How to make your own Telescopes
This part of our web site gives you lots of information about designing
and building your own telescope. The principles are the same whether
you are just building a small telescope, say for your GCSE project,
or a really ambitious telescope.
(If you would like a .pdf version of these instructions, you can
download one here)
This diagram shows the layout of an astronomical refracting telescope.
The objective lens of the telescope is shown at
the left, while the pair of lenses that form the eyepiece
are shown at the extreme right. The only other thing that is inside
the telescope is, well, fresh air.
The ASTRONOMICAL TELESCOPE consists of an OBJECTIVE
LENS, whose job is to gather the light from the distant object
of interest and to produce a sharply focussed image, and an EYEPIECE,
which can be just one lens, but is more usually a couple of lenses.
The purpose of the eyepiece is to magnify the image produced by
the objective lens.
The astronomical telescope, then, is particularly simple. It produces
an image that is inverted i.e. everything looks upside-down when
viewed through such a telescope. Obviously that doesn't matter when
you are looking at the Moon, stars or planets, but it can be a nuisance
if you wish to observe heavenly bodies of a quite different nature.
The TERRESTRIAL TELESCOPE produces an erect image.
Traditionally the image was turned up the right way by an extra
set of lenses called the RELAY LENS SYSTEM. That made the
telescope rather longer, because room had to be left for the extra
lenses. However, the positioning of the relay lenses allowed the
magnification of the telescope to be varied, so that was useful.
If you want information about relay lens systems, how to construct
them and how to position them within your telescope, just get in
touch with us.
Nowadays the image is turned up the right way by means of a roof
prism, also called an Amici prism, either the straight-through version
(PR.8 in our catalogue) or the right-angle version (PR. 6 in our
catalogue) and we will show you later how this is done. The advantage
of using an Amici prism, instead of the relay lens system, is that
the length of the telescope is reduced. Again, we will explain this
later. The next diagram shows the components of the telescope.
| Diagram of an astronomical telescope
For the objective lens you could use, say, our lens
9530, 5900, 9613, 5001, 5003 or 5004.
|For the eyepiece you could use a pair
of lens 5293 or any of our shorter focal length lenses.
The eyepiece ('e') is fitted into the end of the drawtube 'd'.
Usually the end of the drawtube has a small clamp-screw to hold
the eyepiece safely in place.
The drawtube should protrude about 150mm beyond the end of the
main tube ('m') of the telescope. The entire drawtube needs to be
about 230mm (9inches) long, because the plug 'p' needs to be about
75mm long. The drawtube needs to be a neat, sliding fit inside the
plug 'p'. You can line the plug with baize to help the drawtube
You can cut a suitable piece of baize from the middle of a friend's
billiard table. You may wish to provide the plug with a clamp screw
also, so you can hold the draw-tube in a fixed position once you
have focussed the telescope by sliding the draw tube to the position
that gives the best image, seen through the eyepiece.
It is a good idea to do a scale drawing of the proposed telescope,
even a full-size drawing, once you know the focal length of the
objective lens that you are going to use. Remember that the total
length of the telescope will be the focal length of the objective
lens, plus the thickness of the eyepiece lenses.
The purpose of light baffles is to prevent stray light, reflected
off the inside of the telescope, spoiling the contrast of the image.
You can have as many baffles as you like, but two or three is usually
sufficient. In the diagram they are labelled 'a', 'b' and 'c'.
On the scale drawing mark the extreme edges of the path of light
from the edges of the objective lens to the edges of the eyepiece
lenses. These extremities are shown in the diagram by dotted lines.
The holes in the baffles should just come to the edges of this cone
of light, so you can use your scale drawing to MEASURE the size
holes you have to cut in your baffles.
Position the baffles inside you telescope in the same locations
as you chose on the scale drawing. Do make sure that the baffles
cannot be hit by the drawtube when it is pushed right in! The last
job is to paint the inside of the telescope, including the baffles,
with matt black paint.
