10" f/8 Beam me up, potty!

*
[14]

Monopole practicalities: The stiffness of a beam (or plate) rises as a cube of the depth. Point my single beam telescope at the North Pole and it lies as flat as it ever does. Albeit tipped up at 55 degrees to the Pole. Which helps to avoid sag. Now swing the monopole telescope to point south. The beam is now on edge at maximum stiffness. The mirror will now be cantilevered to its maximum. Hanging almost horizontally off the beam to one side. This will try to put a twist into the beam just below the saddle. Which can be resisted by local stiffening carried well into the saddle area. This will also reduce or remove any local flexure. Any pressure applied to the focuser will be trying to flex the beam across its thinnest section.

The most obvious way to increase beam stiffness is to add another beam on top of the other. Preferably bonded together to avoid the "cart spring" effect common to bolted strip and clamped constructions. The beam would now become 1.5" deep and weigh at least 5.6lbs. Still only 1/4 the weight of the cardboard tube but the total OTA weight is rising.

Logically I ought to be looking at a true (double) Serrurier truss at this focal length. Or, using a beam either side of an ultralight Dobsonian. The alternative is to avoid any sag with lightweight trusses added to the single beam. Any remaining flexure between the mirror cell and MkIV saddle can be removed with triangulating struts.Or a single (?) strut run from one end of the OTA to the other. Running in parallel with the main beam. There is also the option of running tensioned cables between each end of the OTA.

My original plan was to use a large, deep saucepan as a mirror cell container/backplate. I want to adhere to the nominal 1" of clearance between the mirror and the internal diameter of the tube.Online searches have pointed me towards a large aluminium stockpot. The sort of thing used by large groups (like the Scouts) at picnics and camp sites.

Large stockpot with curved handle painted out in PhotoFiltre. 

I am now homing in on an easily obtained,  12.5 litres aluminium pot of about 30cm/12" diameter x 24cm (9") high.. Once the curved handle is removed this leaves a clean, fairly lightweight and reasonably attractive cylinder. With deep, parallel sides and a stiff, flat base attached. The simple, riveted straps, which once held the handle, can be used as  a base for a carrying handle for the telescope. Or used as support bases for simple struts. These can be connected to the mounting saddle clamping blocks. Drilling of the beam's narrow edges must be avoided at all costs. Its depth is just not enough to avoid weakening it unduly. (Quite possibly catastrophically!)

The large pot will sit on curved supports resting on a large, sliding block clamped to the alloy beam. The cylinder will help to distribute the loads from the mirror over a large area of the supporting block. The lid may even become useful protection for the mirror. A couple of up-and-over, locking catches might be a good idea. A neat round hole will be cut in the middle of the pot base for a small computer cooling fan.

I believe the pot material is 1.8mm. (1/14")  I have yet to discover whether the pot base is any heavier. A 3-point, plywood collimating "cell" will support the full thickness mirror at the bottom of the pot. Perhaps using the pot base for support if it proves strong enough. Though I might need a disk of plywood for reinforcement if the base is too thin and "floppy". It would be foolish to have the mirror change collimation simply due to cell sag!

I have yet to see the stock pot in the flesh. (so to speak) I'd like to measure the exact weight  (without the handle) and to check the exact measurements before purchase. Though I consider that none of this is exactly critical. I have seen figures of 1.3 and 1.6kg mentioned online. (Roughly 3lbs)  I'm hoping the large and thick metal handle is as heavy as it looks. Which will reduce the weight nicely with its removal. There may be some variation between suppliers offering seemingly identical pots. Any alternative reinforcement to support the mirror cell (without using a pot) will be likely to add as much weight than the aluminium pot by itself. It probably won't provide the mirror protection of a pot. I shall have to go into town (some 20 miles away) to physically eyeball the object at an outdoor sports shop.

It might prove necessary to line the pot with thin, or thick, closed cell black foam insulation. Or even some of the cardboard tube leftovers. I shall decide on what is necessary when I have actually tried out the telescope under the night sky. I have read a number of reports online that bare aluminium does not super-cool by radiation to the night sky. It is only when coated with a typically infra red radiating paint that the thermal problems arise.

Some users of naked aluminium tubes report no tube currents (at all) despite rapid and large temperature drops during evening observation. This is very encouraging, if true. Kriege, in his Dobsonian book, claims that open tubes avoid closed tube thermal problems. As does Royce. A black rip-stop nylon shroud helps. Both to block stray light and to keep the observer's warmth at bay. This assumes that a tubular support is available at each end of the OTA. Otherwise there is nothing to which to attach the shroud.

