The K-20 Project

The K-20 Project

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5/28/2007 - Building the Mirror Box - The "KEEL" of the Telescope

The Mirror Box on a truss-tube telescope is the most important part, second only to the optics.  This is the point where the torque of a 16 lb Secondary Cage hanging at the end of 7 feet of truss meets the torque of 70 lbs. of solid glass and a steel tailgate/flotation cell.  This is also the part of the telescope that the altitude bearings are attached to and thus also carries the entire weight of the Optical Tube Assembly.  In a nutshell, the box MUST be VERY rigid, precisely square and yet be as LIGHTWEIGHT as possible.  There are 3 characteristcs of the mirror box that allow it to satisfy these conditions; HVHC Baltic Birch plywood, corner gussets and braces, and a light baffle.  HVHC stands for "Hard-Veneer, Hard Core" and is a plywood made of hardwood plies on the inside and hardwood veneer on the outside.  Baltic Birch is a cabinet grade HVHC plywood with almost triple the number of plies as general use grade plywood.  The end result is that it is VERY rigid and finishes absolutetly beautifully.  This telescope will look better then some of the furnitur in my house!  Once the box is completed, the corners are stiffened with both gussets and braces.  These allow the sides of the box to remain as thin as possible and therefore as lightweight as possible.  Finally a 1/4 inch thick Baltic Birch light baffles is placed on the top of the mirror box and really locks the corners square.  This is especially important as the fulcrum of the telescope is at the TOP of the mirror box;  it's also where the truss attaches to the box as well.

ENTRY PICTURES


My brother cuts the sides of the mirror box. All that was required for the entire box, corner gussets and all was less than one 5' x 5' sheet of 5/16" Baltic Birch HVHC plywood.	To ensure that the Altitude Bearings are esactly parallel and lined up with one another we clamped the two SIDES of the mirror box together and then clamped one bearing in the proper orientation to them. After marking the location of the mounting holes we then drilled them through BOTH sides of the mirror box at the same time. In the lower picture you can see how the notch in the upper parts of the bearings will allow for the truss pole to pass.

Here is the completed box, note that the material is not very thick but with 11 plies laminated cross-grain it is VERY rigid. In the lower picture you can see the corner gussets being installed.	Corner gussets and braces installed. Cody and book for scale.