The main Sherwood Observatory telescope uses a system of mirrors,
in a Newtonian configuration. A 61-cm diameter main (primary)
mirror collects light from the object being observed. Light reaching the primary
mirros is reflected back along the optical tube in a converging cone of
light, before reaching an optically flat oval mirror inclined at 45° to the
The secondary 'flat' mirror is supported by an adjustable 'spider' system of
This spider is the cause of 'crosses' that can sometimes be seen superimposed
on photographs of stars. The light cone reaching the secondary mirror is then
deflected into an eyepiece focussing assembly affixed to the side of the telescope
tube within the upper section.
Individual eyepieces provide a range of magnifications. Lower
and intermediate powers are most frequently used. Seeing conditions
determine how much magnification can be applied. High magnifications
are effective only when the atmosphere is stable.
A single lens reflex camera body can be used in place of an eyepiece
to photograph the Moon, planets, stars and a wide range of other
objects. The CCD camera captures images more rapidly and these
can be analyzed on a PC.
Video cameras provide a convenient medium for displaying objects
to large numbers of people. This method also shows how atmospheric
turbulence - known as 'seeing', determines image quality.
The telescope is driven to counteract the Earth's rotation, enabling
objects to be kept in the field of view.
Bright objects such as the Moon, planets and stars down to naked
eye visibility can be found by using one of two "Telrad reflex
sights" mounted onto the rear of the telescope. When viewed
from the rear, the Telrad provides a heads-up illuminated target
which is aligned witht he direction the telescope is pointing.
For fainter objects, where pointing accuracy becomes more critical,
the 61-cm reflecting telescope is fitted with a NGC-miniMAX computer
system, again calibrated to the direction of the telescope. This
holds a database of several thousand celestial items, and can
be used to guide the telescope to point at objects that are not
bright enough to be seen with the naked eye.
As well as looking through the eyepiece, a CCD camera, SLR camera,
or Astronomical video camera can also be connected to the telescope
to allow images to be recorded for posterity.
Some of the images we have secured with these devices can be
seen in our online gallery.
Sherwood Observatory is a facility that has become larger than
our original dream; in being a fund of ideas and progress in astronomy,
we envisage it serving our community well in the years ahead.