Before you rush off to get started, here are a couple of pieces
of important information:
1. Choosing the right sort of lens
In the science books used in schools you will find diagrams
that show the layout of lenses in a telescope. These textbooks always
show the lenses as single pieces of curved glass. Such lenses will
work, after a fashion, but the optical quality of the telescope
will be miserable, because rainbow colours will surround the view
that it produces and it will be impossible to obtain sharply focussed
In real telescopes the lenses are ACHROMATIC lenses, which means
they are made from two layers of glass. The types of glass used
in the two layers, and their curvatures, are chosen so that there
are no rainbow colours around the image to spoil its crispness.
When you choose the lenses for your telescope, especially the objective
lens (the big lens at the front of the telescope) you need to be
sure it is an achromatic lens. All the objective lenses we sell
are achromatic lenses.
2. Finding the focal length of a lens
When you use a lens you need to know something about the way it
performs. One of the most important measurements is the distance
from the lens to the point where it produces a sharply focussed
image. That distance is called the FOCAL LENGTH of the lens. The
focal length depends on the curvature of the glass surfaces - a
chubby lens with highly curved surfaces, for example, will bring
the light to a focus a short distance from the lens. A lens that
has a very shallow curvature will bring the light to a focus a greater
distance from the lens. The focal length of the objective lens in
a telescope is important because it is the deciding factor in the
length of the telescope - the total length of the telescope will
be just a little bit greater than the focal length of its objective
The other important factor that is decided by the focal length
of the objective lens of the telescope is the magnification that
the telescope produces. The longer the focal length of the objective
lens, the greater will be the magnification that the telescope will
produce. Later we will show you how to measure the focal length
of a lens, and how to work-out the magnification of the telescope.
BUILDING A PRISMATIC TELESCOPE
Now we are going to give you details of how to construct a telescope
where the eyepiece is set at right angles to the body of the telescope.
The advantage of an angled telescope is that you do not have to
get your head under the telescope when you are observing objects
that are at a high angle, such as the stars that are nearly overhead.
There is another advantage: the telescope is more compact. The
way to turn the light path from the objective lens through 90o is
to use a right-angle prism. You will find them listed in our catalogue.
You need to realize, however, that the view seen through this telescope
will be the right way up, but the wrong way round. Such a telescope
is fine for astronomy, but it is not much use in the daytime.
The prism will need to be held in place. The best method is to
glue the triangular side of the prism, which is usually frosted
glass, to a support that protrudes from the rear plate of the telescope.
You can telephone us if you need some help with this idea.
The diagram shows how the components are positioned. The distance
from the objective lens to the eyepiece is still the same as if
the prism were not there and you were making a 'straight-through'
telescope. That distance is now being bent round a corner, so the
front to back length of the telescope will be less.
When you make such a telescope you still need to have some means
of altering the focus. The choices are EITHER to fit the instrument
with a stubby draw-tube OR to dispense with the draw-tube and alter
the focus by moving the objective lens instead. Either method will
If you want to make a telescope like this, but for terrestrial
observation, such as bird watching, you just need to use an amici
prism, such as the lovely PR.6 listed in our catalogue.
If you are a serious astronomer and are worried about light-loss
through the glass prism, you can replace it with a small, front
aluminised mirror. We sell such items and we list them on a separate
data sheet that we will happily send you on request.
| Diagram of a prismatic
You can select a suitable objective lens for this
project from our lists of lenses that appear in our catalogue
(page 8 et seq.)
|For an astronomical telescope you could
use our prism PR.2 OR pentaprism PR.8 For a terrestrial / astronomical
telescope you need to use our amici prism PR.6 |
| You can find suitable eyepiece lenses for this project
listed in the catalogue. (For the telescope shown in the photograph
below we chose a pair of lens 5293.) |
And here is one we made earlier!
Finally, a few words of encouragement: making
a telescope for the first time is always a daunting task, because
you are not sure which components to choose - there seems such a
huge variety. Then there is the problem of finding the tubing necessary
to make the body of the telescope. Do not worry, we can help! If
you need to talk to someone about the project, just call us, we
may even be able to supply some tube.
It is a fact that the telescope that you make yourself will probably
perform better than those that you can buy ready-made. After all,
you are likely to take much more care in its construction because
you are working for your own benefit. If you do make a few wrong
moves along the way its worth remembering that the person who never
made a mistake probably never made anything.
Good luck, enjoy your telescope building and don't be afraid to
seek our advice if you get stuck..
