furrysocks

Now you can... ;) http://www.smtools.co.uk/shop-virtual-tour.html

Folks, If you've been reading this blog, I hope you've enjoyed at least some of it. I'm going to stop, now. I should be completing and painting the dobs mount later tonight and expecting first light at some point over the weekend. I'll post a first light report and some field photos on the forum, instead. It's been both frustrating and fun, and I accept that it may not be perfect first time - balance, vibration, etc... - but it should at least hold together! ;) Like many DIY scopes, it may never be finished. I had a jumble of ideas to begin with, most of the tools and half the skills required. I changed my mind many times and rushed a lot of it. Too many late nights, too much effort. A lot of money spent, too. But it's got me out the house and back in the workshop, using old tools, new materials, thinking, planning, etc... and I get a scope out of it at the end. :D Clear skies, Matt.

I'll get a photo of this tomorrow, but it has to be the least well executed part of the build so far. I'm certainly putting the hours in on this scope in the evenings, but I'm starting to tire. There's a star party in a couple of weeks that I really want to take it to... I doubled up some 18mm pl with PVA and clamped it for a few hours. I drew the shapes I wanted and bandsawed the external and internal profiles. I used a rasp to 'fit' the internal profile of each of the four half rings to their respective part of the tube, having marked each one first. I cut two 4"x10" stretchers and drilled them for threaded inserts and glued, pinned and screwed the rings to these. I drilled through holes in the tabs top and bottom for the bolts and t-nuts to clamp the tube. I've resined it all and it's now curing. I drilled four holes in each of the soil pipe end caps. There are three positions in the stretchers for the bearings to attach to, giving me 2 inches fore/aft to play with. It's a rough job but it should hold the scope. I hope.

Last night I got the OTA assembled. There's still some painting to do inside, but it's basically there. Tomorrow, I'll get two soil pipe end caps for bearings. The plan is to make up some tube clamps joined together with a couple of plates into which I will fit a dozen or so threaded inserts each to allow me to move the bearings to various positions, as I don't yet know the final weight of the top tube. The rest of the mount will be pretty standard. All 18mm ply, tube rings probably doubled to 36mm for aesthetics and strength around the bolt holes. I'll run the bearings on a couple of squares of kitchen cutting board and go from there. Not much sign of the cloud clearing or the wind easing this week, but the first starlight should be soon... :D

First light!!! I labored over the maths to get the focal length folded to within the range of the focuser, measured my EPs, left some spare room both in and out. I then drilled four struts to connect the top and bottom tubes together using 4 of the 8 threaded inserts on each. With the front end sitting on a folding workbench and the back end on top of 4 spare van tyres, I aimed for the floodlights of a farm a mile or so away. Dropping down from a 30mm to an 8mm EP (both Vixen NPLs), I got a cracking view of both elements in the light as well as the branches of a tree swaying in front of it. Very happy with where the focuser sat with both EPs - unless I've got it wrong, I should have the focuser further in for astro over terrestrial. Next job, cut and drill 16 trusses and put this thing together. It was about as rigid as a banana, tonight. Happy days!

Without a finder scope attached to the top tube, and using the biggest EP I currently have (30mm 1.25" Vixen NPL), the top tube is approximately 2.1kg. The bottom tube with mirror is approximately 4.7kg. Using Robert Royce's truss length calculation spreadsheet, this gives me a separation between the bottom and center tube of only 120mm, about 4.75". That's "not enough" I don't think for it to be aesthetically pleasing. By making the top tube heavier, I can move the balance point up the scope to increase the separation between bottom tube and middle tube. The photo I took in the last entry had a gap of about 12 inches which is a bit closer to what I was expecting. It would be nice if the scope was comprised of two sections of roughly the same height, so they would stand side by side, perhaps in a lightweight crate or something. This happens at roughly 11" of separation between the bottom and middle tubes and in order to get the balance point to the middle of the center tube at this separation, I need to increase the weight of the top tube from 2.1kg to 3.3kg (maintaining it's own center of balance). There's four thoughts in my mind at the moment: 1 - go large with a finder on the top tube 2 - add a sliding weight underneath the top tube. 3 - add a heavier weight on the center tube, somewhere forward of the altitude axis. 4 - mount the altitude bearings further back than the center of the center tube. I may go for a combination of the above. On one hand, I think I'm being daft by ignoring the scope's own center of balance, choosing style over substance if you like - especially when this comes at the price of actually adding weight to it. I think, truss rods included, I'm going to come in at 10-11kg, plus cradle and some additional fiddling. On the other hand, I can choose the look of the scope to be right in my eyes and then make good. I've made the rest of it up as I've been going along, so I'm not too worried.

Tonight, I mounted the focuser and secondary holder. The entire assembly will be about 6 feet long and somewhere in the region of 11kg (24lb).

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.

