Lens telescopes have existed since 1609, when the Dutch
spectacle maker Jan Lippershey sold the first telescopes to
his astonished customers. At first they were only meant as a
curiosity. They had a concave and a convex lens, produced
an upright image, and had a 3.5x magnification. Galileo
Galilei (1564-1642) improved on this invention and was the
first to make astronomical observations with it. Today almost
all lens telescopes are built according to the principles
of Johannes Kepler (1571-1630). His telescope,
based on two convex lenses, produced an upside
down, but much bigger and better focused
image. These are of course the most important
factors for observing the heavens. However,
building larger refracting telescopes, poses
two problems: Firstly, in a lens the light
rays are broken, like in a prism. That
can lead to colour distortion and
poor focus, and can only be correc-
ted at great expense. Secondly, the
bigger the telescope, the heavier the
lens and, as a result, the more
difficult it is to handle.
Isaac Newton (1643-1727), natural
scientist and mathematician, was
aware of these problems and solved
them in 1668 by inventing a telescope in
which the objective lens was replaced by a
concave mirror. The light was now gathered,
reflected, and then directed sideways to an
eyepiece by a small flat slanted mirror. Newton
ʼ
s first
mirror, which he built himself with his own special tools, had
diameters of 25 and 50mm, a focal length of 300mm and 30
times magnification. Thus the groundwork was laid for a
whole new generation of telescopes. Today almost all large
telescopes, including the Hubble space telescope, are built
according to Newton
ʼ
s invention.
The optical properties of this AstroMedia* reflecting
telescope roughly correspond to those of the first instrument
built by Newton. The mirror has a focal length of 450mm. Its
curve is spherical: it has the same round surface as a globe.
A spherical concave mirror has one disadvantage: the light
rays reflected from the edge meet a little nearer to the mirror
than those reflected from the centre, causing a slight focus
distortion. However, since this distortion is not grave and
such mirrors are relatively easy to grind, they are
often used in smaller telescopes. In telescopes
with a larger mirror and a higher magnification,
mirrors with parabolic curves are used. These
are of course more difficult and expensive to
produce, but the result is that all the light rays
meet at one point. The primary mirror
of this kit is made of ground and
polished glass and is vapour-coated
with aluminium. It is a product of
BAADER Planetarium, the leading
German suppliers of astronomical
instruments, and has been specially
designed and produced for this
AstroMedia* kit. The eyepieces are
made with acrylic lenses. Thanks to
the friendly advice of Wolfgang Busch,
Ahrensburg, an arrangement of lenses
was found which reduces distortions and
colour shifts to a very high degree. A
cardboard telescope can of course never
compete with factory made instruments made from
aluminium tubes, but this kit gives you the possibility to
follow in Newton
ʼ
s footsteps, and in the end you will be just
as surprised with the result as Newton was with his first
reflecting telescope.
The Reflecting Telescope
209.NST
1.
Please read the instructions for each step completely before
beginning and allow yourself enough time. You will be rewarded
with a more beautiful and accurate reflecting telescope.
2.
Before removing any pieces, cut the little joints that hold them in
place with a sharp knife, e.g. a craft knife or small carpet knife.
Only remove the piece that you currently need.
3.
There will be a groove wherever the card needs to be folded.
The grooves will sometimes be folded to the back and sometimes
to the front. ”Fold to the back” means folding away from oneself
when facing the groove. ”Fold to the front”, means folding towards
you. After folding, smooth the fold with the back of your thumbnail
to flatten it.
4.
As a rule, the areas to be glued are marked in grey on the front
of the card sheets. Use a good solvent based all-purpose glue
which has the advantage over water based glues that it doesn
ʼ
t
warp the paper and dries faster.
5.
If you want small pieces to stick faster, coat one side generously
with glue, press the two surfaces together so that the glue is
transferred to both surfaces, pull apart and blow two or three times
on the glue until it is almost dry. Then press the two parts together
accurately and firmly. The glue will stick immediately. It is better to
avoid this method with the mirrors and lenses as the glue can leave
strings on the surface.
6.
You also need some sticky tape, scissors, a pair of medium
rubber bands, some paper clips, and about 10 strong clothes pegs.
For fine
adjustment of the main mirror you need a set-square or a
ruler, a thin felt tip pen that can write on glass as well as a mirrored
glass ball (for example a Christmas tree decoration).
7.
If you are a perfectionist you can blacken the white cut edges of
the diaphragms, lens holders etc. with a black felt pen before fitting.
8.
After the glue has set, you can rub down the protruding edges
with emery paper or a fine file, e.g. the AstroMedia* handicraft file
400.SBF.
9.
Advice on the mirrors: The main and the secondary mirrors are
coated with a vapour-deposited aluminium film on the front surface.
Despite a protective quartz film, this makes them much more
delicate than rear silvered mirrors. Avoid touching the mirrored
surfaces with your fingers and, if necessary, clean them only with a
lint free cloth and alcohol. The aluminium film of the small
rectangular secondary mirror is covered by a protective foil. This
foil will not be removed until after construction.
10.
Special note for those experienced in handicraft: It is possible
to make the mounting of the telescope even more stable by cutting
some stiff card to a perfect fit, to be placed between some of the
card pairings in this set. Among these included are the parts [C3]+
[C4], [C5]+[C6], [H1]+[H2], [J1]+[J2] (the size of [J2] without foot
tabs) and [J3]+[J4] (the size of [J4]. However you must bear in
mind that the size of the bridge-piece [J5] and the glue markings on
the base plate [H1] are in relation to the normal thickness of the
walls.
Tips for successful construction - Please read before commencing!
