This Refractive telescope uses two convex lenses. This causes the image to be upside down.
The first lens, called the objective, receives parallel light rays, and refracts it to its focus(we'll call it f1). The second lens, called the eyepiece, is placed in such a way that the image formed by the objective at its focus(f1) acts as the object for the eyepiece, at the eyepiece's focus(f2). The eye piece refracts the rays to be parallel again.
Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.
Step 1: Gather the Materials
You will need
- two convex lenses, one with a long focal length for the objective lens, and one with a very short focal length as the eyepiece.
I used a lens with 82~ cm focal length as the objective. it was supposed to be 100 cm, but turned out otherwise.
my eyepiece has a focal length of 5 cm.
- PVC pipes, same diameter as the lenses, in this case, 5cm. This will be the main body of the telescope.
- A pipe that fits perfectly into the main body, so that you can focus the image.
- Spray paint, i used matte black.
- Hot glue or any other glue to fix the lens in place.
- Hand Saw
-Hot glue gun ( if you're using hot glue)
You will also need some pipe cutting skills, which i clearly didn't have :p
Step 2: The Math
Before cutting the pipe into size, you need to know what that size is. For that, you need to find out the length of the telescope, or the distance between the eyepiece and objective lenses.
First, verify that the lenses are of the focal length they are supposed to be, by practically finding out the focal lengths.To do so, stand in front of a window during the day, and try to focus the scene outside on the wall or screen opposite to the window. Move the lens towards or away from the window until you feel the image is at its clearest. It will be easier if the window is the only light source of the room at the time.(turn the lights off)
Now, the distance between the wall/screen and the lens is its true focal length.
Once that is out of the way, distance between the objective and eyepiece is the sum of the focal lengths.
If you don't want to do all that, get someone to hold the objective in front of a window, and align the eyepiece in front of it and move backwards.Look through both lenses at the scene outside the window. Measure the distance between the lenses once you feel the image is clear enough.
Power of the telescope
Magnification of the telescope is given by
Focal length of objective(cm) / focal length of eyepiece(cm)
according to this formula, my telescope can magnify the image by 17 times (17x)
Step 3: Cut and Paint
Once you know the distance between the lenses, you can cut the pipe to size. For me, this distance was 86 cm.
- I cut the pipe and got it 84 cm long.It's okay if the measurement is a few centimeters off, since the slider can make up for it.
- Paint the pipes
- cut an extra piece of pipe of the same diameter of the main body, and around 10cm in length.
- cut the coupling part of the pipe off. It's the part of the pipe that fixes on to another pipe of the same diameter. See the picture for better understanding
- i cut another piece, about 15cm long, and cut it vertically, so that it opens up, and its over the main body. this will be called the cover piece. cut two of these if you want to attach the telescope to a tripod.
- next, paint all the parts
Step 4: Gluing the Lenses
After everything is painted, its time to attach the lenses.
- I used the coupling part of the pipe to attach the objective, so that its easier to change lenses if needed.
- I first put some hot glue on the brim of the coupling.
- Next, i placed the lens while the glue was still hot (obviously), and added hot glue to the sides of the lens.
Since the eyepiece is a part of the slider, and the slider is slightly smaller than the lens, you will have to attach the extra piece of pipe cut out earlier. This also prevents the slider from getting stuck in the main body. Now, the extra piece of pipe is a part of the slider( and will be called the big end)
Glue the eyepiece to the bigger end of the slider.
(i tried removing the excess hot glue, but the lens fell off, and didn't try removing it when i attached the lens again)
Step 5: Assembly and Conclusion.
The assembly is simple.
the objective lens goes on one end of the main body, and the slider goes into the other.
i used the cover piece over the eyepiece, and it keeps the user's eye in the right place.
to attach the telescope to a tripod, you need to make a hole in the second cover piece, and use a nut to fix it in place.(see the pictures)
The moon's craters are clearly visible through this telescope.