10" truss dob build... yep, yet another one!

[Andy_E]

Exciting times! Finally got the go ahead from the missus to start my first build . I'm going for the (now seemingly ubiquitous) 10” truss 'obsession style' dob... I guess I lose marks for predictability. Unfortunately I don't have much spare time these days so it's going to be quite a long term build, hoping to get it finished in time for C/2012 S1 ISON (fingers crossed). The plan is to strip down a S/hand 10” OTA for parts as finances are also rather tight.

No build progress as yet as most of the last month or so has been spent doing research; my head is about blown so I guess that means it's build time! I've mocked up the plans for the mirror cell (photos to follow) which is a six point support. I was tempted to go for a three point for simplicity but this build is intended as much as a learning curve towards bigger scopes as it is a project in itself (plus I got better readouts in PLOP), and a six point is a nice n' easy introduction towards more complex designs. The cell itself is to be constructed entirely out of aluminium, with the UTA, rocker and mirror box out of birch ply.

I'll try and keep the thread updated as often as possible as it would be great if I could get some 'advice in progress'... I'm sure I'll have plenty of questions!

In fact, here comes the first one . Do you think I would get away with four single truss tubes on a scope this size? Or am I better off going for the eight truss design as per usual?

As a final note, I would like to state that I hold a number of members here on this forum personally responsible for this build! After following a number of threads on the matter, it really is an inspiration to see such wonderful telescopes being built by some of the guys and gals out there in SGL land! If my scope ends up half of what I've seen posted on here over the years I'll be a very happy man!

Andy

[estwing]

good luck with the build Andy..will be following this all the way with you!..clear skies!

[Andy_E]

Cheers... good news is it probably wont be finished for three months or so... that means plenty of clear skies for everyone

[estwing]

ready in time for the PSP then

[swamp thing]

Nice one Andy, you can't beat building your own scope

Personally I see little advantage in having fewer truss poles. Eight allows for great stiffness because trusses are opposite one another and triangulated for strength.

[Andy_E]

It's all about the triangles. I suspected as such but just thought I'd check, cheers

[Rustysplit]

I'm with Steve, I think 4 poles won't cut it. 8 just makes it all nice and tight. Although a 6 truss design would probably be fine as well for 10".

Good luck, I will be following with interest.

[Andy_E]

Thanks Alan. I've seen a six tube design and must say it looked quite good. I suspect I'll stick with the eight tube rout mainly as I have a thing about symmetry... ah the obsessive details begin at the early stages of scope building I see.

[Rustysplit]

I know what you mean about the symmetry. Balance is a beautiful thing.

[JamesF]

Good luck with the build. I'll be interested in seeing how it progresses.

James

[Andy_E]

Right, it's aluminium ordering time soon. Still can't decide whether to go with 3/4" 16swg or 1" 16swg for the truss poles.

[Astrobits]

I used 8 poles 19mm ( 3/4" ) dia for my 16" truss dob and it works fine. 25mm ( 1" ) will be significantly stiffer and would probably be preferable for a 6 pole design.

Nigel

[Andy_E]

Thanks Nigel. I'm going for an eight pole design so I imagine the 1/4" will be fine... I've just reached the paranoia stage as things are about to be ordered. Don't want to under do things but I suspect I'm more in danger of over doing things at this stage. I guess that's just a symptom of first time build syndrome!

[Andy_E]

Here's the rough mock up of the mirror support, and by rough mock up BOY do I mean rough! The measurements are actually pretty accurate but the materials are just any bits and bobs I could lay my hands on around the house. The supporting 'T' will be constructed out of 25mm square alu' tube and the three mirror support bars from 25/6mm alu' flat bar. The mirror itself will be attached with clear silicone. I quite like this 'T' construction as it allows a fair bit of ventilation to the mirror compared to some of the other ideas I've played around with, bearing in mind it's only a 10”. I'm also quite fond of its simplicity... anything simple gets a thumbs up from me at the moment! I was considering mounting the cell on a removable plate, but as the mirror box itself is going to be quite shallow I think this might be over-engineering it slightly.

[Moonshane]

hi Andy

looks like you are getting stuck in! I agree about the poles although for eight poles, I would tend to use 5/8ths OD and 16swg (I think that's a bit over 1mm thick). they are well sturdy at the sort of lengths you'll be using.

re the mirror cell, I'd be doubtful that a six point cell could be fixed with silicone successfully as it would possibly cause stresses in the mirror and create astigmatism. if using silicone then three blobs would be better onto a flat board which then had three bolts through it for collimation. you are then pushing and pulling the board and not the silicone if you get me.

