- Nov 17, 2017
This telescope has served me well, but as I was approaching retirement the year previous to last
I wanted to build another set of telescopes to last me the rest of my life. I had several criterion
a) I wanted a telescope with a shorter focal length--in particular 1/2 the focal length of the 20".
b) I wanted as large an aperture as could be made to work well with the optics available today.
c) I wanted to do more and better machining
As to (c) above, the above telescope was built with nothing more than a hack saw, files, drill
press, sandpaper, and I farmed out the woodwork to a friend with a cabinet making shop.
Being an engineer by profession, an amateur physicist, amateur mechanical engineer, and
a all round driven person, I invented a new mirror cell architecture which one can puruse at
The optical train in a Newtonian telescope is well established. With both (a) and (b) requirements
that largest primary mirror and pin-point star images with coma corrector is in the F/2.75 range
enabling a 14.5" mirror. F/2.75 is the recommended "fastest" Newtonian one should make with
todays optics (in particular a Paracorr 2 coma corrector). At this "speed" not very many kinds
of eyepieces will put up the the <now>F/2.75*1.15 = F/3.15 light cones. Luckily (or on purpose)
I had only been collecting EPs that would put up with such fastoptics.
But I backed down to a primary of F/3 which left me with a 13" aperture. To build the scope
around. At F/3 this telescope has 2× the Field of View as the current 20" F/4 enabling wide
field viewing of <say> the andromeda Galaxy and with 13" of aperture a bright image of same.
The EPs mentioned above all weight in in the 1-3 pound range, the Paracorr 2 is just over 1
pound, and if a Barlow is used for further magnification, we have 6+ pounds of weeight out
cantilevered 6+ inches from the typical focuser mounting position. So, instead of following
conventional reasoning (some might say any reasoning whatsoever) I designed a mounting
system that minimizes the cantilevering of forces, greatly stiffening the ability to hold an EP
at just the correct spot. A summary of this can be found at:
In Jan of 2015 the mirrors were ordered, with a proposed ship data 9-10 months later and
ended up arriving in May '16 for the 13".