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[11]
In search of [ultra] lightness:
My continuing research online suggests that a single tube is lighter and stiffer than multiple tubes. That round tubes are stiffer and less prone to torsional twisting than square tubes of the same dimensions.
Round tube's stiffness increases as the square of the diameter. Which seems obvious when you consider that placing the optics inside the tube offers the greatest stiffness and resistance to twisting about the axis. The only downside to this is the sheer weight involved with any practical, or affordable, tube material available today.
Tube length is a vitally important issue. Not only from the weight point of view but also the very rapid increase in flexure with increased length. Short fat tubes are very stiff. Long, thin poles are anything but!
Aluminium behaves very similarly to steel when used in tube form. Softwood is heavier than metal for the same stiffness in similar but solid sections. Hardwood is much heavier for the same stiffness. Whereas beams are lighter for the same strength in softwood than either aluminium or steel for the same cross section. So make your mirror cell out of pine but your truss (or spar) out of large diameter, thin wall, aluminium tube.
It all rather depends what one is trying to achieve and the dimensions of the required tube. Matching lightness to a long focal length will place the greatest demands on the design of any telescope tube. A tubular monopole offers the greatest chance of lightness but must be suitably sized to match the loads. Otherwise it will sag and vibrate in a breeze or when touched.
An altazimuth (Dobsonian) mounting favours a certain tube layout. Since the tube rises in a simple arc and side loads are quite low. While an equatorial demands stiffness at all possible tube orientations.
The structures attached to the monopole tube want to be very well braced or triangulated to the main spar. Any local flexure between the cell and monopole will be disastrous in terms of the mirror's reflected optical axis. The mirror effectively doubles the angle of deflection. A simple fact which was, and may still be used, to indicate movement or vibration in structures by optical amplification. The tiniest flexure of the mirror support will add up enormously over the 6' distance to the centre of the secondary mirror.
Up at the top of the tube any added weight has to be balanced lower down. So a nominal "ring" is better than a full "cage" where low weight is desired. You can't cheat and ignore the added weight of the lead or barbell weights attached to the back of the mirror cell. The added mass is very unlikely to aid cooling and its moment is still very real!
Lightening the top end of the tube has obvious benefits here. Provided one has adequate shielding from stray light and dewing. There is no free lunch in telescope tube design. I have already touched on the benefits of not allowing body heat or thermal currents to pass through the light path. Nor do we want the tube structure supercooling to the cold, night sky.
For a true ultralight, planetary telescope one ought to avoid a full thickness mirror. Thinner mirrors cool more readily but require more complex support to avoid distortion. (Which can add weight if one is not careful) Since I already own the mirror I have no choice in this matter. So its 8.5lb weight becomes a design factor over which I would seem to have little or no obvious control.
I can speed up cooling with fans rather than leaving it entirely to chance. I can also expose the mirror to allow it to cool more rapidly. I can constructively use its greater weight as far back/low down in the tube as possible. This will help to counterbalance anything added to the top of the tube. Or avoid raising the tube's C of G and thus its pivot point. Which will only add extra height and weight to the base of a Dobsonian and may require a taller ladder to reach the EP at the zenith.
On an equatorial the low mirror position may help to reduce the very long OTA's moment. Making it easier to support well on a marginal mounting. Or offering a greater margin of safety with a consequent reduction in flexure, vibration and backlash.
When I first saw them I was very tempted by builder's straight edges. These 4" x 3/4" [100mm x 18mm] aluminium profiles seemed ideal. Only until I looked at the likely flexure modes across the much narrower depth. Twist might also be a potential handicap. Then I realised that the MkIV's long saddle would reduce the unsupported lengths dramatically. Twist would be reduced to an almost negligible level. The flat, wide, rectangular spar could be readily attached to the MkIV's saddle by clamping.
I am not absolutely certain of the actual weight of these affordable aluminium profiles. I tried lifting a 2 metre example and would put it at roughly 5lbs or about 2 kg. These rather smart looking profiles are available in lengths from 1 metre on upwards. Even up to a staggering 6 metres, or 18', all with useful end caps. They even come in protective polythene bags to keep them looking smart before purchase.
Sliding the focuser/spider assembly along the spar would be relatively easy if non-destructive clamping was used. As would attaching finders and even sliding balance weights to take care of variations in accessory weight at the EP. Monopole minimalism, lightness and incredible cheapness all in one readily available, commercial unit? Surely not all three simultaneously? Usually one benefit must be sacrificed to enjoy any of the others!
Any wood strip reinforcement should be made a fairly tight, push fit to allow for shrinkage over time. Placed as precisely as desired with a suitably long push stick. (With ready access from both ends in case of a mishap) Even end grain Balsa could be employed if one were absolutely desperate for the lightest possible OTA. What about polyurethane builder's foam? It would be a piece of cake to fill the entire profile from end to end. Probably helped along by a suitable length of hose or pipe attached to the aerosol nozzle.
Changes in the pivot point of the spar "tube" could be readily adjusted for. Simply by using a suitable clamp at either end of the MkIV's mounting saddle. As could the primary mirror cell and focuser/spider arrangements. Hardwood clamps with stainless steel coach screws and butterfly nuts seem most useful here. There is no point in drilling and weakening the beam itself.
If it were not for the present snow storm with 50mph gusts and heavy drifting I would now be standing in the builder's merchants. Double checking the weight of a 2 metre straight edge with my digital luggage scales. Before heading for the checkouts with a silly grin on my face. While clutching a 200 Kroner note [about £20] in one sweaty hand and a 2m profile in the other. How could a simple beam possibly be so light compared to my massive cardboard tube? :-)
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