|Project by bbasiaga||posted 12-14-2012 03:37 AM||2141 views||12 times favorited||25 comments|
I built this telescope over the course of about 2 years. I started in 2009 by ordering the 22” mirror, which would take 12 months to get. In the mean time, I worked on the design and then started cutting wood.
The primary structures are made of Baltic Birch plywood. Some half inch, some three quarters, and most of the joinery is dowels and biscuits. The uprights and focuser board (red colored) are made of paduk. The basis for the design is John Dobson’s ‘Dobsonian’ namesake telescope. However I strayed from the basic design which has considerable size and weight for a scope of this aperture. The way the Dobsonian telescope works, in short, is like a giant teeter totter. The heavy mirror is supported on a frame inside the ‘mirror box’. The lighter ‘upper cage’ holds the eyepiece and focuser, and these two are connected by truss poles. You can see this in the second photo.
In the traditional design, the mirror box is built deep enough that its top corresponds to the center of balance of the mirror box and upper cage. Two smaller, semicircular bearings are permanently placed there to act as a pivot point to allow the scope to be pointed up and down. This whole assembly (mirror box, bearings, trusses, upper cage) then sit in a ‘rocker box’ which has the bearings’ mating surface. The rocker is bolted at its center to the ground plate, which then allows it to spin in azimuth. Thus, you can rotate the cage up and down on the bearings, and left and right on the rocker box.
In my design, which like many good designs is compiled from concepts seen in others’ designs, the mirror box is made just large enough to contain the mirror, its mount, and the mounts for the truss poles. Since the mirror box is not tall enough to reach the center of gravity, the bearings become much larger so that the center of the circle they would form reaches the scope’s pivot point, and still allows the scope to function. These bearings are almost 42” in diameter. To make this scope manageable, the bearings were made removable, and the rocker box was sized such that the mirror box could nest in it. With this design, the heaviest component is still manageable by my wife and I, and the scope itself collapses in to a small space. In the last two photos you can see it is actually possible to fit this monster in the trunk and back seat of a mid-sized sedan. Though honestly, we always just load it in our SUV.
Its a unique design and it has allowed us to have the power of a large aperture telescope available to us at home. I hope you enjoy!
-- Part of engineering is to know when to put your calculator down and pick up your tools.