Another [10"] OTA lash-up.


It has long occurred to me that I could use four of the builders straight edge profiles to build a very stiff and light, four sided, altazimuth OTA. 

The images show a full sized mock-up using recycled rings largely held together with cord.  I have a sheet of 12mm birch plywood in stock to make smarter new rings.

The main problem with my earlier spar-type OTA was the torque forces around the doubled beam from the heavy 10" mirror's considerable offset. The beam did however suit a German equatorial mounting at the expense of some potentially odd eyepiece angles. 

The 2 meter, 6'6" long cardboard tube was simply far too heavy and awkward for comfortable handling. This four spar and rings OTA assembly is much lighter and readily provided spaces to grip it securely for transport to the observing site.

Having spars on all four sides immediately increases the stiffness of the 'tube' assembly. The spars have their own considerable depth to resist bending loads. 

The spars are also widely separated. Which naturally increases their 'moment.' So that bending of the OTA is greatly resisted in all planes.

My plan is to throw together a Dobsonian to get the 10" mirror back in action after far too long a hiatus building the 7" refractor and building mountings.  I don't think I ever once had the beam OTA correctly collimated due to flexure. The Dobsonian design provides the minimum possible ground clearance. Which obviously suits the very tall 10" f/8 OTA. The lower the eyepiece the less I need stepladders at modest object altitudes. A Dobsonian has no eyepiece access problems since it avoids odd tube rotation angles.

The spars suit clamp-on altitude bearings to achieve balance without permanent cosmetic damage to the spars. This is useful if I should ever rethink the design in the light of experience.

There are no torque loads applied to the new OTA. The mirror mass and secondary are both at the tube's center of gravity axis. I have simultaneously got rid of the thermal mass of the large and heavy aluminium saucepans and the aluminium plate of the primary mirror cell. I shall use a simple birch plywood cell for primary collimation.

The focuser will be fixed to the cardboard tube between two spars in the secondary cell to minimize the distance from the optical axis. This location will automatically apply a 45° angle to the focuser base for comfortable viewing at any tube angle.

The tube rings have no need to be made of heavy plywood if the short cardboard tubes are glued to become solid units with their pairs of rings. The cardboard tubes will provide some protection against dew and stray light. The plywood rings will each have four rectangles cut out to closely match the spar's outer dimensions. With the rings separated by the length of the cardboard tubes there is considerable natural resistance to any leverage applied by the spars to the ring assemblies. The reverse is also true.

No serious spar clamping is necessary due to friction in the rectangular cut-outs of the rings. In fact it is very difficult to move the cells along the spars with a quite normal tolerance sliding fit. Even with only two spars in rectangular cutouts I needed a rubber hammer to move the rings over and along the spars. Simple screws driven through the edges of the rings will easily suffice to fix the cells and allow easy, later dismantling if required.

I am trying to think how I could use a router to mass produce the 16 rectangular holes without wielding a hand saw. I have several bearing-guided, router bits which might work with a suitable template. Oddly enough I have never made or used a router template until now. Previous attempts to cut the holes with a coping saw were rather 'clumsy.' Requiring considerable coarse filing to smooth things out but left ragged edges from the birch plywood splintering.

I ended up using heavy brass plates with added barbel weights and G- cramps [C-clamps] to "box in" the router base plate as I cut each rectangle. An initial small drill helped to 'sharpen' the rounded corners left by the 1/4" 6mm router bit. Next time I shall make a square base plate out of Tufnol or plastic to better confine the cutter. The rounded base of the Bosch router does not slide perfectly straight against an edge. Nor does it work well inside a template. 

Click on any image for an enlargement.

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