STUDENT
TELESCOPE MAKING
|
Title |
Telescope Making Experiment |
|
Age
Group |
7 – 12 |
Theory:
Telescopes come in three basic designs:
refractor, reflector, and catadioptric, plus some variations on each of the
types. Each has its pluses and minuses, and of course, each type can cost a
little or a lot depending on the quality of the optics and the accessories
needed.
Refractors and How They Work
A refractor is a telescope that uses two
lenses to deliver a view of a celestial object. At one end (the one farther
away from the viewer), it has a large lens, called the "objective
lens" or "object glass." On the other end is the lens the user
looks through. It is called the "ocular" or "eyepiece."
They work together to deliver the sky view.
The objective collects light and focuses
it as a sharp image. This image gets magnified and is what the stargazer sees
through the ocular. This eyepiece is adjusted by sliding it in and out of the
telescope body to focus the image.
Reflectors and How They Work
A reflector works a bit differently.
Light is gathered at the bottom of the scope by a concave mirror, called the
primary. The primary has a parabolic shape. There are several ways the primary
can focus the light, and how it is done determines the type of reflecting telescope.
Many observatory telescopes, such as
Gemini in Hawai'i or the orbiting Hubble Space Telescope use a photographic
plate to focus the image. Called the "prime focus position", the
plate is located near the top of the scope. Other such scopes use a secondary
mirror, placed in a similar position as the photographic plate, to reflect the
image back down the body of the scope, where it is viewed through a hole in the
primary mirror. This is known as a Cassegrain focus.
Newtonians and How They Work
Then, there's the Newtonian, a kind of
reflecting telescope. It got its name when Sir Isaac Newton dreamed up the
basic design. In a Newtonian telescope, a flat mirror is placed at an angle in
the same position as the secondary mirror in a Cassegrain. This secondary
mirror focuses the image into an eyepiece located in the side of the tube, near
the top of the scope.
Catadioptric Telescopes
Finally, there are catadioptric
telescopes, which combine elements of refractors and reflectors in their
design. The first such telescope was created by German astronomer Bernhard
Schmidt in 1930. It used a primary mirror at the back of the telescope with a
glass corrector plate in the front of the telescope, which was designed to
remove spherical aberration. In the original telescope, photographic film was
placed at the prime focus. There were no secondary mirror or eyepieces. The
descendant of that original design, called the Schmidt-Cassegrain design, is
the most popular type of telescope. Invented in the 1960s, it has a secondary
mirror that bounces light through a hole in the primary mirror to an eyepiece.
The second style of catadioptric
telescope was invented by a Russian astronomer, D. Maksutov. (A Dutch
astronomer, A. Bouwers, created a similar design in 1941, before Maksutov.) In
the Maksutov telescope, a more spherical corrector lens than in the Schmidt is
utilized. Otherwise, the designs are quite similar. Today’s models are known as
Maksutov –Cassegrain.
Refractor Telescope Advantages and Disadvantages
After initial alignment, which is
necessary to have the optics work well together, refractor optics are resistant
to misalignment. The glass surfaces are sealed inside the tube and rarely need
cleaning. The sealing also minimizes effects from air currents that can muddy
the view. This is one way that users can get steady sharp views of the sky.
Disadvantages include a number of possible aberrations of the lenses. Also,
since lenses need to be edge supported, this limits the size of any refractor.
Reflector Telescope Advantages and Disadvantages
Reflectors do not suffer from chromatic
aberration. Their mirrors are easier to build without defects than lenses are
since only one side of a mirror is used. Also, because the support for a mirror
is from the back, very large mirrors can be built, making larger scopes. The
disadvantages include ease of misalignment, the need for frequent cleaning, and
possible spherical aberration, which is a defect in the actual lens that can
blur the view.
Once a user has a basic understanding of
the types of scopes on the market, they can focus on getting the right-sized
one to view their favorite targets with. They can learn more about some
mid-range-priced telescopes on the market. It never hurts to browse the
marketplace and learn more about specific instruments. And, the best way to
"sample" different telescopes is to go to a star party and ask other
scope owners if they're willing to let someone take a look through their
instruments. It's an easy way to compare and contrast the view through different
instruments.
Experiment/ Methodology
Method1 Making
a Telescope with Magnifying Glasses
1 Gather
all your materials. You'll need a piece of corrugated paper that is about 24
inches in length (this is a ridge material, easily available from paper stores
or craft stores). You'll need two magnifying glasses that are NOT the same
size. You will also need strong glue, scissors, and a pencil. If the magnifying
glasses are the same size, the telescope won't work.
