furrysocks

Might use nappy liner tissue instead of buying mat as I've got bulk rolls of that. Only 30gsm itself, but much stronger than the surface tissue. I figure another layer of *anything* is going to help - given I can't rely on external rings to maintain the round. The tubes are strong as they are - I just don't think they'll take a knock happily and want to add another layer to be safe.

I've been chatting with astrobeast1 (Dan) over on CN and I've come to a decision whether or not to ditch the external rings completely. The surface tissue I used with the resin to cover the tubes is very lightweight at 30gsm. The ribs/baffles I've fitted on the inside do not quite butt-join together and I haven't bothered to fill the gaps between them yet, either. Both of these things represent weak spots in the current tubes and risk the tubes collapsing or splitting in some way. The external rings were there to serve two purposes: - provide additional rigidity to the tubes (keep them round) - to satisfy the geometrical requirements of flat-end truss attachment to a dodecagon. A layer of 300gsm chopped strand mat and additional resin around the outside will provide more rigidity - I should have used this the first time round. I will attend to the ribs where they butt against one another (or not quite, as it happens), probably with epoxy putty. Together, these two things will (I have convinced myself) hold the tubes in the round under any stresses to which they are likely to be subjected. By dispensing with the rings then, the trusses attached to the top and bottom tubes will mate directly onto the surface of 8 out of the 12 sides, with pairs being attached just either side of a 'vertex' - so two each at top, bottom, left and right. Additional 'meat' will need to be fixed internally to accept the threaded inserts that will take the truss bolts. On the bottom tubes, this may need to be significant, given the weight of the primary which will be about 6 inches away. At the bottom end of middle tubes, I will double up MDF triangles (so 36mm thick) to make up the required geometry of the end-profile and attach these to 4 of the 12 faces - top-left, top-right, bottom-left and bottom-right. These will be screwed from the inside with a strip of mat over the top to 'tie' them to the adjacent faces. The trusses then attach to these, either side of the top 'point'. I want to have a full ring terminating the trusses from the top tube, such that I can bolt the whole lot to the top end of the middle tube - the scope will then be able to be split into two sections. I still need to give a little thought to the placement of those bolts, the attachment supports on the middle tube and the precise hardware I will use. A picture is probably in order... Anyway - getting rid of the rings will eliminate over 1kg of weight, to be replaced with not far short of that again with the mat/resin and triangle blocks. However, the overall appearance of the scopes afterwards should be much improved. I think design-wise, it's probably the optimal solution in terms of a hybrid of the dodecagon tube and Serrurier truss designs. Aesthetically, it trumps the set of external rings by a large margin. I'm confident it will hold together, too - well, I have to be. A niggle I still have is whether the choice of material for the trusses (1/2"x1/2"x1/16" aluminium channel) is going to be sufficient. Royce's 8" open truss newt uses this but my tubes are likely to be about half as heavy again as his. In the meantime, I need to get the hacksaw out to remove the external ring I've already attached and also see about fixing the belt sander. This is what I get for not thinking it all the way through from the start.

They're all black inside, now. 20 year old matt blackboard paint is much less reflective than the "matt" spray. Still quite light, and I'm sure not as "black" and as "matt" as good quality flock but it'll get me started. Some chunks left in the bottom of the can, so I'll see what I can do with some thinners and a fork, or something...

Fair play - look forward to reading your posts. :)

Last night, I finished the resin/tissue coat on the sixth and final tube. Nearly had a disaster with it as it started to come apart and I only noticed 30 minutes into the cure. I got it *just* in the nick of time and patched it back together. They are not as structurally sound as they will be with the external rings to fully hold the form. They're going to take some sanding, though... I got some cheap "matt" black spray paint from the £1 shop and sprayed the inside of one tube but it's very reflective. I'll try the very old matt blackboard paint I have and see if that's better. The red is £1-shop tile/paving paint. It spreads on lovely with a kitchen sponge (pack of £20 from the £1-shop). The external ring took a bit of hand-rasping to fit and I skewed a couple of pins out the back but after flooding the slight gaps and letting it cure, it seems sound. The ones on either end of the middle tube are going to have to be as solid as possible. I may use a few pins to tack it in place and then pop in some 1/2" or 5/8" screws from the inside. Then flood with resin. I'll go round the inside mating surface at least 2 or 3 times with resin and let it soak into the MDF end-grain. Need to get a wriggle on with designing and building the mirror cell, secondary mount and focuser so I can get these tubes assembled, weighed in order to start cutting and fitting trusses. Then it will start looking more like a telescope. With the added weight of the paint and extra resin that the MDF soaks up, I'm upping the estimate to 12kg for each OTA.

