10" f/8 More potty training.

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A visit to a large DIY store produced some heavy gauge stainless steel washers and two 50mm long coach screws.

The coach screws are of two different sizes. The larger is nearest the primary mirror and will provide more compression load than the smaller one further away.

A collection of stainless steel M6x20mm Allen hex-socket screws have replaced the ugly, hex-head screws between the beams.

It is a shame I did not wait until I had the larger washers before cranking down on the furniture screws. The very thin walls of the beams are slightly dimpled where the smaller heads of the furniture screws applied more local pressure.

Update: The scrap heap came to the rescue once more when I found some lengths of rectangular box section aluminium which slipped perfectly inside the hollow beams. Once centralised in each channel, I drilled through the box sections and refastened the screws with the sides no longer bulging inwards.

 Here is a view between the beams, beneath the primary cell, showing the smarter, stainless steel, hex-socket screws in place.

The neater arrangement of  shorter [50mm/ 2"] coach screws and wing nuts now clamp the cell to the beams via the channel sections without the need for spacers.

Still no attempt has been made to clean up the heavy, beam-bridging channels from their years of lying in the scrap heap. I am quite tempted to make some large-ish holes to reduce their weight a little. Thinner, box-section material would have been better but beggars cant be choosers. Large holes would look smarter and more "techy" than the usual, Swiss cheese look of drilling many smaller holes.

The smaller, cross-ways channel is to accept the axle of the simple trolley wheels salvaged from an old sack truck. The wheels make moving the cumbersome OTA around the garden to find clear sky almost effortless.

The secondary cell, seen from below, showing the thick Tufnol plate to spread the wing nut's clamping loads into the beams.

The  last piece of studding will be replaced with socket head screws and furniture nuts.

The four legged spider, with offset vanes at the central hub, has stiffened up the secondary mirror nicely compared with the rather floppy, curved spider I made first. I have yet to shorten the central stub. Though I may make a new, shorter one and keep the original intact.

The cells have still not yet been painted matt black inside. Nor lined with matt black Funky foam as a possible alternative.

I checked the latest weight of the OTA without finders or primary mirror at 9kg or just under 20lbs. I just checked and a 2 meter length of 315mm spiral wound ventilation duct would weigh only ~6kg. This would increase with the addition of the spider, focuser and mirror cell, of course, but it makes you wonder. Is all this effort with alloy beams and channels really worth the effort? I could have finished a lighter truss tube by now. The heavy duty pot I used for the secondary cell is adding unnecessary weight.


General view of the newly smartened-up OTA relaxing against the hedge. At over 6'6" or 2metres tall the OTA is unusually long for a humble 10" Newtonian. Just the price of using an F:8 mirror to maximise the image quality potential on the Moon and planets. A smaller secondary mirror is possible when using a long focus, primary mirror.

The disadvantage of the offset, alloy cells, relative to beams, is to make the assembly feel very lop-sided when it is carried about. It is fine once fitted to the trolley wheels, but any attempt to lift the OTA by the beams produces a very strong torque effect. Which makes the cells want to drop to their lowest position beneath the beams. This pendulum effect is greatly exaggerated when the primary is in place in its simple cell.

The handle on the top of the cell makes carrying the OTA very much easier, since it supports the offset weight of the primary mirror directly. The secondary cell is much lighter and needs no support when I am carrying the OTA via the handle and middle of the beams.

The twin finders have been removed while I work on the OTA. Telescope "tube" hardly qualifies for this rather unusual, optical support system.

Talking of which: I flatted the domed heads of the collimation  [coach] screws in the lathe. The domes were so high when a self-adhesive [protective] pad was fixed that they were far too close to the height of the true support pads. There was a real risk that the mirror could find itself supported out at the rim. I keep looking at Plop [software] and trying to decide if I should provide 6 or 9 point support. Though at 40mm the mirror is probably thick and stiff enough to be well supported on 3 points at 0.4x the primary's radius.

Click on any image for an enlargement.

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