The
Telescope
 |
possibly
by G. F. Sargent
courtesy Corporation of London ©
|
The
telescope was of a novel design - slung on a pulley
on the outside of a tall brick tower.
By
carefully reconstructing the plans of the basic telescope,
it is possible to begin to understand how the telescope
operated and discover some interesting facts about its
limitations.
The
tube was constructed very much in the manner of a steam
ship of the time, having separate panels of steel riveted
together along seams. The great length of the tube would
have required internal strengthening otherwise it would
tend to sag.
 |
A
photograph of Wandsworth Prison today.
|
In
some of the journal and press reports of the time there
were references to the angles of inclination to which
the telescope could be positioned. However, on examination
of the construction plans, it becomes clear that the
telescope was unlikely to have been able to point to
the horizon or indeed any higher than about 74°
from horizontal: even after having allowed for some
licence in the dimensions of the tower (the brickwork
may have measured 64 feet in height, but this figure
may not have included the rotating wooden roof structure).
Indeed the telescope could not be raised any higher
whatever the height of the tower, because the dimensions
of the cradle hoop supporting the telescope tube would
have become prohibitive!
The
tower was made of brick, but what type and colour. On
the site of the old assylum north of the telelscope,
Wandsworth Prison was built just a few years before
the telescope. The bricks for this came from Frying
Pan brickworks. It may be that for simplicity's sake
the bricks for the tower came also from this source.
back
to top of page
The telescope shown in elevation.
|
The
Accuracy
back
to top of page
Since
the telescope appeared not to have any means of calibration
with reference to the sky, all the target objects to which
the telescope was pointed needed to be visible to the naked
eye. There did not seem to be a system for acurately pointing
the instrument - there was no finderscope to speak of. Although
not shown in the limited illustrations, there may have been
some kind of simple viewing tube or primitive "gunsight".
In
order to set the telescope to a particular azimuth, the observer
may have placed marker flags around the perimeter of the azimuth
rail. However, the accuracy of this particular method would
only have been made less uncertain if the altazimuth housing
at the top of the tower also had some kind of calibration.
The
Movement
back
to top of page
Moving
the 'scope around would probably have required a "knack".
Since the eyepiece end of the tube rested in a sling on a
wooden "dolly", small pointing corrections would
have been undertaken by simply slewing the 'scope "left
and right" and "up and down" one or two degrees
without having to move any other part of the instrument. Indeed,
with some training, tracking something like Venus or the Moon,
would have become second nature.
It
seems that from the little technical information we have,
there are several ways to move the whole telescope. It would
certainly have been easier with several people to assist,
but essentially it could just about have been operated by
just one individual!
Firstly,
the telescope would have been readied for use. This would
have required removing the lens cover, releasing the "brakes"
on the azimuth armature and those at the top of the tower
in the rotable wooden altazimuth housing. After attaching
the large 3-metre steel dewcap (this may have required assistance!),
the operator would then have winched up the tube from its
near-horizontal "parking" position and attached
the eyepiece unit.
 The
operator would then have rotated the whole instrument, bringing
it to within a few degrees of the correct position by pulling
on "reigns" attached to the altazimuth housing while
nudging the azimuth armature as he / she walked around the
tower. Once this course setting had been made to the azimuth,
the "reigns" would have been tied up to the azimuth
"dolly", this in turn would have been pushed either
away from the tower or toward it - depending on the altitude
of the observation. It has to be remembered that the inward/outward
movement of the "dolly" has to be accompanied by
the raising or lowering of the telescope tube - this is achieved
by turning the handle on the small winch on the opposite side
of the tower. Although the telescope is said to have weighed
about three tons it was counterweighted, therefore the effort
in changing its inclination would have required little effort.
If
the winch "reeved" rope onto a drum, it would only
have required two or three turns at any one time. Indeed the
rope would have passed from the underside of the telescope
tube down to ground level, passing through a system of guide
wheels attached to the underside of the track around the tower
to the opposite side, onto the winch and then up vertically
to the underside of the counterweight.
Once
it was ascertained that the target object was nearly in the
telescope's field of view, the azimuth armature could have
been moved perhaps as much as ten degrees without having to
rotate the tower (much more than this, the telescope tube
would have made contact with the tower, the torque on the
instrument would then have become dangerous!). The smallest
of adjustments to the eyepiece "dolly" would then
have brought the target object into view.
At
the top of the tower is the altazimuth housing, this is designed
to hold the telescope tube while allowing it to be raised
and lowered and also rotated to the correct azimuth bearing.
Around the outside of the structure there appear to be four
grilled openings, spaced at 90° intervals. These could
have been used as communication ports. So, to facilitate the
easier use of the 'scope, an assistant would have remained
in the tower during use. Instructions would have been "yelled"
up from the ground. Instead of using the "reigns"
to position the tower in azimuth, the assistant would have
simply moved the tower around by hand. A good idea would have
been to mark compass bearings on the static rail inside the
housing, sadly this would not have been very useful as the
telescope itself was not directly in contact with the tower!
The
Difficulties
back
to top of page
As
has already been seen the telescope was not the easiest of
instruments to use. The telescope could only be operated during
fair weather with extremely light winds.
Although
the tower could have its floors weighted to provide vibration
damping, the external chains supporting the tube may have
resonated much in the manner of a guitar string.
It
is interesting, but in the illustration by Sargent, small
flags are shown attached to the "reigns" at regular
intervals. These could be for many reasons. Apart from decoration,
they may have been to scare birds so their acidic droppings
did not rot the ropes, the may also have been for crude calibration
of the telescope tube in some way or they may have been to
dampen vibration from wind.
One
aspect of the 'scope that might have played a role in damping,
was its shear weight!
The
Lens
back
to top of page
As
the lens' whereabouts is unknown at present it is only possible
to use references from publications at the time to guess as
to what may have been so wrong with it.
 The
lens was made by Mr. F. Slater of Somers Place in Euston Square.
It was a composite of two pieces of glass, one of flint and
the other of plate. The plate glass lens had a positive focal
length of 9,182mm, the fractive index being 15103. The flint
glass component had a negative focal length of 15,201.9mm
with a refractive index of 16308. Combined, they formed an
achromatic lens (reduced colour fringing especially at the
edge of the field) with a focal length of 23,164.8mm.
We
know it was 24-inches in diameter - a huge size for the time.
There have also been references to the fact that on many occasions
in order for it to work at all, it needed to be stopped out
(masking the central part of the lens and only allowing light
to pass through the outer circumference). Certainly
any error in figuring could have easily been corrected. Alas,
this appears never to have been underatken!!
back
to top of page
|