2 Hold
one magnifying glass (the bigger one) between you and the paper. The image of
the print will look blurry. Place the second magnifying glass between your eye
and the first magnifying glass.
3 Move
the second glass forward or backward until the print comes into sharp focus.
You will notice that the print appears larger and upside down.
4 Wrap
the paper around one of the magnifying glasses. Mark the diameter on the paper
with the pencil. Make sure that it is pulled tight.[3]
5 Measure
along the edge of the paper from the first mark. You will need to measure about
1 1/2 inches from the mark. This will create the extra length to glue around
the magnifying glass.
6 Cut
down the marked line on the paper to the other side. You should be cutting
across the width of it (don't cut lengthwise). The paper should be about 24
inches in length on one side. Cut a slot in the cardboard tube near the front
opening about an inch (2.5 cm) away. Do not cut all the way through the tube.
The slot should be able to hold the large magnifying glass.
7 Cut
a second slot in the tube the same distance from the first slot as was written
down between the two glasses. This is where the second magnifying glass will
go.
• You should now have two lengths of
corrugated paper. One piece should be slightly larger than the other.
8 Place
the two magnifying glasses in their slots (big one at front, little one at
back) and tape them in with the duct tape. Leave about 0.5 - 1 inch (1 - 2 cm)
of tube behind the small magnifying glass and cut off any excess tube
remaining.
9 Glue
first length of paper around one of the magnifying glasses. You'll need to glue
the edges of the paper together as well, since you've left about 1 1/2 inches
of paper.
10 Make
the second magnifying glass tube. This one will need to be slightly bigger than
the first one. Not too much bigger, only so that the first will fit into the
second one.
11 Slot
the 1st tube into the 2nd. You can now use this telescope for looking at things
farther away, although it will be difficult to view the stars clearly. This
type of telescope is really good for viewing the moon.
• The images will be upside down, since
astronomers don't care about up and down in space (there is no up or down in
space, after all). if you wish to align the image with gravity, you can use two
prisms aligned in an "N" shape to correct the image, but you will
have to re-position the lenses.
Method 2 Making
a Telescope with Lenses
1 Gather
materials. You'll need two lenses, a mailing tube that has an inside tube and
an outside tube (you can get this at the post office or office supply store; it
should have a diameter of 2 inches and a length of 43.3 inches), a coping saw,
a box cutter, some strong glue and a drill.
• Lenses should be a different focal
length. For best results get a concave-convex lens with a diameter of 49 mm,
and a focal length of 1,350 mm and a plano-concave lens with a diameter of 49
mm, and a focal length of 152 mm.
• The coping saw is the most effective
for making clean, straight lines, but you can use any other kind of saw or
cutting device if you need to.
2. Cut
the outer tube in half. You'll need both sections, but the inner tube will act
to space them out. The lenses will go in either section of the outer tube.
3. Cut
2 pieces from the inner tube of the mailing tube. These will be your spacers
and they should be about 1 to 1.5 inches in diameter. Make sure you cut clean
and straight with the coping saw (or other tool).
• The spacers hold the second lens in
place at the end of the outer section of the mailing tube.
4 Make
eye-hole in mailing tube cap. Use the drill to apply light pressure to the
middle of the cap to create your eye hole. Again, this will need to be as
smooth and as clean as possible to create the best viewing results.
5 Drill
holes on the outside of the large tube. You'll need to make the holes where the
lens is going to be placed in the outer tube, because the holes allow you to
put glue into the inner part of the tube. Near the end of the inner tube is the
best place, about an inch in.
• You'll also need to the make holes at the
end of the outer tube for the eyepiece and the cap.
6 Glue
eyepiece lens against removable cap. The eyepiece lens is the plano-concave
lens and the flat side needs to be against the cap. You'll glue through the
holes you made and turn the lens to spread the glue. Press tube against lens
until the glue is dry.
7 Cut
off closed end of outer tube. You'll end up sticking the inner tube into the
outer tube through this hole.
8 Insert
first spacer into outer tube. The spacer will need to lie flat on the inside of
the outer tube to hold the concave-convex lens in place. You'll need to drill
the holes and put the glue in like you did with the eyepiece.
9 Insert
lens and second spacer. You'll need to make the holes, put the glue in and
spread it around. Press firmly until the glue has dried.
10 Insert
inner tube into outer tube. You can slide the pieces as necessary to get the
right focus. Since this is about 9x you should be able to see the moon's
surface really well and even Saturn's rings. Anything else will be too far away
for your telescope.