Here is some technical information if you need it:
You can measure the focal length of a lens by the following
method: In daytime stand indoors, opposite a window.
Hold the lens against the wall opposite the window and move
the lens away from the wall until it produces, on the wall, a
sharply focussed image of the outside world. Measure the distance
from the lens to the wall. That distance is the focal length (approximately)
of the lens.
The magnification of the telescope depends upon the focal lengths
of the objective lens and the eyepiece. The magnification is calculated
by DIVIDING the focal length of the objective lens by the focal
length of the eyepiece.
Magnification = Focal length of objective lens divided
by Focal length of eyepiece
Another way to find the magnification of a telescope (or a
pair of binoculars) is as follows:
(1) Measure the diameter of the objective lens.
(2) Hold the telescope or binoculars a few centimeters from your face,
with the instrument pointed towards the bright sky (NOT THE SUN!).
Look at the eyepiece(s) and measure the diameter of the bright
circle of light that you see in the centre of the eyepiece(s).
(3) Divide the diameter of the objective lens by the diameter
of the circle of light in the eyepiece. The answer will be the
magnification of the instrument.
Example: The objective lens of a certain pair
of binoculars has a diameter of 30mm
The circle of light seen in the eyepieces is about 5mm
The magnification produced by the pair of binoculars will be 30mm
/ 5mm = 6
So this pair of binoculars will give a magnification of 6 times.
You can use this method to find the focal length of an eyepiece.
First you find the magnification of the telescope by the method
we have just described. Then you find the focal length of the
objective lens by the method given at the top of the page.
Focal length of eyepiece = Focal length of objective lens /
Now that you have got the gist of what objective lenses look
like and how they work, the next thing you need to understand is
how to make the eyepiece of the telescope. Eyepieces are seldom
just one lens, usually they are several lenses grouped together
to make the eyepiece easier and more comfortable to use than would
have been possible with a single lens.
The Plossl Eyepiece.
This is one of the commonest eyepieces used in astronomy, and certainly
the easiest to make. It consists of two identical achromatic lenses,
placed with their most curved surfaces facing each other. The lenses
are positioned so that these surfaces are almost (but not quite!)
touching. The same design is also called the Symmetrical or Dial
Sight eyepiece. It is obvious why it is called a symmetrical eyepiece
when you look at a diagram of its construction.
This type of eyepiece is also called a Dial Sight eyepiece because
it was commonly used in the dial sight (the aiming instrument) on
artillery pieces. The focal length of the Plossl eyepiece is just
over HALF of the focal length of each of its lenses.
Example If two of our lens 5293 were used, the
eyepiece would have a focal length of about 35mm., because each
of its lenses has a focal length of just over 50mm.
Building the Plossl Eyepiece
next diagrams show the components of the Plossl eyepiece.
First make the spacer ring 'c' the same diameter as the lenses.
This ring keeps the two lenses from touching. The spacing of the
lenses is not critical but try to get them as close as possible.
Assemble the two lenses and the spacer ring and hold them together
by a twist of adhesive tape. (This is shown as 'f' in the lower
diagram.) The body of the eyepiece ('e') should be bored out to
such a size that the lens assembly slides in smoothly.
Make a retaining sleeve 'a' that is a tight fit in the eyepiece
body. Push this sleeve into the body of the eyepiece to stop the
lenses from falling out. The lower diagram shows the complete eyepiece.
If you are a more advanced metal worker you can make the components
of the eyepiece so that they screw together. This photograph shows
such a Plossl eyepiece that we made, using a pair of our lens 1401.
It is most important that the exposed metal parts inside the eyepiece
are painted matt black to minimize reflected light within the eyepiece.
The Ramsden eyepiece is made from two
identical plano-convex lenses, spaced apart by about 75% of their
focal length. Again, the focal length of the finished eyepiece is
just over half of the focal length of the lenses from which it is
made. The lenses have their curved surfaces facing each other. Again,
the lens arrangement is symmetrical.
Example: If two of our lens 20 were used they
would be spaced about 3-5cm apart.
How to make a Plossl eyepiece
We suggest that you can use any of our achromatic doublet lenses.
The focal length of the eyepiece will be about HALF the focal length
of the lens that you chose.
This diagram shows the assembled eyepiece