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Example Build Plan

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.

Calculating Plywood Parts Sizes

6-inch f/8 Mirror        8.25-inch Tube Outside Diameter

ADJUSTABLE CRADLE

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):

  • [2] Cradle Sides 10.25” × 12”
  • [2] Cradle Top/Bottom 8.75” × 12”
ALTITUDE BEARINGS

Diameter = 1.2 TO 1.8 × Tube OD = 1.7 X 8.25” = 14” Diameter)

Altitude Bearing Parts (¾-inch Plywood):

  • [1] 14" Diameter Disk cut in half makes two Altitude Bearings
ROCKER BOX & GROUND BOARD

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):

  • [2] Rocker Box Sides 10” × 24”
  • [1] Rocker Box Front 11.375” × 23”
  • [1] Rocker Box Bottom Circle of 16" diameter
  • [1] Ground Board Circle of 16" diameter

Plywood Layout

Cradle End View Layout
This plywood layout diagram allows us to estimate how much plywood we need to buy, and how the parts will be cut out of the sheets. We show a 4-foot by 4-foot sheet of plywood as two sheets of 2-foot by 4-foot size to aid in dimensioning the parts, it could easily be a single sheet in reality.
Plywood Layout Diagram

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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