# Q & A: How to build a telescope

Q:
Hi! I'm in 6th grade and I am trying to build a telescope for the upcoming Science Fair (Which is in 5 weeks.) I really don't understand how a telescope works or how to build it. Please help! P.S. Some helpful resources would work also!
- Niki (age 11)
Champaign, IL, USA
A:

Telescopes basically do two things:

1. They collect more light than your eyes can. This lets you observe objects that are too dim to see with the naked eye, like distant galaxies or the planet Neptune. The larger the diameter of the main lens or mirror inside the telescope, the more light it can "catch". Nocturnal animals like owls have large eyes for the same reason. The largest telescopes have mirrors that are about 10 meters (30 feet) wide! But a telescope that is even a little bit bigger than your eye (a few inches) will help you see all kinds of dim objects.

2. They magnify objects, making them look bigger. This is useful if you want to look at something like the moon or the planet Saturn, which appear small in the sky. Magnifying these objects reveals details, like the moon's craters or Saturn's rings. However, a distant star will look basically the same whether you magnify it 2 or 200 times—it's just too far away to look like anything other than a point of light.

To get magnification, you need to use two lenses with different "focal lengths" or strengths. (If you've ever tried on eyeglasses from a few different friends, you've seen the effect of different focal lengths.) To calculate the magnification of a telescope, divide the focal length of the "objective lens" (the one at the front of the telescope, facing the sky) by the focal length of the "eyepiece" (the lens you look though with your eye). For example, a telescope with an objective focal length of 40 millimeters and an eyepiece focal length of 20 millimeters will make objects look 40 / 20 = 2 times bigger. The objective lens must have a longer focal length than the eyepiece does. If you turn the telescope backwards, objects will look 2 times smaller!

The easiest kind of telescope to make yourself is probably a Galilean telescope, named after Galileo who helped design it. It has two lenses in a tube with an adjustable length. Here are some for building one, and you can find many others by searching online for "build a Galilean telescope." Good luck!

Rebecca Holmes

(published on 01/07/2015)