Making a Galilean Telescope

Galileo: “In this matter, it behooves all those who wish to make such
observations to be forewarned. For
it is necessary first that they prepare a most accurate galss
that shows objects brightly, distinctly, and not veiled by any obscurity,
and second that it multiply them at least four hundred times and show
them twenty times closer.” Sidereus Nuncius tr. Albert Van
Helden, p. 38.

What is a Galilean telescope?

A Galilean telescope is defined as having one convex lens and one concave
lens. The concave lens serves as the ocular lens, or the eyepiece, while
the convex lens serves as the objective. The lens are situated on either
side of a tube such that the focal point of the ocular lens is the same
as the focal point for the objective lens.

How does a Galilean telescope work?

The Galilean telescope was innovative in that he was the first to expand
the range of magnification of the new spyglasses beyond 3X, using his
particular set of lenses. In Sidereus Nuncius,
Galileo described how these two lenses served to magnify an object.

“When there are no glasses in the tube, the rays proceed to the object
FG along the straight lines ECF and EDG, but with the glasses put in they
proceed along the refracted lines ECH and EDI. They are indeed squeezed
together and where before, free, they were directed to the object FG, now
they only grasp the part HI” Galileo, Sidereus Nuncius tr. Albert
Van Helden, pp. 38-39.

Actually, Galileo could not explain how his telescope magnified
precisely. He did not understand, as we now know, that the magnification
of his telescope can be computed by F/f (see top figure). Increasing the
magnification requires lengthening the telescope. Our 10X telescope is
about 4 feet long.

From the
above picture, you can see that an image, HI, will be viewed upright, making
the Galilean telescope useful for terrestrial purposes as well as
astronomical. Keplerian telescopes, in contrast, invert the image.

What are the disadvantages of a Galilean telescope?

The Galilean telescope’s biggest disadvantage is its small field of view.
A Galilean telescope typically has a field of view of about 15-18 arc
minutes. The moon has a diameter of about 30 arc minutes, so the Galilean
telescope only reveals approximately one-fourth of the moon’s surface at
one time. In the Houston skies, a typical field of view has only one star
or no stars at all. This makes it very difficult to map a constellation.

Increasing the magnification on the Galilean telescope, like
all telescopes, reduces the field of view. Perhaps Galileo built a 30X
telescope, but it is doubtful that he used much in his observations. The
field of view must have been very tiny.

How do you make a Galilean telescope?

Building the
Telescope Tube: Last Year’s Group’s Work

Parts List (With approximate cost):

• Cardboard Telescoping Mailing Tube (1), $3
  • Diameter = 50mm (or 2″), Length = 1100mm (or 143″)
  • Should be comprised of an inner and outer tube with closed ends on
    the outer tube.
• Concave Convex Lens (the “objective lens”) (1), $16 for this and the
  next lens as a pair.
  • Focal Length = 1350mm (0.75 diopter)
  • Cut to our specification of 49mm diameter.
• Plano Concave Lens (the “eyepiece”) (1)
  • Focal Length = -152mm (-6.6 diopter), Diameter = 49mm
  • Cut to our specification of 49mm diameter.

Suggested Tools:

• Coping Saw
  • Alternatively, any other instrument that will make a relatively clean cut
    through the mailing tube.
• Drill (bit sizes discussed below)
• Super Glue
  • Alternatively, any other kind of glue that will firmly hold the inner
    and outer mailing
    tubes together. It must be of a thin consistency.
• Greenlee Punch (optional)

Instructions:

The basic premise of the telescope tube is to align two lenses the
appropriate
distance from each other. For this telescope, the lenses are a concave
convex (one side
curved out and the other curved in) and a plano concave (one flat side
and one side
curved in).
The plano concave lens is used as the “eyepiece” with the plano (flat)
side facing
the eye. The concave convex is used as the “objective lens” that is
aligned with the
eyepiece and with the convex side facing the sky. Notice that this lens
is actually
different than the plano convex lens used in the original Galilean
telescope, but
still gives the same results.

The following design uses pieces of the inner tube of the mailing tube to
hold the lenses
in place inside the outer tube. This is best illustrated in the
following diagram,
which shows the cross section of the telescope tube:

The outer tube of the mailing tube should have a short end that pulls off,
and this can be used for the split in the outer tube shown above. This end
will be used to hold the eyepiece. The inner tube must have two pieces
(about
1″ to 1.5″ each) cut off of it that will be used as spacers to hold the
objective lens in place. Make these
cuts as straight and clean as possible, which will be difficult since the
tube
is made out of cardboard. A coping saw works pretty well for this.

Take the short piece of the outer tube and cut or drill a hole (from
3/16″ to 5/16″
should be fine) directly in the center
of the metal cap on the end. This will be the eyehole. It is important
that this hole
be as clean as possible (no metal protrusions) so that the the flat side
of the eyepiece
will fit snugly against the metal cap. An electrician’s hole punch or
Greenlee Punch works
well for this task. If a drill is used, drill with a light pressure,
then smooth out
the inside surface as much as possible.

Place the eyepiece flush (flat side) against the inside of this eyehole.
The large
piece of the inner mailing tube left will be used to hold this in place.
To do this, drill small holes around the outside of the eyepiece tube.
Then, with the
eyepiece properly in place, slide
the inner tube into it, put glue into the holes, and turn the tube a
little bit to
spread the glue inside. Hold the tube snugly against the lens inside the cap
until the glue dries.

