10" f/8 An alternative (ultralight) tube?

*
[10]

Discovering the sheer weight of the full length, double thickness, cardboard tube had me looking for an alternative tube design. Something more practical for moving around the garden. Many interesting objects are too low or hidden by obstacles to allow a fixed telescope position in my tree-lined garden. The raised platform is not yet built and unlikely to be any time soon. Particularly given the latest and endless Danish winter we are enjoying yet again this year.

The most obvious tube design would be a Serrurier double truss. The need for accurate optical alignment over such a long focal length demands considerable care in design. Trusses with small included angles, which hardly extend beyond the clear aperture, are more likely to droop in use. Such a long focal length ideally needs a larger central (cradle) box. The truss tubes will then be forced into more steeply pyramidal forms with much wider splay angles between pairs of truss rods. (Or tubes) Cones and pyramids are inherently stiff compared with long, slender cylinders.

As the connection between the mounting and optical tube assembly the central /cradle) box itself needs to be stiff and free from deformation. Longer telescope tubes have a considerable moment arm. Which will seriously stress almost any affordable mounting. Even a Fullerscopes MkIV. It is not simply the physical support of such a long tube but its braking, steady tracking and acceleration. Wind effects are considerably magnified by the greater tube length. Trying to use high powers, when the seeing allows, will be fraught with difficulty. Particularly if the tube is not itself as stiff as its connection to the mounting saddle.

Much of this is why the Dobsonian mounting is so popular with amateurs for larger apertures. It avoids many of the pitfalls and bottlenecks of the typical German equatorial mounting. Though at the high cost of being an altazimuth. Thankfully equatorial platforms and drives now allow the Dobsonian to track the sky. Apertures are still increasing with previously unheard of sizes in amateur hands. Imagine the size and cost of a German equatorial mounting to support these huge instruments!

Many Dobsonians now use only one truss running the full length between the mirror box and focuser/spider support ring, or cage. This places even greater demands on the compression stiffness of the truss poles. These must inevitably be far  longer than a true (double) Serrurier truss. Moreover they are unable to match the sag of the now missing, mirror box trusses of a true Serrurier (double) truss.

The stiffness of a tube is a function of diameter rather than wall thickness. Which suggests that chasing lightness in the form of thin (carbon fibre?) rods will not lead to success. We all know the difference between waving a long cane and a stiffer tube of the same length. The willowy rod will often bend under its own weight. Leaning on it in compression will quickly bend and snap it. Shorter rods will resist compression to a far greater extent. Larger tubes will bear massive weights. Try lifting a long length of 2" PVC drainpipe by a central sling. Now do the same with a 6" diameter pipe. The 6" may be very much heavier, for the same length, but it is much stiffer. Its restive surfaces are much farther from its axis.

I looked at the sheer scale of my cardboard tube and decided that a truss system central box wants to be about a 24" cube. This matches the length of the MkIV's cradle. It will also help to make the most of any double truss design in these longer tube lengths.

How to build a strong but lightweight cube of these dimensions without welding? Make a central box frame with 'Porsa' square profile alloy tubing from their furniture building system. Then use their matching, metal reinforced, plastic corner joints to hold it all together. The sort of stuff used by aquaria enthusiasts for supporting their seriously heavy fish tanks. Once I had built my central cube I could  "skin" or plate over the box exterior in aluminium sheeting, pop-riveted onto the box form. 'Porsa' tubing is sold with different flanges to the nominal 25x25mm square tubing at rather higher cost. It is also available in a low reflectivity black.

Plating over the box will provide geometric stability and remove the danger of tension unseating the joints. These tubular, furniture building systems are designed to work largely in compression. Something which cannot be remotely guaranteed to occur in the middle of a constantly moving telescope tube. The trusses both push and pull depending entirely on the tube's orientation. Tension must be allowed for in the design of the "box". The stressed skin effect of the plating will add considerably to the central box's resistance to distortion under such push-pull situations. The 'Porsa' corner joints are designed to be hammered into place. Strongly suggesting that separation in use is extremely unlikely without brute force.(i.e. A heavy rubber hammer)

The potential weakness with amateur truss constructions usually lies with their inability to resist compression. Even thin wires can handle considerable tension loads provided they do not stretch beyond their elastic limit. Resisting compression loads requires very stiff truss members. Any play between the truss joints and tube structure must also be avoided. Any tube form can be tested for overall stiffness with a laser collimator. As the tube is moved between horizontal and vertical any flexibility will shift the reflected light cone across or even right off the secondary mirror!

*