This page shows an example of calculating parts sizes and laying them out on plywood sheets. Our example is for a 6-inch f/8 mirror in an 8¼-inch outside diameter tube - one of the standard sizes of telescopes we encourage our first time telescope makers to produce in our telescope making class.
The prototype yellow scope shown throughout this section has a 6-inch f/7 mirror, and it's rocker box is a bit shorter than this example uses - it's rocker box sides are 18½-inches tall due to the shorter tube length of an f/7 - otherwise, most other dimensions are essentially the same.
Of course, you need to calculate the variable dimensions for your specific mirror and tube sizes - this page is intended to help you in that task by showing a completed example.
Thanks to club member Glenn Jackson, who leads the mount making part of our Telescope Making Class, for the content of this page.
6-inch f/8 Mirror 8.25-inch Tube Outside Diameter
Cradle Side Height = Tube OD (8.25”) + (0.25” Bottom Gap) + (0.75” Top Gap) + 2× Plywood Thickness (2 × 0.5” = 1”)
Cradle Side Height = 8.25" + 0.25" + 0.75" + 1" = 10.25"
Cradle Top Bottom Width = Tube OD (8.25”) + (2× 0.25” Gap = 0.5”)
Cradle Top Bottom Width = 8.25” + 0.5” = 8.75”
Cradle Length = 2× Mirror Diameter (2× 6”) = 12”
Cradle Parts (½-inch Plywood):
Diameter = 1.2 TO 1.8 × Tube OD = 1.7 X 8.25” = 14” Diameter)
Altitude Bearing Parts (¾-inch Plywood):
Rocker Box Width = 1.2 × Tube OD (1.2 × 8.25” = 9.9”) = 10”
Rocker Box Height = Balance Point + 2.75” = 21” + 2.75” = 24”
Rocker Box Front Width = 8.75" Cradle Width 2×0.5” Cradle Side Thickness
+ 0.125” Clearance + 2×0.75" Rocker Box Side Thickness
Rocker box Front Width = 8.75” + 1” + 0.125” + 1.5” = 11.375”
Rocker Box Front Height = Rocker Box Side Height – 1” = 24” – 1” = 23”
Rocker Box Bottom Diagonal = Square Root( Rocker Box Width² + Rocker Box Front Width² ) = Sqrt( 10² + 11.375²) = 15.13"
Rocker Box Bottom Diameter = Round Up (Rocker Box Bottom Diagonal) = 15.13 => 16”
Ground Board = Rocker Box Bottom Diameter = 16"
Rocker Box & Ground Board Parts (¾-inch Plywood):
Once you have calculated parts sizes and have a parts list, you need to figure out how much plywood to buy.
The best way to do this is make a scale drawing of the parts, packed as tightly together as possible. We show this for our example in the diagram at right.
A standard sheet of plywood is 4-feet by 8-feet, or 48-inches by 96-inches. Many home centers in the US also sell partial sheets in the plywood aisle. For our sample, we need one half sheet of ¾-inch plywood (4-feet by 4-feet), and a 2-foot by 2-foot sheet of ½-inch plywood.
Our layout also shows two pieces of a mirror cell for the OTA.
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