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The HAS 12" Truss-Tube Dobsonian Project
PART II: A Draft Design
A Dobsonian telescope is actually a Newtonian telescope on a simple alt-azimuth type mount. The proposed truss-tube telescope for the HAS 12" mirror is exactly that with, as an additional feature, a truss construction instead of a fixed, fully enclosed, tube. This is in order to make the scope lighter and more portable.
HOW A NEWTONIAN TELESCOPE WORKS
Instead of a lens, a Newtonian type telescope uses a slightly hollow mirror which reflects the incoming light back into focus. That's why these type of scopes are often referred to as reflectors. Once the light is reflected back from the main mirror, it hits another mirror, called a diagonal or secondary, at the top of the tube. This mirror reflects the light sideways, out of the incoming light path, into the focusser, so that you can see the image in an eyepiece without having your head in the way of the incoming light. You can read more about Newtonian telescopes here.
A PROPOSED DESIGN
To the left is a sketch of the proposed design of a reflecting telescope on a Dobsonian mount, incorporating the 12" mirror currently in the possession of the Society. The sketch indicates the light path and the position of the main structures of the scope, such as the mount, the mirror and mirror cell, the secondary mirror and spider (the construction which keeps the secondary mirror in place), and the focusser.
It is proposed to make part of the 'tube' out of 18mm marine plywood and to join the top and mirror sections together using a construction consisting of aluminium trusses.
In the image to the left, the telescope is pointed to the zenith. The overall length of the 'tube' is just over 6 feet, and it is quite likely that steps will be required for those who are slightly vertically challenged.
