Jump to content

Banner.jpg.b83b14cd4142fe10848741bb2a14c66b.jpg

furrysocks

Members
 View Profile  See their activity

  • Posts

    141

  • Joined

  • Last visited

 Content Type 

Blog Entries posted by furrysocks

  1. furrysocks

    Mirrors acquired

    By furrysocks,

    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.
  2. furrysocks

    End of Blog

    By furrysocks,

    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.
  3. furrysocks
    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.
  4. furrysocks
    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
  5. furrysocks

    First light!!!

    By furrysocks,

    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!
  6. furrysocks
    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.
  8. furrysocks
    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
  9. furrysocks
    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.
  10. furrysocks
    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.
  11. furrysocks
    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.)
  12. furrysocks
    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.
  13. furrysocks

    First light...

    By furrysocks,

    ...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.
  15. furrysocks
    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.
  16. furrysocks
    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.
  17. furrysocks
    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?
  18. furrysocks

    Polyester resin woes

    By furrysocks,

    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.
  19. furrysocks

    Making the tubes

    By furrysocks,

    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
  20. furrysocks
    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.
  22. 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/).
  23. furrysocks

    Optical Tube Assembly

    By furrysocks,

    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.
  24. furrysocks
    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.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.