Latest Project - 18" f3.9 Truss Dobsonian

[Moonshane]

I am starting this thread to show people how I get on making a large truss dob. The scope will house a new 18" f3.9 mirror which has a good figure I think of 1/8PV.

As always I start with the upper assembly which will house the focuser (probably a Feathertouch), the finders (a telrad and a 9x50mm RACI finder) and the secondary mirror and spider (obviously the eyepiece too eventually. The weight of this fully loaded secondary cage determines the balance points of the scope and therefore it needs to be done first. As always I use only birch plywood which is more stable and much better quality than most external grade plys. It's a little more expensive but well worth it; the L'Oreal of the plywood world!

It's a basic construction of two rings of 15mm ply (40mm wide and with id of 19.5 inches), four struts of 18mm ply and a focuser board and finder board of 15mm ply. You could potentially reduce the weight a (very) little by using thinner ply for the two boards but I wanted one a little more solid due to the increasingly heavy eyepieces being used these days.

I drilled and screwed in some threaded inserts (M6 for the struts and M4 for the boards) and all went well. The joints are nice and tight and it's light and strong.

Next will be the side/altitude bearings. I'll make a template first and then router the shapes from that. The template will be in 12mm plywood and the bearings themselves will be two 15mm sheets bonded together to create bearings 30mm thick x around 600mm diameter.

Here's some pics so far with the obligatory can of beans for scale.

[Naemeth]

F/3.9? That will produce a lot of coma and will be a great test of eyepieces . Look forward to seeing it completed, eventually .

[Moonshane]

It certainly will but it also allows someone of about average height to observe with feet on the ground or on a single small step.

[Naemeth]

Are you going to replace your current 16" F/4 with it, sell it or have it as your 4th?

[Moonshane]

Four scopes is at least one too many - ask my wife! I am making this for a fellow SGLer but hope that adding the build thread will inform and inspire others to have a go themselves

[Naemeth]

Four scopes is at least one too many - ask my wife! I am making this for a fellow SGLer but hope that adding the build thread will inform and inspire others to have a go themselves

Ask mine (my wife to be)!

I hope the SGLer has a Paracorr or is intending on getting one (If you are reading this and you are the SGLer and haven't got one, you might want to add it to the shopping basket )

[8kids]

I hope we'll have no more of those inferior beans in future pictures Shane. I think you'll find Heinz to be the product of the discerning scope builder.

I shouldn't be too far behind you, I'm just waiting for my aluminium poles and some odds and ends for my own 12" truss build.

Gary.

[SnakeyJ]

Sounds like another fantastic project and off to a great start - will enjoy following the progress on this. Out of interest where are you sourcing the mirrors for this one?

[Moonshane]

cheers all,

I hope we'll have no more of those inferior beans in future pictures Shane. I think you'll find Heinz to be the product of the discerning scope builder.

I shouldn't be too far behind you, I'm just waiting for my aluminium poles and some odds and ends for my own 12" truss build.

Gary.

ha ha I knew someone would pick up on the corale beans! I save money on beans so I can buy more greens. good luck with your project mate - have you got a thread?

[Moonshane]

Sounds like another fantastic project and off to a great start - will enjoy following the progress on this. Out of interest where are you sourcing the mirrors for this one?

cheers Jake

not sourced by me but it's a John Nichol mirror set - nice!

[8kids]

cheers all,

ha ha I knew someone would pick up on the corale beans! I save money on beans so I can buy more greens. good luck with your project mate - have you got a thread?

No thread yet, I'll be putting router to ply next week, and I'll post on here as soon as I have something to show.

Gary.

[Moonshane]

look forward to it matey

[SnakeyJ]

cheers Jake

not sourced by me but it's a John Nichol mirror set - nice!

John needs no intro around these parts - this should be a fantastic build and optically excellent. Look forward to reading more as the build develops.

[Astralstroll]

Well I'm off to a good start as I've got the same garden table. Just need to stock up on beans now and I will be well on my way!! As soon as my indoor DIY chores are finished I can devote some time and effort to creating something more fulfilling than a nicely decorated house. On a more Astro related note, are you using the "clear" birch ply?

Dave.

[Moonshane]

good luck once you get going

not sure what you mean by 'clear' birch ply? it's a good quality birch ply with almost no voids and tight layers (well under 1mm per layer) and a nice face both sides.

[swamp thing]

Good start matey i'll be watching this with interest.

[Moonshane]

cheers Steve, as always we'll compare notes.....

the UTA had to be a little longer than I would normally make them to ensure the dual speed dials on the focuser can be used in any position. it's still only about 14" tall though so not huge. in the end it provides more protection for the secondary from dew, less likelihood of stray light getting into the light path and as the truss tubes will be a little shorter as a result (I anticipate only about 46 inches long), more rigidity.

[8kids]

good luck once you get going

not sure what you mean by 'clear' birch ply? it's a good quality birch ply with almost no voids and tight layers (well under 1mm per layer) and a nice face both sides.

In woodworking terms clear means defect free, i.e no knots or splits etc.

[kirkster501]

Well I'm off to a good start as I've got the same garden table...

Lol... Me too! Needs oiling when it comes out of the shed for its summer outing.

Back to the topic. Looking forward to how this develops. Boy, f3.9? That is going to be tricky to collimate.

[Moonshane]

not really, just the same as any other newt

[Moonshane]

based on the above description then yes, I am using 'clear' birch. any occasional knots are very small (and in my view add to the beauty) or replaced by an inlaid piece of birch laminate. even these I usually hide when bonding two sheets together or on the inside of the focuser board etc.

[estwing]

wow is that a half-tin of beans!!!something that big may need a wheelbarrow carry system? good luck with the build.

[Astrokev]

based on the above description then yes, I am using 'clear' birch. any occasional knots are very small (and in my view add to the beauty) or replaced by an inlaid piece of birch laminate. even these I usually hide when bonding two sheets together or on the inside of the focuser board etc.

Out of interest Shane, where do you get your birch ply; can't say I've seen it around these parts, and it looks excellent stuff.

Your off to a great start, and I'm really looking forward to seeing it come together. Lots of pics please - they really inspire.

[swamp thing]

Birch ply is available from most refutable timber merchants. The 15mm thickness is not often a stocked size, but most merchants can get it in a few days.

[Moonshane]

Indeed, I have to order mine in even the 12mm and 18mm I have used. usually if I order by Monday/Tuesday it's there by Saturday and they cut it to approx (or exact) size too. if buying more than one sheet ask for discount.