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End of Blog

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
furrysocks

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Tube clamps and bearings - what a pig's ear!!

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
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OTA complete - dob mount next

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 3
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First light!!!

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
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Weight, balance and aesthetics.

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
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Sneak peek

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).
furrysocks

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Another night's work (part II)

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
furrysocks

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1

Another night's work

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
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Finished cell, and truss supports

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 an
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Shortcut - not so much fibreglass

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
furrysocks

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Primary mirror springs

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.
furrysocks

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2

First light...

...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 an
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Another change of plan

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 r
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2

Finished resin coating the tubes

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 refl
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1

Carry on, regardless...

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 we
furrysocks

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2

Polyester resin woes

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 ho
furrysocks

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3

Making the tubes

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 a
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3

Maths - weights, etc...

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 valu
furrysocks

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3

Slight change of plan

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 orig
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1

Optical Tube Assembly

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
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Secondary mount and focuser

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.
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Mirrors acquired

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
furrysocks

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