A telescope on a raised platform?

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The first of a series of essays on telescope design. Not remotely meant as lectures. More of my own "thinking aloud" about a forthcoming project.  I am indebted to Clive for sharing his knowledge and patiently acting as a sounding board for my own ideas.

They say that desperation is the mother of invention. My neighbour's security lights are a constant pain in my usual observing position. The trees surrounding the garden and nearby are all bad news for a clear view to east, south or west. The house is on the southern border of my garden. Further blocking the most interesting part of the sky. And, the damned lights!

So I decided on a raised platform shielded from the lights by the house. Possibly with a simple DIY observatory to keep me more comfortable in the harsh and constantly windy Danish winters. Snow is often lying for months and it is not a pleasant surface for extended standing about.

So I propped the ladder against the shed and went up to admire the view. With my feet only eight feet off the ground and level with the shed eaves, I had a panoramic view! Impressively so. Now I had to make the idea work in practice. No expensive white elephants for me. The Head Gardener would not approve either.

My massive welded steel pier is as solid as a rock standing on the ground. 8-10' up in the air on a wooden platform? Not very likely at all.  So I needed a tall new pier rising from the ground and sticking up another 7' above the platform somehow. Seven feet plus eight feet to the platform floor is over fifteen feet tall! But how to do it? No cast concrete piers on cubic metres of foundation block for me. It would take enormous resources (like a JCB) and completely wreck the drive, parking area and garden! We live on soft clay, with a high water table and have annual permafrost for literally months on end. I'd need a pump to keep the water low enough in the newly dug, bijou swimming pool while I dug furiously for weeks on end!

I needed something on much more of my puny, human scale. Something I could erect working entirely alone. (as usual) Timber is good. Timber is very manageable in suitable units. But you can't just prop a 15'+ wooden  pier up against  a hole in a raised platform. Isolation between the two is absolutely essential. Otherwise every movement by the observer and the slightest  breeze will lead to vibration up through the pier, the mounting and on into the eyepiece.

Tapered concrete post anchors are readily available at a remarkably low price at Jem&Fix (a discount Danish builder's merchant and DIY store) These are roughly 2' high x a foot square at the base. See the image alongside of a stack of the things lying on their sides in their outdoor department.

Once buried, and the hole properly backfilled with lots of ramming, it would take an awful lot to lift these lumps of concrete out again. These things reminded me of tent pegs on elephant-sized doses of steroids. Even skinny tent pegs can take enormous (steady) loads in a decent lawn. Imagine what pulls (or pushes) one of these huge, concrete anchors can resist. Now I had a plan!

I'd use these adjustable concrete anchors to support my platform legs. Not only holding the legs perfectly still but avoiding rot from exposure to the soil. Then I'd make a wooden pier 15' high. This would be anchored to the ground. Then I'd "guy" the diagonal timber struts to other concrete anchors just below and independent of the platform. The holes for the concrete anchors would be easily manageable. I have a long, narrow, post hole spade! While the adjustable ironwork straps on the anchors would solve any inaccuracies in height and position. Crackers? Quite probably! :-)

The stay anchors could even be leaned inwards to face the load. The square base would resist the pressure loads and the adjustable screwed rods and brackets used to apply even more pressure.

The relatively small area required by these concrete anchor blocks is far more friendly to the garden than a single, vast, cast concrete slab and a towering concrete pier. The anchors are far more user-friendly. What about those who end up inheriting the huge slab and pier if we should ever move away? The anchors can quickly disappear if the platform is no longer desired as a picnic spot with a view.

First I thought about using an old telegraph pole for the pier. Or perhaps buy a disused lattice radio mast. Or even buy a massive steel tube in a scrap yard. However, all of these would be well beyond my powers to lift them into place without the help of  a crane. Though there are boat winches, pulleys and timber tripods... Nah! Building the tripod would be as much work as lifting the pier itself.

Then I thought of using four, 4" square, pressure treated, vertical  posts. Each length of timber would be relatively easy to handle by one person. The four posts could be separated with a very strong spacers just below floor level. Right where the diagonal bracing timbers are fixed for maximum resistance. Then the top and bottom of of the pier could be pulled in with a metal strap to achieve a fair degree of triangulation. Festival of Britain 1951 'Skylon' style. (see image to the right) That should take out any resonant modes. Or any pier flexibility above the platform floor level!

If the raw, pier timbers above the platform offend my eye they can be simply clad in smart plywood. For even greater stiffness in a stressed skin, tapered box section. Nothing wrong with a bit of bling. Provided it has a useful purpose. Supporting a reflector would be much easier, of course. Requiring only a stubby extension above the platform. Instead of a 7' tall one!

The platform could also support an observatory of some kind. This would be built in place rather than craned in as a unit. A crane would be just as damaging as an excavating machine. And probably twice as expensive! There is no gang of willing astronomical society helpers here. Nor even the funds to hire a JCB and driver.

A hemispherical dome is a difficult chore of many, differently shaped gores. With (usually) far too much weight concentrated in the reinforcing rib. All just to support the cheapest possible roofing materials. So what about a semi-cylindrical, rotating 'dome?' No need to try and force a 3D curve onto flat materials. No buckled and ugly aluminium sheeting or soggy, heavy and disintegrating hardboard. No need for fancy geodesics either.

Suitably thin plywood can be easily bent to follow the gentle curves of a self-supporting half cylinder. The slit reinforcing arcs could form the central supports. Similar, outboard arcs can support the outer edges before the semicircular sides are added first.

If 4 mm waterproof plywood was chosen for the first curved surface then a second layer could be glued on top of the first. To form an incredibly strong curved laminate of even more layers. Though gluing such a large area might be better achieved on the ground. Or thicker plywood used in the first place. The gentle radius of the curve is hardly a difficult proposition up to say 12mm (½"). Vertical ribs could be added to the half circle "sides"of the turret to help improve stiffness in strong winds. Side cooling vents, or even an access door are a possibility.

 An up and over slide (shutter) can cover the viewing slit. Just like a dome but much more easily arranged. Or a shutter could slide horizontally on stainless steel or aluminium bars. Even allowing some variation in opening width in windy weather. Or to block stray light. The shutter will follow the line of the cylinder's curve without effort or wind-catching projections.

The half cylinder still needs a support ring and rollers to rotate on. Much like a round dome needs them to rotate on itself. This can all be kept safely within the footprint of the semi-cylinder easily enough. The supporting walls can be cylindrical or any variation typical of DIY domes. The semi-cylinder "turret" need not catch the wind much more than a true, hemispherical dome. A coat of white paint and the usual thermal problems are avoided. As well as improving the longevity of the plywood. A semi-cylinder also has plenty of storage space and headroom compared to a hemispherical dome of the same nominal diameter.

Click on any image for an enlargement.

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