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Building an 8" Open Tube Reflector

Calculating Strut Length

The following spreadsheet program will calculate the distance between the ends of the three sections of the telescope for any given focal length mirror and properly place the center box for accurate balance. The process involves answering a few questions and performing some activities.

1. Weigh the bottom section. Then determine the balance point the bottom section by placing it on a pencil placed crossways under the box or even just placing it on your arm and roughly judging the balance position. Measure the distance from the balance point to the upper end (the end toward the center) of the box.

2. Measure distance from center of eyepiece to lower end of box.

3. Weigh the top section with all hardware attached and even a typical eyepiece inserted. Then determine the balance point as you did with the bottom box. Measure the distance from the balance point the lower end of the box.

4. Measure distance from face of mirror to upper end of box.

5. Measure the length of the center box with both upper and lower sections screwed together.

6. Measure the distance from the focal plane at the eyepiece to the center of the tube.

You will now have the information you need to answer the questions on the spreadsheet. To bring up spreadsheet go here

NOTE: The spreadsheet will give the distance between the boxes, not the length of the tubes.

7. Calculate actual strut length as follows:

The triangle shown above explains the basic geometric principles involved. Where:

S = Strut length
D = Distance between tube ends
O = Amount that truss length extends over tube end
B = Base or length of truss offset from center line

Process: Add the amount the truss extends over the tube ends to the distance between the ends of the tubes. For example, if the distance between the tubes is 20" and the truss extends over the ends 3/4" then: D + O = 21.5". Assume the amount B is 4". Then, using the Cartesian equation, the length S is the square root of the sum of the squares of the other two sides; or, 462.25 + 16 = 478.25. The square root of 478.25 = 21.87" or about 21 7/8" - close enough.