10" f/8 N-OTA lot of progress!

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[15]

I tried various horizontal sandwiches using both beams. Despite clamps at both ends and at each end of the saddle the beams bent and vibrated when agitated at the focuser end. Even with a half length 1" plank between them, and the tube rebalanced, they just weren't rigid enough. I fitted the old Declination axle and tightened it in the work stand to ensure it wasn't just the saddle rocking. That wasn't the problem. The beams just lacked stiffness across their narrow dimension. There wasn't enough depth.

So then I laid the two beams on edge with various spacers between them. Instant satisfaction! The "tube" would neither bend sideways nor wobble vertically. A spacer at the mirror end of about 4" (100mm) worked a treat. I clamped the far end to achieve a reasonable length of parallel beams to allow the focuser assembly to slide up an down. It would be rather hard to slide accurately if the two beams came to a sharp point just here. In retrospect though I can fit a projecting rail pointing back towards the primary mirror. The focuser/spider assembly can be clamped to this rail.

At first, I just used scraps of 4"x2" for spacers at the mirror end to see how that looked and worked in practice. Now I need to decide how best to include support for the pot/mirror cell enclosure using the spacers themselves. No point in adding curves on top if the whole thing can be made as one unit. Fabricating them into dual purpose units should be both lighter and prettier too. I also need to allow some clearance for the light path to keep it at least an inch (25mm) above the beams. Really solid saddle attachment also comes into the picture. Flexure must be be avoided without adding extra weight.

Here's a quick mock-up for scale. The  box section spacers are clamped at each end of the MkIV saddle and one at the tail end of the OTA. The beams are still 2m or 6'6" long. This spar structure will lie below the optical path. I shouldn't have used the ladder to prop up the OTA. It only confuses the eye due to their similarity. 

It would be nice to use aluminium for lightness in this part of the construction. I don't have anything remotely large enough in suitable cross sections which aren't massively thick. I have both girder and angle profiles lying about but they must be nearly 1/4" thick! Ideal for the saddle fixings, I suppose. I'll have to weigh the longer lengths and work out how much they will add in much shorter sections. Do I really need to duplicate the entire length of the MkIV's cast saddle when short pieces can be simply bolted at each end?

In case you are wondering why I'm rambling on, like this, it is because I don't want to forget anything. I might make holes in the beams and then bitterly regret it afterwards. A much better idea might suddenly pop into whatever remains of my addled brain.  Writing it all down, as it occurs to me and making simple drawings, helps to avoid ugly mistakes in rather expensive materials. I have the memory of a chip shop sieve.

Well, the heavy channel section  proved to be absolutely ideal! 1.8lbs per foot didn't really matter. It measures 100mm x 50mm x 5mm thick. I'm cutting 60mm lengths which make perfect squares when one is inverted and placed on top of the other. This exactly matches the depth of the beams. So now I have strong fixings to the saddle and very functional, square tube spacers between the beams. Is that serendipitous, or what? Each 60mm box-shaped spacer is costing me another 8oz. So I'll have to allow another 1.5lbs in total. One box at each end of the saddle and one at the bottom end. I suppose I could drill large holes in the sections with a hole saw to reduce their weight slightly.

Here I have laid the mirror cell pot onto the spar to give a sense of scale. The pot is about 11" inside diameter.  The beams are 6'6". When the OTA is completed the pot will sit slightly higher. The poor grass looks awful after months under snow!

The only downside is having to saw the heavy channel stuff by hand with a hacksaw. Despite the freezing weather and roaring wind I'm getting hot and tired! This morning, while I was standing about, thinking desperately, I wore two down filled jackets. One inside the other and I was still cold in the chilling wind. The aluminium also needs a lot of cleaning up with abrasive paper after lying outside for many years.

Still, needless to say, I am delighted with this suddenly rapid design progress. Imagine what it would cost to source 100mm square aluminium tube in Denmark! I think I will drill and then pop rivet the channel sections between the beams to achieve a clean external appearance. I can't do that until I have found the finished OTA's balance point. I'm still thinking about how to slide the OTA along the saddle. I could use long studs through the box spacers into fixings in the MkIV's saddle. Then suitable plates on top of the spacers would clamp the beams down onto the saddle but they would slide with difficulty. It doesn't have to be too easy to adjust. As it probably won't be necessary unless major changes are made to the OTA.

I have some heavy aluminium angle which I think will support the cell pot nicely. I just need to put a curve on one side of the angle to match the pot's curvature.  The other side will still be straight and probably riveted to the beams. Or the rearmost spacer. Given enough length they will provide the strength and stiffness required to spread the mirror loads evenly through the pot's cylindrical structure. So that it all ties into the beams without flexure.

Using the twin beams and spacers is taking their toll on the scales. The all-up weight of the 10" F/8 OTA with mirrors, focuser, pot and spider must be sneaking towards 21lbs by now. Still an incredible difference compared with 24lbs for the bare tube in cardboard!

                      Lbs.
Beams       =    5.6
Spacers     =    1.5
Pot            =    2.0
Mirror       =     8.5
Focuser     =    1.0
Spider       =    0.5?

Total          = 19.10

I still have to add the pot support brackets, mirror collimation board and cooling fan. Perhaps adding another pound or two? I may need to buy that hole saw at this rate! I could quite imagine a row of holes in the beams for a really "techy" appearance. Or (perhaps) not. :-)

I weighed the mirror for the first time on the kitchen scales. 8.5lbs is lighter than the 10lbs I had been working with.

Ever onwards....

Click on any image for an enlargement.

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