A Cheap Astronomical Telescope
Lenses held by retort stands
A colleague asked me to devise a way for students to make a simple telescope. Convex lenses are more readily available than concave ones so I chose to investigate making a simple astronomical telescope using lenses obtained from a $2 shop.

The photo shows the lenses held by retort stands. This makes it easy to move them around when finding the best distance between the lenses.

I used one frame from a pair of $2 reading spectacles for my objective lens. The spectacles were rated at 2.00 Dioptres which means their focal length is 1/2.00 metres = 50.0 cm. (I measured the focal length to be about 46 cm using the ‘focus the image of a distant object onto a screen’ method.)

For the eyepiece lens I used the 60 mm diameter lens from a $2 set of magnifying lenses. I estimated this lens to have a focal length of about 18 cm.
The “Easytorecall UK Web Directory” has a clear explanation of telescopes at: http://www.easytorecall.com/how_telescopes_work.htm
My telescope should have a length equalling the sum of the focal lengths. This should be about 64 to 70 cm but my arrangement above seemed best at about 75 cm.
Optical arrangement of telescope

The magnified image

The two photos show first the optical arrangement and then the magnified image. The object is a yoghurt container.

When viewed 5 m away through the telescope the object looked to be the same size when viewed by the naked eye about 1.5 m away. This gives the magnification at just over 3x which corresponds fairly well with the formula: Magnification = focal length of objective divided by the focal length of eyepiece = 48/18 = 2.7x

Better magnifications can be obtained using eyepiece lenses from your optics teaching resources that have much smaller focal lengths.

However, this simple apparatus using $4 of materials is useful for showing students the principle of a refracting telescope.
Telescopic cardboard telescope

This last photo shows my final telescope made using cardboard tubes and masking tape. The tubes can be ‘telescoped’ to adjust the length when focussing on objects at different distances.

Please feel free to email me if you have any questions or to suggest improvements in this resource.
Cheers, Denis.