furrysocks

Now you can... ;)

http://www.smtools.co.uk/shop-virtual-tour.html

On closer inspection (!) one of the primaries is not full thickness. They are 40mm and 24mmm respectively. I found this out when I removed one from the cell it came in, in order to wash it. It has the better surface of the two and is the one I am using in the first OTA.

Oh, and I need to decide what I'm going to use for a finder and account for its position and weight when I'm calculating strut lengths, as it will affect the weight and balance point of the top tube.

Cut and sanded the excess back on this center section. Ready to mark and drill bolt holes and fit threaded inserts. Will trace round it to cut a ring for to which I can attach the top trusses.

Touched up the matt black inside the bottom tube. Ready to fit the threaded inserts and paint the outside. Then fit primary mirror, weight, measure and find the balance point.

After that, I'll work on the top tube - fit internal support blocks for the truss threaded inserts, fit focuser and secondary mirror, then paint, weight, measure and find the balance point.

After that, I'll be ready to calculate and cut the trusses! :D

Short of drilling the inner ventilation hole(s), sanding, sealing and painting, and marking/drilling the glue dimples, I now have a Stellafane mirror cell top part.

http://stellafane.org/tm/dob/ota/cell2.html

I still need to determine where the glue dimples should go. Stellafane recommends 71% of the diameter. I've heard from someone else it should be 40%. About 2/3 of the way down the page here (http://www.atmsite.org/contrib/Holm/Plop_optimized_cells/), it quotes deformation values for both (based on an 8 inch, 1.1inch thick f/8 pyrex primary). It appears that 40% is correct (Thanks, Chris!)

The mirror cell bottom part is next. This is where it gets ugly.

I have 12-segment tubes (hardboard + fibreglass/resin). The strength of these is unknown, and the weakest point is no doubt the longitudinal butt joints between the segments. The Stellafane mirror cell bottom part has three points, which sit inside the tube and would be screwed from the outside. I've got a rib right at the bottom so I can't put anything inside the tube - it's going to have to go on the back. I've got mending plates (75mm long) that I intend to bend into an L-shape roughly 1:3. The short part will support the bottom of the cell and the long part will attach an inch or so up the side wall of the tube.

To hang the mirror from three tube segments will put shear force on the butt join - I know I'm not going to pull the thing apart with a few kilos hanging off it like this but it's making me think. Would a 4 or 6 point bottom part be better, to distribute the load across a greater number of tube segments? I start getting into difficulties balancing the need to have some meat for placing the collimation and lock bolts and trying to remove material for ventilation and wight reduction. I think I'll stick with three, and where I screw/bolt into the side wall, add internal bracing in the form of 3 small internal blocks, each secured to the respective adjacent tube segments.

I'm also half considering modifying the bottom part. Rather than using a single 3/4 thickness of ply, using two 3/8th thicknesses of slightly different sizes. The internal diameter (round) of the ribs at the bottom is about 1/2-3/4 of an inch less than the outer (12-segment) diameter of the tube itself. By having one thickness of ply cut to fit that internal diameter and the second thickness extending all the way out to the side to accept the bracket, I'll move the weight of the mirror 3/8" further up the tube and I think it would improve the aesthetics slightly as well. I want the weight forward as it's a three tube double truss construction and as the middle tube goes at the center of mass, I don't want the bottom tubes to be too close to each other. 3/8" isn't going to make much difference, but I've got more 3/8" ply than 3/4". ;)

Just been down to Screwfix and now have a bunch of 60mm M6 bolts, wing nuts, and mending plates. I'm essentially making the Stellafane primary mirror mount, though rather than fitting inside the tube and screwed in from the sides, this will be mounted on the back with L-shaped brackets secured a couple of inches up the sides. Will probably be a 4-point mount, with three sprung collimation bolts and lock bolts.

Might have been a badly measured or mixed batch of resin, bit the tissue is not good. I'll get some mat, probably 225gsm, perhaps lighter.

Secondaries received.

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.

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. :)

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.

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

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.

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.

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...

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?

Mocked up a 4 inch section last night using one baffle and three sections of hardboard, taped on the outside with adhesive on the inside. Quotes 24-48 cure time on the tube of adhesive and I left it on the radiator for a 2 hour blast of heat this morning. It held together well, but the waffle side of the hardboard delaminated when it underwent destructive testing.

I reckon that with a sufficient number of internal baffles/support (perhaps 6 or 8) the adhesive will hold the tube together well. I will, however, secure the hardboard to the supports from the outside using 15mm finishing nails.

Still unsure exactly how I will re-enforce the tube for the primary and secondary mounts, but I'll work something out.