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Submitted: Wednesday, 11th June 2008 by Douglas Parkes
A few years ago I constructed an Obsession-style 381mm (15 inch) Dobsonian following the ideas suggested in David Kriege & Richard Berry’s great book The Dobsonian Telescope: A Practical Manual for Building Large Aperture Telescopes.

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!

Open Excel and load the file: begin by entering all of the required parameters into the pink-coloured cells. These will be fairly self-explanatory to anyone who has read the Kriege & Berry book: the weights of various components (primary & secondary mirrors, primary mirror cell, secondary mirror holder, completed secondary cage); diverted secondary distance; focal length of the primary mirror; primary mirror thickness; distance between top & bottom secondary cage rings; and, the thickness of primary mirror cell.

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:
• the torque provided by the mirror box section;
• the torque of the truss tube end of the telescope;
• the convergence angles of the truss tubes change;
• length of the truss tubes alter;
• side bearing height off ground changes;
• the height of the eyepiece off the ground alters (I didn’t want to use a step-ladder when I observe so eyepiece height was critical)
• and also the spreadsheet provides the sizing for the altitude and azimuth Teflon pads, again automatically adjusting as the parameters are varied.
For torque calculations I have used newton-millimetres  (Nmm) in lieu of newton-metres  (Nm) to produce a larger result for easier interpolation when altering the various parameters. The Imperial conversion is inch-pounds (in lieu of foot-pounds) for the same reason.

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.

##### Notes:
1. I used cam-action pole sockets (as per a Kriege & Berry suggestion in the book) and heartily recommend them. A pole is placed in its socket and given a slight twist and it locks tight (until it is untwisted during disassembly—it couldn’t be easier).
2. Since, for Dobsonians under 16 inches mirror diameter, the spokes of the side bearing arcs normally interfere with the poles, I used solid cast aluminium side bearings purchased from Obsession. I also used cast aluminium pole seats for the secondary cage attachment.
3. 12 mm ply is more than adequate strength for a 15 inch Dobsonian mirror box and rocker box.
4. I used self-aligning azimuth bearing pads (Teflon) to give an ultra-smooth azimuth motion. I designed the pad holders myself.
5. I used a secondary mirror holder that provides an X-Y adjustment (not the fiddly three-screw adjustment) for rapid secondary mirror collimation. The holder also provides for adjustment of the mirror offset independent of the X-Y adjustment.
6. The mirror box sides are different dimensions to allow for the wooden biscuit joints that I used in the construction process.
7. My Dob now has an Argo Navis Digital Telescope Computer and Stellar Cat Go-To fitted and instead of the usual drive wheel driving directly on the rim of the ground board, I used an Acetal (industrial plastic) flexible rack and gear mechanism which works well.
8. I regret any errors that may be in the spreadsheet.