the caveat that the
concrete base should not extend more than an inch from the joists. The wall
should overhang the end of the joists so rainfall splash back
runs onto the base pad and drains away. The under floor must also
be well ventilated, otherwise the joists will rot. I would also
use tanalized timber, and paint the side in contact with the concrete
with a bitumen mastic. I had sealed the base pad with epoxy floor
paint to prevent it dusting, and seated the joist battens on hardwood
wedges. However, although I had successfully sealed the wall cladding
to the base using a ruberoid mastic, when I dismantled the structure,
the joists had begun to rot, even though there was no obvious
ingress of water beneath the floor.
The wall and rail tube form an integral assembly of twelve 2"x2"
SHS stanchions rawl bolted to the base upon which is bolted a mill rolled 47-mm
CDS Schedule 10 tube, plugged and butt welded in five sections.
The wall cladding is 22SWG (0".028) 'ZINTEC' lined with 'SUNDELA'
The pier cap is surmounted with a slate bedplate into which socket cups are let.
Into these the bedplate sockets and levelling bolts rest. Altitude and azimuth
alignment are effected by differntial screws. The altitude differential
screw constants are 412 and 37.2 arcsecs per rev. The azimuth
differential constants 247 and 27.5 arcsecs per rev. By means
of these fine adjustments the hour axis has been aligned to the
true pole within ±15 arcsecs using Vezin's method.
The azimuth and the altitude (not shown but identical)
alignment sockets are hour-glass shaped in order to spread
the pressure of the levelling bolts through the slate and
transfer the load directly to the pier cap. The sole purpose
of the bed plate is to locate the levelling bolts of the base plate,
it merely rests on the pier cap due to its enormous self weight
and resists thermal creep and vibration induced movement.
Differential screws enable extremely fine adjustments without
the drawback of very fine screw threads that would not
withstand the force needed to move the massive instrument.
The observatory is equipped with a matching pair of brass
reproduction marine chronometers with 4MHz quartz movements
that are regulated to approximately ±0.1s per day by
Comparison at Coincident Beats. There is also a pair of digital internal/external max/min air
thermometers and a mercury max/min recording thermometer and a thermo-hygrometer.
The dome being well ventilated, finished with a special heat reflecting
paint, and lined with polystyrene sheet, maintains a differential
air temperature of less than 0º.5 C.
Engineering Assembly Drawings & Fabrication Details [html 814kb]
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