furrysocks

The materials I've got... I have 11 lengths of 3mm hardboard, 75mm wide with a 45deg bevel down one side. The nice man at B&Q tried his best with a second sheet and just one came out wonky. From a 9mm laminate/mdf sheet about 2'x2', I have cut 18 ribs, 1/3 of a round each. These provide an internal aperture somewhere between 240 and 245mm. These all have a 45deg bevel on the inside diameter to about 7mm - not quite knife edge, but I needed some meat to run on the router bit bearing. The original plan... I was going to cut to length at 1.7m and fit 6 sets of ribs along the length. This was my 'least cost' option - one sheet of hardboard, a tube or two of high strength grip adhesive, a few rolls of masking tape and some spare paint/varnish. However, in mocking up a round using 5 inch 'test pieces' of hardboard cut from one end of the full lengths, I see a rather nice secondary/focuser stage for a truss newt. The new plan... I can save myself a lot of handling of long, floppy lengths of hardboard by going for a truss design. I'll save wood too, so I can probably get both primary and secondary tube stages for the binoscope made out of the materials I've got cut and prepared already - perhaps just a few more ribs! The extra money spent now in making this scope (or these scopes!) lightweight and portable will surely pay off in the end. When I run the bead of adhesive between the outer sections and then fold them to 150 degrees, I'll wet-finger the bead down into the joint and then lay on top a length of bamboo skewer (£1 for >100), cut to fit between the two baffles, and then run another bead of adhesive over them once the first has cured. It'll add weight, not a lot, and should increase the strength of these edge-edge joints. The inside surface of the hardboard is already "waffly" - my "poor man's flock". Inside will be sealed and painted matt black. Outside is still a few coats of exterior varnish, for now. Now to source a dozen truss rods - budget £20 or so. The focal length is >1.5m, so a Serrurier truss might let me use slightly shorter lengths, like floor brush handle tubing or something. Had I seen this page (http://astronomyasyl...ultralight.html) before starting all this, I would have easily put the cost of a sheet of 18mm ply to this build (£28) - but the hardboard only cost about £5, which I've already spent so I'll continue with that, but go 'truss' with them. I may take some inspiration from here (http://www.rfroyce.c...russ_tube_newt/).

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.

Secondaries sent off for re-coating.

The OTA will be based around a 'coopered' hardboard (masonite) tube, 12 sided. I know that this is not going to be the best material but at <£6 for a sheet, it's going to be cheap. The idea came from Toshimi Taki's ply tube, with an added pinch of thrift. Using the timber cutting service at B&Q, I have 12 full length strips, each 75mm wide. I will run one edge of each over the router table fitted with a 45 degree chamfer bit, using fence, featherboards and a second pair of hands to assist. Each internal angle must be 150 degrees - one surface cut to 45 degrees and butted together in a cradle or sorts will give me a 15 degree gap to fill. I've got a couple of tubes of Hard As Nails from the pound shop to put in the caulking gun and a sheet of scrap laminate from which to cut the knife-edge ribs/baffles. Masking tape on the outside and a wet finger to smooth/smoosh the bead should do the rest. Variations in the technique might include the use of PVA or epoxy to seal the edges first - I'm planning a couple of test joints but impatience may get the better of me. Outside, I'll give it several coats of quick-drying exterior Dark Oak varnish and on the waffle side (poor-man's flock), I've got some matt black chalkboard paint that might do the trick once I've combined it again - it's not been opened in over 10 years. I might give the inside a coat of primer first to help seal it. I'll re-enforce the ends, bolt holes, handle mounts, etc with ply, as necessary, either inside, outside, or both. If I can get away with building the tube for under a tenner and it lasts two or three seasons, it will be a success of sorts. If it's a false economy, at least I tried not to spend too much.

I've been tinkering over the last few weeks and making progress towards constructing suitable hardware for the secondary mount and focuser. With the mirrors, I received a secondary fixture with a round aluminium 45, topped with an elliptical brass plate to which the mirror itself is mounted. I epoxied three large washers together, marked, drilled and tapped three holes for collimation bolts and three further holes for the vanes (will use bike spokes). Pending bike spokes and re-coated secondary. I've knocked up three focusers so far, all Crayford. First used small sections of plastic chopping board planed down to thickness and fixed to vertical blocks of wood - lacked the ability to fine tune. Second used four skateboard bearings mounted on an extruded aluminium section - too tall. The third attempt is a Helical Crayford type. Four bearings angled at a couple of degrees, around a central hole with a friction screw opposite. Each bearing is secured with countersunk bolts, with two small washers to back the inner race and give the OD of the bearing clearance from the surface. The washers sit in counterbores perpendicular to the angled through holes. I set the angle with a small block of wood under one end of a platform placed on the drill press table - no idea what the angle is. I get something like 1cm travel for a full revolution but I don't imagine that even repeatability is that critical, provided it's secure, friction is more or less consistent and doesn't bind. I should just be able to screw the bolts straight into an appropriately sized through hole without the need to tap or use threaded inserts, and just epoxy the bolts back in if the threads come loose. The whole assembly should mount to the OTA with three bolts to permit alignment. I can get this down to about 2 inches in height. Plus, it's only something like 9 holes and 4 counterbores. Will make it from two thicknesses of 13-ply birch plywood. One thickness would do if I offset the bearings from each other, but a greater vertical separation of the bearings should reduce any cam-effect in the event that the focuser tube is not 100% round along its length. Planning to use 39mm ID PVC pipe into which I can press fit and align the crappy 2x barlow I got with the Aldi dobson to serve as an EP holder, shortened to ensure no vignetting. Preliminary design in newt-web with the 43mm secondary and 216mm primary in a 265mm ID tube gives me a tube length of about 1.6m, 20% obstruction, 17.5mm 100% illumination, 31.4mm 75% illumination and no vignetting. Pictures of the secondary mount and helical focuser to follow.

The better of the two primaries measures somewhere between f/7.57 and f/7.575.

I now have the mirrors - both are 8.5" and around 40mm thick. There are many surface scratches and scuffs on both and light will shine through from the rear in a number of places. Though I have an untrained eye, one appears just good enough to use in the first instance. The two flats both definitely need to be done before use. The flat that is fixed onto its holder appears to be done so with a layer of green felt/baize and glue and there is sign of the same on the back of the other mirror. I think that carefully running a hacksaw blade back and forth down between the mirror and holder will slowly remove this material and separate the mirror. When I send the first primary away for testing, I am also considering whether or not to obtain a test report - primarily out of interest, but it may prove useful if/when I come to sell.

A mate has today collected my mirrors from the seller in the North East. They were advertised a couple of weeks ago on preloved.co.uk and I have paid £100 for two 8" primaries, one mirror cell and two secondaries. The primaries are approx f/7.5 and full thickness and the secondaries measure approx 43mm and 47mm along the minor axis. I have been told that the secondaries definitely need re-coated and that one of them has been permanently fixed to its mount. The primaries may be usable until such times as they too are re-coated. I believe these to be a working set and a spare set used in a Charles Frank reflector which had been installed in a gentleman's garden in Stockton for a number of years before changing hands, being subsequently damaged and the mirrors salvaged. All the parts I have bought have been stored on top of a wardrobe for the last 4 or 5 years. No other parts from the original telescope remain. I hope to receive these myself in the next week or two.