There is an interesting potential connection between a warm mirror and a cool but lightweight, ventilated metal cell of large surface area. Will the mirror cool  more rapidly by radiating to the cooler aluminium than if the cell is insulated or thermally neutral? Perhaps it is not important when a forced draught is provided? Does it mean the fan can be switched off once the mirror has cooled to the ambient air temperature?  A fully exposed mirror can radiate to its cooler surroundings. Why not radiate to the large, cool aluminium surface instead? Only direct experience will tell whether any thermal differential will lead to warm air currents.


Clive has kindly sent me some fascinating details and images of some open telescope structures. Horace Dall had an open tube construction on very unusual triangulation principles for his Dall-Kirkham. This was on a large fork mount. I don't think the design would be possible on a German mounting without separating the lower struts on either side of the saddle.

Henry Hatfield (of Photographic Lunar Atlas fame) had a unique wooden beam 12" telescope. More recently the Italian Lazarrotti "Gladius" uses two or four, thick carbon fibre rods in multiple clamps. Again, for a Cassegrain optical system. You can see the basic resemblance to my 10" F/8. Except that the Italian design is very much shorter than mine. I seriously doubt that even four CF rods in a compact bundle would cope with a structure 2 metres long.

Of the three, I think I like Henry Hatfield's box beam simplicity where form strictly follows function. It is deepest at the saddle and tapers towards the secondary support. Potentially simple to copy even using alloy beams. It would need some side plating to tie it all together for an updated and hopefully more lightweight example. I will keep it firmly in mind for a backup design if my single beam prototype fails to perform.

A partial image of the wooden telescope appears on the linked Hatfield Obituary below: I shall continue my search for a better picture showing more detail of the entire instrument.

britastro.org/journal/pdf/121-1hatfield.pdf

PS: The first online dealer messed me about and had no stock of the cooking pots. It would be a fortnight from ordering before they could supply! I call that fraud if they don't warn buyers on their website! Luckily I found a real outdoor shop nearby which happened to have some stock. What I paid extra was saved in petrol not having to go to town.

The pot turned out to be more attractive than I had anticipated and nicely light at a smidgin over 2lbs with the handle still fitted. It was slightly smaller than anticipated at 11" inside diameter x 8" deep. The dimensions I had seen online were obviously the maximum external measurements. Still large enough to provide clearance for the primary mirror and the forced draught from the fan.

The only downside is that the aluminium is very easily marked. BTW: I now have two alloy beams. Having bought another in case the first proves too flexible. I had to wait for the pot before I could make the mirror cell, beam clamps and pot supporting brackets. Without knowing the true external radius I would have been completely in the dark. For my first attempt I shall saw the clamps out of 3/4" plywood. Possibly laminated to produce 1.5" for greater strength. I just need to have a working telescope to discover the balance point. It can always be made much smarter later on. The cardboard tube still awaits if it should prove worthwhile on the MkIV mounting.

PPS. I made a start on the OTA. Having removed the saddle from the MkIV I placed a 15lb weight on one end of the beam. The beam balanced with the "mirror" substitute almost on the saddle. With a very long flexible neck as the beam stretched off into the distance. Adding weights to simulate the focuser, secondary mirror and spider had surprisingly little effect on the balance point.

I shall now have to stiffen the beam considerably. Initial plans are to double up the entire beam. Parallel and separated at the mirror end and saddle and only joining together again at the secondary. This will probably require side plates and pop rivets. Though I am also playing with ideas for clamping. Which would allow the tube and its various components to be slid up and down. The mirror cell needs no mobility but the OTA does if additional weights are to be avoided.

I cut out some Tufnol plate to make clamps and regretted the nasty stink and dust it caused in the shed. Tufnol is much stiffer and stronger than wood. Allowing a smaller OTA overhang beyond the saddle. A large overhang seems almost normal on modern mountings. Inevitably leading to flexure and massively increased moment. The OTA has to be balanced by counterweight. Braking and accelerating all this mass will quickly overload the mounting. Particularly with long OTAs.

It may well be the reason long OTAs have virtually died out in the race to compact, coma overdosing designs. Not to mention the compound Schmidt Cassegrains. If they were as long as Many Newtonians nobody would ever bother with them. So the continuing popularity despite all their weaknesses are the result of poor and ridiculously expensive mountings.

Click on any image for an enlargement.

*