CAD render 3ring construction

Frodo - a 10 inch travel Dobson

10 inch, f/4.8 newtonian


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basic considerations


hogging out
smoothing the curve
foucault tester


mirror cell
wire spider
middle ring
rocker box


travel and assembly





mirror cell

PLOP calculations suggested a six point mirror cell would be quite sufficient for the 254 mm (10 inch), 24 mm thickness f 4.8 mirror.
PLOP output graphic   PLOP results table

What follows is the original version, I have since found out that there is a reason glued aluminium cells haven't found widespread use in the ATM community, yet:

To unite the threefold rotational symmetry of the mirror support with the fourfold rotational symmetry of the truss assembly I chose a hexagon, made from 20 mm square aluminium tubing with four arms that connect to the lower ring.

cell hexagon with arms

Since I wanted to make as much as possible myself and it would have been quite expensive to have the structure welded I decided to try and glue the mirror cell. I used a high strength epoxy (brandname UHU Endfest 300) and the following procedure:
    - first I degreased the parts with acetone,
    - then applied the epoxy
    - assembled and fixed the parts with duct tape
    - and baked the whole thing in the oven at 180 °C for five minutes
This resulted in reasonably strong connections, although I have to say that in two cases the connections failed and had to be redone. How strong the pieces bond together can be seen here:

stress test for glued connections

The toolbox weighs around 15 Kg and applies the force perpendicular to the glued connection.

However, it cannot take a sharp blow easily. If you really want to break it, it is quite easy to do by knocking the part off with a hammer. But I hope within the usual mode of usage it is sufficiently strong. Time will tell…

And time did tell...

After a couple of observing sessions over the last autumn and winter I had another connection failure (one piece of the hexagon including the arm broke off) of and I decided to redo the holder for the bars from 9 mm plywood from my scrap box. This has since withstood all my destruction attempts (including rough handling of the OTA, bumping the cell rather sporty on our porch floor etc.).

wood/aluminim mirror cell

The three bars to hold the mirror are made from 5 mm thickness aluminium. They are glued to roller skate bearings (ABEC 5) which serve as the pivot to minimise stiction. The bearings have very small axial play so I hope when the mirror is rtv’d to the cell it won’t need lateral support. This is a necessity as I want to be able to rotate the whole OTA around the longitudinal axis, which cannot be done with the conventional cell design with a separate lateral support (sling, wire etc.)

aluminium bar on roller skate bearing   lower ring with mirror cell

An additional benefit is the negative altitude capability for daytime observations (imagine your watching ships along the coastline of the Pacific ocean somewhere in New Zealand…)

negative altitude capability




   © 2005 by Andreas Derwahl •  contact