One more tube done outside, two more inside - both split a little. One external ring attached. Carry on, regardless... :S

Second tube tissued. Feels solid, fewer air bubbles but I simply can't get the tissue folded right inside over the end baffles. I pull the brush away and it pulls fibres with it, which peels the tissue off. I found if I can get it flat against the end surface then trim to the internal diameter when cured, it's good enough. Got one MDF external ring (250g) marked, cut and matched with a rasp to the first bottom tube. I need to find my straps to secure this tight while I air-pin it in place from the inside with 15mm finishing nails. I may fillet the back-side with caulk where it meets the tube before soaking the joint in resin. I will fill and sand the faces once cured. I'm currently estimating 11kg + mount for the finished scope.

Coating complete, paid for. In the post next week.

I haven't done anything over the last week, except clear out a tall metal cabinet to serve as a curing cupboard. With a fan heater, I can get the air temperature up to 30 degrees with plenty of ventilation. So as soon as I've done some other jobs around the house, I'll get back out there and apply resin and tissue to some more of the tubes. Aluminium channel has arrived - I deliberately over-ordered but won't start cutting until I've got the external rings, mirrors and focuser mounted and can weight the various parts and determine the balance point for each. Then using Robert Royce's spreadsheet, I can work out the strut lengths required. Fitting the external rings will be a case of starting with a template-cut ring, marking for intended position and then hand-fitting each ring to each tube and pin/resin them in place. Then drill and fit the threaded inserts and start to fit the struts. The resin/tissue on the first tube is not the smoothest. It would take a lot of sanding to improve this. One concern is that due to the way that I clamped the tubes together, the sections are, if anything, slightly concave. Over sanding would risk removing thickness from the edge joints where strength is needed most. Once painted up, it's not going to be the prettiest of finishes. But scopes are typically used in the dark, correct?

Easy to snip/trip the cured tissue, in fact it's very brittle and tears easily. Bubbles suck. All in all, not great. The external rings will serve one purpose well, namely to hold the tubes in the round at the ends but I'm concerned about the other forces to which the tubes will be subjected. I'm on the fence as to whether or not I'm wasting my time. I've over-ordered the aluminium channel so I'll have options - I may roll hardboard tubes around a form, instead. In the meantime I'd love to buy a pair of 15x70 bins, but given the spend on these scopes, I can't justify it to myself - let alone to the other half. :(

Wrapped one of the short tubes - messy job. Trying to fold the snipped overlap at the ends round in behind each baffle is very tricky - the fibres that come away from the tissue cause it to pull away when you take your hand away. Will try a brush rather than a sponge - should be a bit better. Will leave this one as it - a ragged inside edge and a couple of air bubbles. I want to see what it's like to work with once cured - sanding, snipping, rewetting, etc...

With the heater off overnight, the first tube has cured to a non-tacky surface. I can see along most of the edge joints that the resin has penetrated by a mm or so and darkened the hardboard. In addition to wrapping the tissue around the circumference, I will cut and fold around the end baffles and daub it in place. With the tissue and after sleeping on it, I'm quietly confident... I've screwed a length of twin-slot shelving support to the door frame with a few brackets on which to rest the tubes as the resin cures. I will seal off the door and frame up an enclosure around which to wrap a tarp. I'll add a small shelf at the bottom in order to pre-warm the resin before adding the hardener. I'm currently using kitchen sponge scourers to wipe on the resin - I don't have any styrene thinners and so I'm not going to waste brushes. In hindsight, I should have cut a few extra baffles and made up the 5-inch test tube, to trial the various stages - it's the first time I've used resin, for example. With any luck, it's all systems go. I'm away camping over the weekend so I'd like to get the tubes done tonight and give them at least an hour or so each in the "warm room" :D

I'm becoming less confident in this build... The £1 high strength adhesive is rubbish. I've got 6 lovely looking tubes, but they are fragile. Perhaps the temperature plays a part. I have started to apply polyester resin to one, and it is curing with a heater, but I don't believe the joints will be any stronger for the application which is the crux of the assembly. There will be external rings on all tubes, save perhaps the top end of the top tube, simply for aesthetics. In this way, they will hold their form at the ends at least. With the remaining bare 5 tubes, I plan to apply the tissue to the outside and stipple coat with resin without a base coat of resin, my first weight saving move. The first tube will therefore be >100g heavier. The tissue/resin is going onto the smooth side of hardboard. It should provide a protective shell but tissue (not mat) is not meant to be structural and I read that polyester resin tends to peel from wood. I don't know what strength of tube will result from this approach The weight will be hanging off the external rings, so the bond between ring and tube must be sound. I plan to pin from the inside with twelve 15mm finishing nails as I assemble them, having saturated the mating face of the ring with resin, and then flood the joint with more resin where necessary. I will clear a corner of the garage where I can pin up a tarp to enclose a space for curing, into which I can feed warm air from a fan heater and monitor the temperature within. I don't mind rebuilding the tubes - I just don't want a primary mirror to drop on the ground when I finally get to observe through one (or both) of these scopes. If the tubes feel okay with the tissue and rings, I'll be sticking a couple of bags of sugar in first and see what happens. ;) Flying by the seat of my pants... thankfully, the learning curve is something I enjoy.

