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How to Make an Eight Inch Reflector
Part 2: Finding the focal length of a mirror

In this part of the tutorial I show you how to find the focal length of your telescope mirror.

Part 1 (the beginning) of this tutorial is here




focus from mirror to eyepiece


Finding the focal length of the mirror is important because that is where we put the eyepiece of the telescope. And the eyepiece is what we look through. And it does the magnification!

We build the telescope so that the focal length ends up right at that eyepiece.

Diagram of the test setup

We can find the focal length of any telescope mirror with a simple setup . You just need a lightbulb and a flat white surface.

Notice here what happens. The light from the bulb bounces off the telescope mirror. Goes past its focal point then creates a sharp image on the flat surface.

Diagram of twice the focal length

This distance will be twice the focal length.


The bulb and sheet setup

So here is the setup. We have a sheet of white foamboard. And we have a lightbulb. It is important that they are on a plane with each other. They both need to be the same distance from the telescope mirror.


Then we place the mirror at a distance from bulb and foamboard. And notice the yellow tape measure so we can get an accurace measurement. Move the mirror back and forth until the lightbulb comes into focus on the foamboard.

The distance to the mirror


The light bulb will come into sharp focus. You will even be able to read the printing on it. And you can see that it also works with a fluorescent bulb. Kind of neat.


Bulb in focus  Bulb in focus


589 inches

So that's it. We bring the bulb into sharp focus and we have twice the focal length. You can see here that for my mirror it is 58 inches. So... our focal length is half of that which is 29 inches. Yay! Now we can build our telescope.

NextOkay! Let's get to building it!




Celestron 127EQ PowerSeeker Telescope

CES1071 Features: -Telescope. -PowerSeeker collection. -Color: Black. -Material: Aluminum. -127 mm (5'') Newtonian reflector. -3x Barlow lens triples the magnifying power of each eyepiece. -Quick and easy no-tool setup. -Slow motion controls for smooth tracking. -Erect image optics - ideal for terrestrial and astronomical use. -Fully coated glass optical components with high transmission coatings. -Enhanced image brightness and clarity. -Accessory tray for convenient storage of accessories. -''The Sky'' level 1 planetarium software with 10,000 object database.


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