Balancing Your Homemade Dobsonian Telescope
Submitted: Wednesday, 11th June 2008 by Douglas Parkes
In order to achieve correct balance of the Kriege & Berry design, the fulcrum point is coincident with the top of the mirror box. Although not being at all good at mathematics, I worked out this Excel file to get the balance right and to provide other useful parameters as well.
All dimensions are entered as Metric units and the spreadsheet automatically calculates the Imperial dimensions as a confirmation for those of us who are confused by the Metric system yet are trying to adapt to it!
Next, decide on the thickness of ply to be used. The marine ply that I used was available in three thicknesses: 12 mm, 15.5 mm and 19.5 mm. I used the masses quoted in the timber supplier specifications sheet (for the brand of ply that I used) for the various ply thicknesses (also weighing one test piece as a confirmation). Enter the thickness of the ply that you have decided to use into the yellow cell E7 and the mass of the chosen thickness ply in yellow cell E11.
Now, all that is needed is for you to enter various mirror box heights into the green cell E10 and compare cells E34 and E36 until they become equal to achieve correct balance.
For Instance: with reference to the grey-coloured cells beginning at cell I7 (I8, I9 et cetera), you can see that a 15 inch high mirror box (a lower mirror box height is best) is a metric dimension of 381 mm. Enter 381 into cell E10. Now compare cells E34 and E36. Note in cell E36 that the torque of the secondary cage plus the weight of the truss tubes far out-weighs the torque of the mirror box, primary mirror and its supporting cell (E34). If you now enter a taller mirror box dimension, say 17 inches (enter 431.8 into cell E10), you see that the torque of the mirror box is still heavier but a much better situation exists. If you continue to vary the height of the mirror box you will find that a height of 468.118 (18.4 inches) almost perfectly balances the telescope. Sorry to be pedantic but the “468.118” is only to demonstrate the principle.
As the various figures are entered into cell E10, the following parameters alter:
When making my Dob, it was comforting knowing that it would be correctly balanced at the end of the construction process.
Feel free to use this spreadsheet. Alter it to suit your own purposes.
Article by Douglas Parkes. Discuss this article on the IceInSpace Forum.