The main design objective of this project was to make the largest aperture telescope with an
OTA that can be transported in the back seat of a small car without disassembly.
In addition, it should be light enough to be easily handled by one person.
The design is basically a classical Newtonian with a few "twists". The optimal aperture was
determined to be 14.5". This is because the tube length must be less than 4 feet to fit a
small car and that means an f/3.3 system. This is about as fast as practical to build a
Newtonian. Also, a 14.5" mirror weighs just over 20 pounds, which is getting near the upper
limit of easy handling in an OTA. Going up to a 16" would have meant a 30 pound mirror and an
f/3.0 system. That would likely be too fast an optic for satisfactory performance. In fact, at
f/3.3 this telescope is pushing the limits of the typical eyepieces currently on the market!
The optical figuring was challenging as well. Making an f/3.3 is substantially more difficult
than an f/4.0, the usual "fastest" mirror. Mike Spooner did an outstanding job on this mirror.
Many other opticians were frightened away by the challenges of this fast focal ratio, but Mike
took on the task and made it perform excellently. In fact, it is a very fine planetary scope!
An f/3.3 planetary is almost unheard of, but this design proves there is no reason a planetary
scope must be slow.
Some key features of this design are:
The primary mirror is made from a conical blank. This is a superior design from a structural
perspective. The central 4" is 2-5/8" thick, with a gradual taper to 3/4" at the edge. This
shape provides greater rigidity to the surface compared to a standard uniform thickness
mirror, and is lighter as well. The result is no heavy, complex multi-point floatation
mirror cell is required. The mirror is simply RTV'd to a 3-arm support and a metal plate.
(See the rear view image)
The tube is kept as small as possible, 15" in diameter. This in conjunction with a low profile
focuser and image plane near to the maximum in-focus range means a relatively small diagonal
mirror is required. The diagonal is only 3.16" minor axis, which is a 21% central obstruction.
That is an acceptable value which does not contribute to any significant reduction in image
contrast.
E-mail: k7hx "at" qsl.net