[Andy_E]

Thanks for that Moonshine. Yes I see what you mean about the board. If possible, I'd like to stick with a six point (though I'm not hell bent on it if it means compromising mirror support) as a precursor to more complex designs further down the line. I'm quite happy to go back to the drawing board and get things right... I'm under no time pressure to get things finished quickly. If I were to stick with the six point do you reckon edge clips would be the way forward? PS; I probably should have mentioned, the collimation bolts are going to be placed as far out in the square tube of the 'T' support as I can get them within an equilateral triangle.

[Rustysplit]

I agree with Shane re the three points for silicone. Simple and proven design. I would still use some edge clips just for security. They should not touch the mirror edges or surface. They are just there to stop it falling out basically.

[Andy_E]

Just to let everyone know the build has been put on hold for a few weeks due to some financial problems (nothing serious but cash will be tight for a while). I'll update the thread when the build is underway (don't want to start a new one and clutter up the forum). Hopefully shouldn't be too long.

[Andy_E]

Right then!!! Finances seem a little better (ah the joys of being self employed) meaning I've managed to get hold of a S/H 10" OTA. Plans are all drawn out (Lord knows I've had plenty of time to do them) so it's time to get cutting and drilling. I'm still unsure as to which material to go for mind. I was originally planning on using 12mm and 18mm birch ply but can't help wondering if I may as well lump for some standard hardwood sheets. I really want to keep the weight down and my is birch ply heavy... beautiful, but heavy. I'm also a little concerned as I'll be using a jig not a router for this project; might 18mm birch ply be a bit hard to cut with this? I've used the 6mm stuff before and have to say it was as tough as old boots (especially to drill) so I'm a little concerned. As I'm no doubt going to end up painting it is there really any point shelling out the extra for the birch?

[swamp thing]

You can use other plywoods if you want Andy. It's up to you. I used birch simply because I wanted the stiffest possible structure to support the optics, so observing at high power is easier. The less flex our scope has the better it will move, therefore the easier it is to track at high power. Normal ply will work fine.

[Andy_E]

Thanks Steve. I keep having to remind myself it's a 10" and not some monster dob... keep over-engineering things in my plans.

[Moonshane]

I agree with Steve that there's nothing wrong with hardwood exterior ply but birch is far superior.

if you are painting and accept some filling etc then it's fine to use. in fact my 16" dob base is made in exterior ply, painted satin black and is going strong with no issues at all; super smooth and sturdy.

[Andy_E]

Finally managed to get saw and wood together over the last couple of days (I'll update further with pics tomorrow). Ended up opting for standard WBP ply... pleasantly surprised with the quality; the majority of the 'major' cutting has been done and only one tiny spot that needs filling (which I suspect was my fault anyway). Unfortunately the garden is my workshop, so suspect I wont get much more done over the next couple of days (rainy here), but I've had permission from the missus to do some drilling in the kitchen so long as I clear up afterwards

[Andy_E]

This is the work so far. UTA rings, mirror box, alt bearings and base all frustratingly sat in the living room waiting for the weather to clear a little. Hopefully off to the local hardware store tomorrow for a handful of fixings.

Still need to fine sand all the edges and seal the ply... I'll be sealing it all before it goes together to make sure no moisture can sneak in anywhere. It never ceases to amaze me how big sheets of ply end up in tiny piles of parts!

The eagle eyed of you may have noticed the retro jig saw... it's a hand me down from the father in law and has seen me through many a task, just can't bring myself to replace it!

[Andy_E]

Managed a few to steal a few hours over the last couple of days to get on with some construction. Nothings fixed yet so just dowelled in place... still it's nice to see things coming together. All the little holes have been filled ready for the final sand and first coat of sealant (going to give everything a single coat before construction to ensure no sneaky moisture gaps).

I hoped to have got a bit further to be honest, but I've been waiting on a delivery of fixings for the spider connectors. Bit of a pain that SW use M5 thread for their spider meaning all fixings are going to have to be 'made'. All part of the fun of building I guess.

All the parts are due to arrive in the next day or so, so hopefully I should get a bit of work done over the weekend. That's if I dont get in to too much trouble; the missus has started to see through the veneer of “oh I only need some wood and a few aluminium poles” spiel. When asked if I'd got everything I replied “ yes. Well I need some angle for the upper truss connectors... and some PTFE sheet... and some thin ply or plastic sheet for the UTA...oh yeah and some flocking material”. The list goes on!