About 4 hours on it again, tonight. Decided to finish early and get some sleep! The center tube has been painted - just need to touch up some matt black inside. The ring that holds the top set of trusses and attaches to the center tube is done. I started with an 18mm ply ring to which I fitted the threaded inserts and through holes, but I decided to laminate another 9mm ring on the front of it to hide the t-nuts. It's also now painted. I've fitted the threaded inserts in the top tube and applied resin to the little blocks inside. I drilled a 56mm focuser hole (eventually I want a 2" focuser) and re-resined the exposed hardboard. That was probably the most nerve-wracking bit of the build so far. I used a cheap split-ring hole saw, and kept backing it off to make sure I wasn't putting too much pressure on the tube and letting the teeth do the work. The last thing I wanted to do was split the tube. It went perfectly. I trimmed a few mm from the secondary mounting block to keep it all within the outline of the mirror. I traced the outline of the top tube on a board and drew a line between opposite corners to find the center. I then drew a circle of diameter equal to the internal distance from face to face. Drawing lines parallel to the outside, I defined the internal 12 segment profile. In the center I drilled a 5mm hole and left the drill bit in. I slid the focuser mount attached to the steel rule onto the drill bit and picked two faces to which I would attach the steel rule. I tapped in a couple of pins and bent and captured the steel rule between them, then read off the measurements for each. The first steel rule I used snapped when I was bending the acute angle for the tabs, so I picked up another (slightly thicker, unfortunately) and started again. I was 1.5mm off center with the first so did the maths and drilled, heated and bent the next one on my camping rocket stove. I should have drilled the holes in the lugs first, but I reckon I can do it... I'll post a picture of that process! I appear to be ignoring 'offset'. I'm thinking that anything can be shimmed, tweaked, etc... so I'm just going for it. So, left to do now: - attach secondary to its mount - measure, drill and test fit the secondary mount in the top tube - make a mounting plate for the focuser and test fit - paint the top tube - weigh and balance the lot - calculate and cut trusses, and assemble - make a dobs stand

Got the bottom tube painted with £shop matt black - it's more a dull satin compared with the blackboard paint on the inside, but looking good. Few drips and the surface wasn't totally prep'd but I don't care too much. Got the threaded inserts for the truss rods superglued into lower end of the centre tube, having made another jig for accurately placing drilling the holes. Also reinforced the wedges with a couple of screws each from inside. Also got an 18mm ply ring traced and cut for the top end (to let me split the scope in two) and coated it in resin. Got the little internal blocks cut, glued and clamped in place for the top tube threaded inserts to bite into. Then set to work on the secondary mount. I already had the 45deg mount with threaded rod so just a small cube of wood a la Stellafane mount. Drilled a steel ruler and epoxied a penny washer to it to keep the mating surface flat. Despite my best efforts, neither of the holes through the block were perfectly square, but I should be able to compensate later. The silicone sealant I have says not to use on brass, but that's what my secondary mount is made of. I may mix some JB weld and spread it over the plate with a threaded bolt to give the silicone a keyed surface to adhere to that's not brass. Good progress for one night. Next on the list: - threaded inserts in the top tube - drill focuser hole - test fit the secondary mount - mount secondary mirror - fit threaded inserts in the ring - drill bolt holes through the ring and centre tube top-end 'lugs' - sand the ring flush to the center tube - paint the top and centre tubes and ring Then: - assemble, weight/measure and calculate truss lengths I've got 1/2" x 1.6mm M6 rubber washers coming - one under each truss to let me elongate the holes in the trusses in order to align the tubes accurately without them shifting. It'll save the paintwork, too. ;) But there are soooo many things that can still go wrong. I may need to use a different silicone for the primary. How well the superglue is going to hold the threaded inserts is a big unknown. Getting the tubes aligned when I fit the truss rods may be a nightmare. Etc.

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

I've put the primary mirror in the cell with silicone - doesn't feel like it's hardening up much at the minute but I'll leave it for a good 24 hours and see what's what. I've been drilling the truss bolt holes in the bottom tube with a little jig made from angle aluminium. But the main thing I got done tonight was to re-glass the center section with the truss supports. I PVA glued two bits of 18mm ply together to make it double thickness. Then cut a small triangle from hardboard as a template and made eight in double-thick ply. Applied 225gsm mat with polyester resin to the center tube, placed a triangle in each of the appropriate places and resined a thin strip of mat over the top. I will fire a screw in from inside and then drill for the bolt holes and threaded inserts.

On reflection, the top tubes which will take focuser, secondary and finder feel strong enough. With some internal bracing, the bottom tubes which house the primary mirrors feel like they should well be strong enough, taking into account the fact that the primary mount itself is going to add some rigidity, too. The center tubes are the longest, have the other two hanging off them and will need to accommodate whatever mounting rings or cradle I choose to make. Therefore, I will only re-glass those center sections. (The two parts of one primary mirror cell are curing in the warm cabinet right now. I'll paint them black tomorrow and assemble later on. I'll glass at least one of the center sections tomorrow, too. Mounting the secondary and focuser is next. I have a feeling I'm going to want to use my 3" reflector as the guide scope, which means another EP is required.)

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.

Going for a simple mirror cell using steel angle brackets, silicone, ply, threaded inserts, 4mm-hex head bolts and springs. The whole lot will be mounted to the bottom end of the bottom tube with external brackets. The springs I've chosen are one inch in length, half inch internal diameter, 2mm wire diameter, 5 turns. Using the kitchen scales, it takes about 5 kg of force to compress each spring to about 2/3 of its length. They will compress down to about 1/2 an inch, unknown force to get there. The mirror is about 3kg. I think these springs should be fine, as I'll preload them to very nearly half their length. If I go with threaded inserts in the mirror cell and bolts through the outer mount, then I may have to preload the springs before I can engage the threads. I may instead have captured bolts in the cell and wing nuts at the back - then nothing is going to pyoing if I undo a bolt too far.

...strictly speaking, perhaps. But not really. I placed one of the mirrors on a cloth on some ply and put it on the ground. With a 35mm eyepiece taped to a washing line prop, and standing on a stool, I tweaked the twist-grip 'focuser' and saw some stars. Quite a few considering the <1deg TFOV. Some of them looked more or less like points - the bright ones looked like a mosquito wing presumably because I was viewing off-axis. I think I proved nothing more than that the mirror reflects light and focuses it to 'some' degree. My mate's got a photo, I'll post when I get it. ;) Going to try and restart this project over the next few weeks. I reckon there's maybe another 20-30 hours to go before I can get one OTA complete. I'm still making it up as I go along. I have a plan for the primary, an idea for the secondary mount and spider, a prototype helical focuser and a bunch of structural work to do.

Only just...

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.