10" f/8 More thoughts on my Ultra-lightweight Newt.

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It has been ages since I last added anything on my long focus Newtonian. I can make all sorts of weak excuses for not having used the telescope but I don't even convince myself.

Setting aside the effort in moving to a spot in the garden where the sky is visible there are far deeper problems. My ultra-lightweight design, based on builder's straight edges in the form of twin spars, had weak spots. I now realise that the lower 'pot' containing the mirror in its cell is very poorly attached to the spars. When I move the top end of telescope around, while the OTA is resting on its cell on the floor, I can see the flex in the aluminium 'pot'. It is only attached by one original handle bracket and a thin, curved strip of aluminium. Flexure of the mirror support is unforgivable in anything like a remotely serious instrument design. I considered all sorts of reinforcement options but disliked the likely added weight. Nor the likely crudeness of appearance from making any modifications. I wanted to keep the design as clean as possible.

Had the telescope been designed from scratch as a Dobsonian I could have trapped the pot[s] between the two widely spaced, straight edge profiles. However, the whole idea is to mount the ridiculously long 7' OTA equatorially. High powers on planets or the Moon require the object be followed. Otherwise the long and unwieldy OTA will need to be nudged every few seconds just to keep the object in the tiny field of view. Photography becomes ridiculously complicated without a motor drive to keep the object centered.

An equatorial mounting tilts the OTA in several planes. Requiring stiffness of the OTA in all 3 dimensions. While the Dobsonian just tilts up and down in altitude but is not heavily stressed laterally. Except when being nudged in azimuth of course. The Dob is really a rotatable cannon with smooth bearings. In comparison, the equatorially mounted telescope is an acrobat. Where the OTA is attached asymmetrically only to the declination cradle. Rather than being supported evenly, on either side, in the trunnions of a typical Dobsonian.

A full English mounting in aluminium would make real sense with my very long OTA. Such a design provides support on either side of the tube. Though the entire assembly would be a huge object to move about the garden. Not to mention requiring considerably more work to build from scratch. Nor would the OTA be easily removable from the declination bearings. Making it suitable only for permanent mounting in an observatory. Since I already have the Fullerscopes MkIV equatorial mounting I wish to continue using it. Not least for its relative compactness and strength.

I do not much like the mirror cell in its present form. It still requires stronger coil springs to ensure collimation remains fixed when the OTA is trundled about the garden on its truck wheels. The secondary spider is still a weak point and I may yet go with a four vane design. Curved vanes may make diffraction effects invisible but many users have commented on smearing of the image. The jury still seems to be out on this subject.

First efforts must be concentrated on reinforcing the primary cell connection to the twin, main beams. I need a curved cradle to spread the support as widely as possible into the pot. The present lash-up is certainly lightweight but quite simply inadequate. One or two inverted channel sections of aluminum would work. Though I would need to cut curves in the channel's heavy webs to match the curve of the pot. The pot itself might need to be further stiffened to carry the loads into the curves of the channels.

Unfortunately the channels would add an extra couple of pounds in weight over the present set-up. Not a serious problem given the trolley wheels for transport. But, still very undesirable from a purely ultra-lightweight point of view. It will certainly increase the moment of the OTA. Though thankfully at the shortest leverage for OTA balance. Starting with one channel section at the lowest point of the pot may be all it needs to support the primary mirror cell successfully. This is where the base adds stiffness and the 10" mirror is situated. Further up the pot there are not such heavy loads to worry about. A simple bolt-through clamp to a curved section will do. Just to ensure the pot cannot twist on the spars.

An alternative would be to rout a suitable curve into a length of 2"x 4" to support the lower end of the pot evenly. I can use the router radius bars and fix the timber to the workbench at a suitable distance.

After further thought I decided to cut a piece of softwood batten to the correct curve with the band saw. Then checked the improvement which a curved alloy channel might bring. The problem proved to be the original  cooking pan's handle bracket. It is perfectly adequate in vertical tension while safely supporting any likely contents over the campfire. However the bracket proved to flex too much laterally. As did the wall of the pan through which the handle bracket was riveted.

After experimenting with two curved battens I shall probably make a curve on each web of the heavy alloy channel which I have used elsewhere in the design. Then bolt through that and the existing channel already sandwiched between the spars. Though I do think a pair of solid battens would provide much greater area of support than the webs of the channel profile. It was just that I hoped to avoid wood in the construction.

Lightening of all the channel profiles by drilling remains a distinct possibility. Though it would need to be done carefully to look well rather than end up looking like an afterthought. Large holes made with a metal hole saw would look better than hundreds of much smaller holes and probably save more weight. Drilling the main spars would save very little weight due to the thinness of the box section material.

I spent an hour dismantling the OTA with the shed interior at -2C, 28F. Thanks to the use of hex socket, furniture screws I was able to use a rechargeable drill and hex bit. This allowed me to run the Nyloc nuts along the bridging studding with a spanner/wrench holding the nuts. Removing captive fixing screws proved easy with the help of a magnet. Which captured the screw and was then slid up the outside of the alloy 'pot.'

I may have found some suitable box section alloy in my collection. These could have curves sawn into them to support the primary cell far more solidly to the twin beams. I have no idea why I persevered with my original design. Probably inertia making dismantling seem like a much greater burden than it was in practice. I was quite surprised at the overall weight of the complete secondary cell. Since the OTA already balances very close to the primary mirror I shan't worry about its weight for the moment. Something to consider in future.

Another update. -1C, 30F in the unheated workshop. I have now removed the existing fastenings for the primary mirror cell. Then I pop riveted lengths of asymmetric alloy angle along the lower end of the beams. Just as I did but with the secondary cell but with the asymmetric profile reversed. The greater height of the angle profile for the primary cell allows for the difference in diameter between the two cells. The upper cell being larger in diameter with a clean aperture stop. [See earlier image above.] The considerable thickness of the cell at the aperture stop could be reduced to save some weight.

Fixing longitudinal angle strips seems so obvious now. The solution to the lower cell flexure/fixing problem had been staring me in the face. Provided I tipped my head back far enough to look up at the secondary cell, of course. It was sheer luck that the asymmetric angle fitted perfectly when I offered up my curved, wooden pattern to the main beams.

The appearance is now much improved and the lower [primary] cell can now be simply clamped to the beams to make it easily removable. The next job is to drill the channel section profiles trapped between the twin beams for a suitably large clamping screw and matching wing nut. The ability to dismantle the entire OTA into 3 pieces has obvious advantages for transport. Though storage is simple enough with the OTA resting on the base of its primary cell. The OTA sits pointing straight up at the workshop ceiling. Unheated storage ensures that it takes far less time for the blank mirror to match the outside air temperature during use. The cooling fan will sped up any necessary cooling. As will the open arrangement of the primary mirror cell. More pictures to follow as further progress is made.

Click on any image for an enlargement.

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