building a dobsonian telescope - finding focal length of mirror
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VIDEO QUICKLINKS
"Making the Spider Video"
Time (1:54)
"Tailgate"
Time (1:57)
"Focal Length of Mirror "
Time (1:30)
"Mounting Blocks"
Time (1:35)
There are three parts to the finished telescope:
-
Telescope Tube
- Making the spider
- Eyepiece Holder
- Making the Tailgate - Rocker Box/Ground Board
- Tube Box
- Inserting Mounting Blocks
- Finding Focal Length of Mirror
- Positioning Eyepiece and Spider
- Using the Telescope
Finding the Focal length of the mirror
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"Focal Length of Mirror " (.mov) |
After inserting the mounting blocks in the telescope tube, you will want to determine where the eyepiece and spider go. This will be done with the students at the high school. Please see the building telescope lesson plans for ideas on how to present this to the high school students in an inquiry fashion.
The first step in determining the position of the eyepiece is determining the focal length of the mirror. This measurement needs to be VERY precise or you will have difficulty focusing your telescope when it is complete. To find the focal length of the mirror you will need a flashlight, two meter sticks, a sticker and a piece of white cardboard.
Put a sticker on the flashlight near the middle.
Line up two meter sticks end to end so you can measure distances up to two meters.
Position the mirror at the end of the two meter sticks.
Put the flashlight at the other end and line up a piece of white cardboard next to the mirror.
Move the flashlight and the cardboard back and forth until you have a clear image of the light and sticker on the cardboard. It is important that the cardboard is lined up with the front lens of the flashlight so you get an accurate measurement. If you move the flashlight ahead of the cardboard or behind it, your measurement will be off.
When the image of the light and sticker come into focus, take note of the measurement. This measurement is the radius of curvature which is twice the focal length. Divide this measurement in half to get the focal length. For a mathematical explanation of the relationship between radius of curvature and focal length, see Eric Hoopers Notes.
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