My Ideal Telescope
© Brooke Clarke, 2005
This is a scope for remote only use
although some provision needs to be made for local alignment and
testing. It is intended for use with remote computer control and
imaging my means of a Silicon based imager, uncooled TV camera, cooled
TV camera, uncooled astronomical camera or cooled astronomical camera.
The human eye can see light in the 400
to 700 nm range with peak sensitivity in the greens,, maybe 550
nm). Silicon based imaging chips have a sensitivity curve that
covers 400 nm to 1100 nm with a peak around 700 nm. Most
telescopes have been designed for eyepiece viewing and so have
an optical system that works in the 400 to 700 nm range. The more
expensive scopes do a better job of this than the lower cost scopes.
If the 700 nm and longer wavelengths are filtered out then an optical
system using one or more lens elements will be able to focus, but a lot
of photons have been taken away decreasing the sensivity of the
socpe. Also a multi spectral analysis will be lacking in the near
To make a lens that will focus light in the 400 to 1100 nm range all in
one plane is extremely difficult both in terms of design and
manufacture (very expensive). But mirrors with the proper
coatings will reflect light over this range without a problem, so a
scope design that uses all reflecting elements will cover the
wavelength range. Common scope optical systems that are made this
way are the Newtonian and Cassegrain.
f Number & Image Size
When using an imaging chip it's
important that the image size of a star is about the same size as a
pixel where the seeing is as good as it's going to get. If a star
image is spread across more than one pixel then the sensitivity is
reduced. If a pixel is much smaller than a star image then
resolution suffers. For poorer seeing conditions caused by the
atmosphere the camera should have the ability to do binning like 2x2 or
Since the primary focus image size is about the same as the f number
the scope should have a small f number without adding a focal reducer
(that has a lens).
Az-El or RA-Dec
To support satellite tracking and
terrestrial use (Az-El) or astronomy (RA-Dec) the mount should be
convertible by means of installing a wedge. The scope should not
have to be "flipped" like a German Equatorial.
Fixed Scope Position
When a digital camera is added to the
scope there are a lot of wires and cables that get tangled as the scope
moves. To avoid this problem and also to avoid optical problems
caused by the changing scope orientation, like mirror flop, the scope
should be held in a fixed position. Where in the sky it's looking
should be controlled by computer controlled mirrors. For example
if the scope was pointing straight up a flat mirror at 45 degrees would
allow the scope to look at any azimuth in a level plane and the
rotation could be on a continuous basis, i.e. like turning clockwise
without any stops. If now a second mirror at 45 degrees is added
turning on a horizontal axis this mirror would allow viewing in a
continuous vertical circle.
If the above scope was put on a wedge so that the main scope was
pointing at the North Celestial pole then the movements would
correspond to RA and Dec.
This mount - scope combination could be
made weather proof thus allowing the scope to be on a permanent pier
that's outside all the time.
Self Contained Dome
The purpose of a dome is to shield the
scope from the radiant heat loss to a black sky. This heat loss
causes the scope to cool below the dew point and then there's a dew
problem. The scope could have shields built in that would act
like a dome.
The mount should have a way to know if
the scope is aimed at or near the Sun and when it is insert a filter in
the light path.
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