My 14" f/5 ultra portable dobson telescope project

At last I decided to give the 14” f/5 optics I bought more than 10 years ago a real life: I’ll be building an ultraportable dobsonian telescope.

I start with the following parts list which I already have acquired over the years:

• Primary mirror: 14” f/5 with focal length of 70.2” (1783mm) (1 inch thickness, to be precise: 24mm at the edge)
• Secondary mirror: 3” (75mm) minor axis
• Secondary holder and 4-vane offset spider vanes (manufactured by Gary Wolanski)
• Low profile 2” aluminium helical focuser (manufactured by Gary Wolanski)
• Finderscope: 8x50 (TAL; heavy but excellent)
• Plywood from my previous ATM attempt (classical dobson, but very heavy and bulky)

Design decisions

Mirror cell

The mirror cell for this 14” f/5 telescope has been computed with PLOP. From the simulations with PLOP I have decided to build an alternate 9 points cell that considerably outperforms the classical 9 points cell for my primary mirror. The design consists of 1 central point and 8 additional points spread evenly on a ring. I have written input files for PLOP for this particular cell design since there was none. The cell does not use 3 triangles, since 2 of them would be very flat, hence making an accurate positioning of the center of gravity very difficult (a 0.5mm tolerance on 20mm is 5 times worse than the same tolerance on 100mm). Instead I designed the cell to use one triange supporting the central point, and 4 hinges (3 identical ones plus one for joining 2 of the former).

Some relevant links:

• Preliminary study for my 14” f/5 mirror with PLOP (many common and special cell designs, up to 18 support points)
• Final study for my 14” f/5 mirror with PLOP, with extra attention to the role of the central obstruction in the simulations (narrowed down to cells with maximum 9 support points)
• PLOP - software tool for optimizing the support of a mirror in a floating mirror cell (David Lewis)
• Twelve point cells (Jeff Anderson-Lee)
• 12 point mirror cell - design ideas that may be helpful (Tom Krajci)
• Some more exotic PLOP Mirror Cells (G.S. Vegos) - 7 points (6+1), 9 points (8+1), 11 points (10+1)