10" f/8 In defence of pots and pans for OTA construction:

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A note to those who sneer at my "pots and pans" construction: My cooking pot cells have a strong, integral base. Which naturally reinforces the tubular section of the body. Making a far stiffer and stronger unit than any rolled tube of remotely similar weight of material. This is still true even when the base has a large circle cut out of it. Most ATMs [amateur telescope makers] have to add plywood or metal rings to reinforce their tubular structures. Or make them thick [and heavy] enough to be stiff in themselves. My "tubular structures" include forged alloy rings as a standard feature of the unitary construction. Which sounds like the sort of empty hype you see in adverts. But is merely a flowery description of the fixed base of the cooking pots. It may be that these "cooking pots and pans" are spun rather than forged. But the base reinforcement argument still holds. 

Meanwhile the spars, or beams, are very lightweight, hollow structures with internal, stiffening ribs. Being duplicated and firmly spaced apart by the large, alloy, channel sections adds enormous lateral and vertical stiffness. The spars easily provide the necessary stiffness required for the widely spaced primary and secondary cells to maintain optical collimation in all pointing attitudes. There is normally a serious risk of flexure with such a long OTA. [10" F:8] A wooden structure would need at least four lengths of substantial timber at the corners of a nominal tube to achieve the same degree of stiffness. Greatly adding to the overall weight. Steel is heavier, rust prone and ugly without priming and regular painting.

The entire construction of my OTA is of aluminium in low cost, familiar structures. These are easily obtained and can be replaced in the event of damage or a desire for change to suit updated components. So, instead of using plywood, with all the accurate cutting work that entails. Or, spending literally hundreds of Dollars/Pounds/Euros for a manufactured OTA and all its components: My design is readily applicable to a range of apertures and focal lengths. 

The telescope builder simply chooses the size of pot or pan to match the clear aperture required. Which is fixed by the optics to give suitable clearance to the components and a clear optical path without vignetting. The alloy beams/spars are easily cut to length with a hacksaw with a fine tooth blade. The bases of the pots can be cut out with a pad saw handle fitted with a fine tooth hacksaw blade. Or an electric stick saw with a fine metal cutting blade. Power tools are no longer as expensive as they once were thanks to [slave wage] Chinese manufacture.

A [possibly] easier but time consuming alternative is to mark the required circle. Then "chain drill" right around inside that circle with lots of small holes. A series of ever-larger drills is then run through the same holes. This will have the effect of narrowing the gaps between the holes until the pot base will eventually drop out. Leaving only a bit of filing necessary to smooth the edges of the large hole. Marking a smaller circle inside the required circle will allow a range of larger drills to be used without breaking through the desired, finished circle. The closer to each other and more accurately the first ring of small holes is drilled the better.

A centre punch [or substituted masonry nail] will ensure accuracy of drilling at the expense of rather more work. Some pans have a useful series of circles on the base from the spinning process. This will aid centring of the large hole for a pair of compasses. Taping two masonry nails together makes a handy and accurate double punch. The point of one nail rests in the first punched hole while the second marks the next with a light tap of a small hammer. A hacksaw blade can be run between the elongated, drilled holes if the cut disk should "hang by a thread" in only a few places.    

In my very simple OTA design the primary and secondary cells can be easily slid along the beams and re-clamped to match any major changes in eyepiece or camera position required at the focus. Meanwhile the entire OTA can be slid up and down, along the equatorial mounting saddle, for balance. This is simply achieved by loosening the two large wing nuts. Then re-tightening them when the desired balance point is achieved. This saves carrying the extra weight of a sliding OTA balance weight. Or having to fit one later after carrying the OTA out to the mounting.

Anything added permanently to the OTA may end up being carried for some distance. The whole point of an ultra-lightweight OTA is to avoid adding unnecessary weight! My entire OTA including the 10" primary mirror, collimation cell, focuser and spider is almost lighter than a bare, 12" diameter, cardboard tube of the same length! At 197cm/77.5" long the entire OTA presently weighs 12.7kg/28lbs. [sans Finder] This seems quite comfortable to lift by the new handle where I saved some weight by choosing a hollow stainless steel example. Rather than the usual, but heavier,  matt chrome plated brass in this common form. I have just checked back to an earlier post and the bare cardboard tube I made actually weighed 22lbs. I thought I remembered it being heavier because it was so difficult to handle! I haven't destroyed the cardboard tube because I may press it into service later.

The twin beams/spars I employed are builders [plasterers and flooring screed layers] straight edges. These nicely finished, rectangular, alloy profiles are readily available in huge range of lengths at remarkably low cost in all the Danish builders merchants I have visited so far. They are further improved from the bare, rectangular profile by rubbery/plastic end stops. Mine were black but red is an option on some makes. The end stops allow them to safely stand on end for vertical OTA storage. It also avoids anything taking up home inside.

I used some scrap 4" x 2" x 2"channel section alloy to join the two beams together. Though a number of alternative spacer options exist. Alloy tubing spacers with internal lengths of studding [screwed rod] could be easily made. Even by somebody who owns nothing more in the way of tools than a junior hacksaw.

I used furniture screws [actually hex-socket nuts] in an attractive "climate" gold finish. These have an internal thread [hence the term nut] which matches suitable electroplated zinc, galvanised or stainless steel studding. The zinc will eventually corrode so stainless steel is probably best for appearance if it is permanently exposed to dampness. Hiding the studding inside alloy pipes will protect them. Adding suitably large washers to the studding will keep them concentric within the chosen tubing. A nut on either side of each washer will ensure the studding remains perpendicular to the studding. All without so much as owning a picture of a lathe.

The length of the spacers obviously sets the distance between the beams. The further apart the beams are fixed [within reason] the stiffer the overall OTA structure. Anyone building an ultra-lightweight Dobsonian could space the spars to go on either side of matching cooking pots. They would have to be bolted directly to the cooking pots since solid spacers would obscure the aperture. Concave curved spacers cot on a rectangular cross section material fitted between spar and pot would ensure alignment and joint strength. The Dob's altitude bearings could then be attached directly to the outside of the spars. Making for a very stiff, but lightweight unit easily carried around.   

The nice thing about pots and pans is their wide availability in a huge range of sizes, finish, weights and appearance. [Not to mention price!] I sourced my primary mirror cell cooking pot from an organised camping outlet. Scouts use these pots for large quantity cooking at their camps. The material is very light, quite thin and has an attractive brushed appearance. The secondary cell pot came from a charity/thrift shop. I searched in a number of such inexpensive outlets until I found exactly what I wanted. It was of slightly heavier material than the primary cell but balanced the OTA perfectly. Had the primary cell been any heavier the OTA would have balanced too low on the twin spars. The addition of a finder is very likely to move the balance point even higher on the OTA. Possibly requiring a slightly taller pier but having no effect on the total height of the mounted telescope. The pier height is chosen for the OTA to clear the ground and no more.

Here is a YouTube video showing the remarkable skills of aluminium, metal  spinners making various utensils:

BTW: Just in case you didn't know: OTA = Optical Tube Assembly. This is the accepted term even when the tube doesn't look anything like a "normal" tube in shape. OTA is useful shorthand and a recognised term amongst astronomers for all forms of telescope. OTA is always understood to NOT include the telescope mounting. One often sees adverts for OTAs where the owner wishes to keep a desirable mounting but wants to sell only the optical assembly. An image alongside the advert will often show both the mounting and the telescope together. Hence the term "OTA only for sale" to save lots of extra, descriptive text. Conversely "Telescope for sale" is expected to mean both the OTA and the mounting are being offered together.

Click on any image for an enlargement.

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