Ordered new 3/8" wood bandsaw blade, plus a 3/8" 24tpi for cutting the aluminium channel.

Ordered twelve 2.5m lengths of 1/2x1/2x1/16 aluminium channel. Should only need eight for the two scopes.

Marked and cut the template for the external 'rings'. With a 1/2" bandsaw blade instead of the 3/8", it was tricky to cut the internal polygon, but it fits 'ok'. I will round a couple of the corners. Photo to follow. Got two more tubes glued and clamped and sanded the sharp edges off the first. Resin arrives tomorrow.

See "Scope build" album for photo(s).

Left the tube in the cold for 24 hours - not a perfect bond but hopefully the polyester resin will sort that - I may dilute the first coat with acetone to help it flow and reclamp with wax-paper shims. Nearly cut all the remaining baffles/ribs but the bandsaw started 'ticking' so I stopped it and rotated the wheel by hand a couple of times - blade snapped on me. I've got a photo of parts and materials for this stage that I'll post shortly. Will likely not do any more until I can try resin on the first tube.

Tonight, I crosscut the hardboard to length and assembled the first tube section with adhesive. I will need to cut around 30 more baffles/ribs and mark and cut 10 outer rings. The first upper section, I've put together without yet the outer ring. I laid out two lengths of 2" masking tape sticky-side up and carefully put down 12 of the 12" strips. After turning them over, I fully taped each joint and then turned them back. I applied adhesive in to all the joints and down both sides. With a mate and 30-odd small spring clamps, we formed it round the 1/3 ribs bit by bit, offsetting the ribs one side to the other by 60 degrees. I pressed bamboo BBQ skewers into the excess adhesive that squeezed out of the edge joints inside. I may add another rib internally when I come to mount the spider - position tbd. I will have to notch the corners to provide clearance for the skewers. Next time, I would avoid a 45 bevel on one edge in favour of symmetric bevels. This would give a cleaner line on the outside. I'll resin and sand these tubes to smooth the edges instead. I hope to assemble the rest of the tubes over the weekend, ready for resin sometime next week. The mirrors have a long focal length (>1.6m). The upper tube sections will have around 8 inches in front of the focuser, making them appear even a little longer. The additional weight up front (making no attempts to reduce weight, at all) will serve to move the center of mass up the whole assembly. This will ensure that the lower struts are not overly short and it should look more balanced for it. This design is a hybrid of polygonal tube and Serrurier truss - which means a lot of work. I'm considering something along the lines of the mount linked below, for a single scope at least. The binoscope is another project, altogether. http://www.cloudynig...5/o/all/fpart/1 I am very excited! :D

Ordered the resin. Need to do more maths before I decide on what size of tubes to build and where the hardware will mount.

£33.50 for 5kg of resin and 300g of catalyst, delivered. From fibreglassdirect.co.uk - not ordered yet...

I've started to put together some numbers. With a foot long top tube and a foot long bottom tube (weighing in at roughly 2.2kg and 5kg, respectively) and 1.5kg of aluminium struts, the center of mass of the 5'8" OTA is approximately 2 feet up from the back end. I guess this is "normal" as I know folk weight the back ends of ultralight dobs I did some rough calculations myself first and checked this using Robert Royce's spreadsheet for the 3-section open newt. I came within 1.5 inches of the value given for a 0-length center tube. This is all just on paper at the moment. Looking at Toshimi Taki's 8' newt, he's about that from the back. Though materials/etc are different, he weights in at 7.8kg for f/6, I'm around 8.7kg for f/7.6. The Explorer 200P f/5 is 10kg/8kg with rings and dovetail on/off, the Skyliner 200P f/6 is about 11kg. This makes my 8.7kg seem reasonable - it passes the sanity check, anyway. If I'm sticking with a single truss design, I may therefore go for a 2'6" bottom tube and a 1' top tube, something a little more like the proportions of the Skywatcher open truss, or the Meade Lightbridge. However, this raises two points in my mind. Firstly, the key benefit of the Serrurier design is that, if I have understood correctly, any sag experienced front and rear will not greatly affect the collimation, as it can with a single truss design. Secondly, if I'm going to be hanging several kilos off my tube, I'd best be confident it's all going to hold together. I will continue to consider whether I will choose a single or double truss design - I can still bulk buy the aluminium broom handles at £1.40-odd each. But as I'm planning to construct the tubes with hardboard and laminate stuck together with a £1 tube of "high strength" adhesive, I could do a lot worse than to add outer rings and coat the assembled parts in polyester resin before loading them.

Washing line props? About £3 each and telescopic, ie two sections. Thicker from center tube to primary tube, thinner from center tube to secondary tube?