Hello, I would like to do an activity with the cub scouts.
I would like to have them build a telescope. I read up on the
Internet and it appears to be a simple thing, but so far my
telescope is not working. The telescope focuses, but objects in the
distance are not magnified and appear upside down. I am using a
telescoping mailing tube, 3 inches in diameter and up to 40 inches
in length. The objective lens is a double convex 50 mm diameter, 500
mm focal length lens. The eyepiece lens is double convex, 50 mm
diameter and 50 mm focal length. I attached the lenses to the tube
as described in the NASA web site, where they describe a way to
build a telescope. I am told that I should use a concave lens to
make the image right side up, but how do I fix the magnification?
That is the essence of the telescope. Can you give me some advice on
how to make this work?
The big objective lens in the front end must remain convex.
Replacing the small eyepiece lens in the rear end with a concave lens
would make the image right-side up.
You could look in Edmund Optics for a concave lens
of, say, 10mm diameter and -50mm focal length.
That is a negative number for concave lenses,
which cannot really bring a light beam to a focal point.
Comment: having an eyepiece so big (50mm) probably will not help
or even work easily.
A 50mm lens with 50mm focal length is so fat it does not really focus well.
It probably looks like a glass toadstool, i.e., at least 1 inch thick.
They are only used for herding lamp-light into one general direction,
so they are called "condenser lenses".
And they can be a little expensive because they are so fat.
Eyepiece lens 10mm diameter and +50mm focus (convex) would be nice.
Even 5mm diameter and +50mm focus would probably do.
Either could be concave, i.e, -50mm focal length.
You will need to make an end-cap with a hole in it to help mount
these lenses because they are smaller than the big tube.
If instead you use a cone or step down to a narrower cylinder,
that is good too but a little fancier to construct.
Convex or concave, the magnification will be that ratio of focal lengths.
For 500mm objective and 50mm eyepiece, that is 10x.
The scope is in focus when parallel rays entering the objective
taper down to a focal point in the tube
at same point the (convex) eyepiece lens focuses on.
So for clear-focus, the distance between the lenses is given by
the sum of their focal lengths.
For convex, 500mm + (+50mm) = 550mm.
I hope your telescoping tube-sets go down to 22 inches!
When you get it that short,
you should see some degree of focused image with your present lenses,
and it should have definite large magnification.
Let's hope that condenser lens does not shmoosh the image completely.
You may want to cut the tubes to 30-36 inches total maximum extension,
so they are only about 1/2 overlapped when set to 22 inches long.
For concave, 500mm + (-50mm) = 450mm.
Hmm, 4 inches shorter (18 inches), more tube-cutting....
Another tip- if your convex lenses are single-convex (flat on one side),
or one side is less curved than the other, (maybe your condenser lens?)
put the more curved surface outside where the light rays are parallel
and the flatter surface inside where the light rays are angled to converge.
That way tends to make better focus than the other way around,
for a given lens.
good luck, I think you are almost there-
If you truly have a 500 mm focal length lens as the objective and a 50 mm
focal length lens as the eyepiece you should have a factor of 10 magnification.
Please verify that you don't have magnification by comparing what you see when
looking through your telescope with what you see when looking through a tube
of the same length and diameter but without lenses. With a factor of ten
magnification on the telescope you should see much less through the telescope
than through empty tube.
You can verify the focal lengths of the lenses by focusing a distant light
source (more than 20 feet -- a street light at night would be good) and
measuring the distance from the lens to the focused image of the light.
Click here to return to the Physics Archives
Update: June 2012