Now, put this aside and take the large outer tube and the two spacers cut
from the inner tube. Cut the closed end off of the outer tube, then use
the other end
to mount the objective lens (since that end already has a clean cut).
Again, the “drill
holes – put in glue” technique will be used to hold the spacers in
place. First, check
how far the inner spacer needs to be placed inside the tube so that the
lens and
other spacer will be able to sit inside the tube comfortably. Then drill
holes in the
outer tube around this area and glue in the spacer as before.

After the first spacer is in place and dry, place the concave side of the
objective lens
flush against it,
and put the other spacer snugly against the lens to hold it in place
(again using
the drill – glue method).

Now there are two pieces, each containing one of the lenses. Slide the
mailing tubes
together as shown in the drawing above, and the telescope is done. By
leaving these
two pieces unglued, the
telescope may be focused simply by sliding the eyepiece part inside the
objective part.
After a desired magnification/focus
is found, the two pieces may be permanently attached (or some tape will give
a semi-permanent attachment).

Two images of a finished tube are shown below. The first picture shows
the telescope tube from the objective side while the second image shows
the ocular end of the tube:

Making the Mounting: This Year’s Work

For the first half of the semester, we used the mountings built by last
year’s group. It looked like this:

This image was produced by the star jumping in the field of view. We
tried to have one person hold the telescope tube steady, but it takes
very little movement to cause a star to move across the field of view
when the field of view is only about 15 arc minutes. Moreover, our telescope
would blow over often and required one person to
hold it as still as possible, but this never really worked very well.

So, we built a new mounting and stars looked like this, with very little
distortion.

The new mounting was constructed on a Saturday morning and afternoon,
based on plans by Tom Williams. It looks like this:

This telescope mounting is composed of

• a base
• a support box for the telescope tubing and its bracket assembly
• a bracket assembly encasing the telescope tubing with trunnion
  bearings which fit into the
• trunnion notch plates attached to the inside of the support box

Parts List:

• Polybutylene Pipe
  • Diameter = 4″, Length = approximately 5′
  • These can vary. The length is approximately equal to the height that
    you desire.
• Polybutylene pipe for the trunnion (swivel) for the telescope tubing
  • Diameter = 2″, Length = approximately 2 pieces about 1″ long
  • This is glued to the tube bracket assembly with
• Wood glue
• Plastic furniture tacks
  • These are used to stick into the bottom of the legs of chairs to
    protect the floor.
  • They provide the foundation for the trunnions to turn.
• Wing Screws
  • These are used for the end plates of the tube bracket assembly.
  • They allow for the different telescopes to be used, just by
    unscrewing these and inserting a different telescope.
• Plastic floor flanges which can hold the big tubing and be
  nailed to the wood
• Nails, Bolts, Screws
• A whole lot of plywood!

Suggested Tools:

• Power Saw
• circular saw
• Power Drill
• glue
• work bench to cut out various shapes of wood

Instructions:

The base is constructed by
attaching the plastic holder to a square piece of plywood and then
adding four legs which extend outward. The big plastic tubing can be
glued into the holder. At the top of the tubing, another plastic holder
is put on, but not glued, so that the entire top of the mounting can be
removed for adjustments and travel purposes. The top plastic holder is
attached to a round piece of plywood which has a whole cut out of the
middle and a screw inserted there. The support box will be attached here
and can swivel 360 degrees.

The support box is constructed much like a shoe box except that
one side is missing. The bottom has a rectangle cut out of end (see where
Travis’s hand is). This allows the bracket assembly to rotate all the way
to a position perpendicular to the ground. Also, on the end of the box
opposite Travis, cut out a semicircle to allow the telescope to rotate
all the way to a position parallel to the ground.

The bracket assembly encases the telescope. It looks like this:

It is made up of a rectangular three-sided box with end plates on either
end (bottom). The end plates (at the top) have wing screws which allow
the top half of the end plate to be removed from the tube bracket
assembly, releasing the telescope. This is useful for making adjustments
to the telescope itself or for using a different powered telescope in the
same mounting. Notice also the round wooden pieces attached to either
side of the support box in the above picture. Wood glue is
applied around the circumference of the circle and the smaller
plastic tubes, only about an inch in length, are squeezed on.
These are the trunnions.

The unique part of the mounting is the trunnion. Trunnion notch plates
are attached to the insides of the left and right walls (when looking
through the telescope) of the support box. Plastic Furniture tacks are
stuck into the insides of the triangular cut-out and support the
trunnions. These tacks can be moved at any time to increase or decrease
friction so that the trunnions will move smoothly, but also not allow the
tube bracket assembly (and thus the telescope) to slip.

The tube bracket assembly, telescope, and support box combined can be removed from the base.

The result is that our telescope moves freely through an entire
hemisphere giving us the ability to look at anything in the sky. It was
with this mounting that we were finally able to recreate some of
Galileo’s observations.

If you would like more information on how to build a relatively cheap
mounting for your Galilean telescope, just e-mail Tom Williams or Jessica Williams.

Return to the astronomy group’s home page .