10" Portable Truss Dob for Skywatcher mirror set

[Moonshane]

Today I managed to start a new project and this will be a truss dob system designed to allow this 10" mirror set to be carried in a light and compact package. It will be constructed from birch plywood of 12mm and 18mm thicknesses. The idea is that it can be packed up and put onto a trolley or similar and carried on public transport (although not flights). I hope that even those without a car will be inspired to consider a good aperture and visits to darker sites.

The first job with this sort of project is to create the top cage which carries the secondary mirror, the focuser and eyepieces plus any finders used.

The size and weight of the secondary cage determines the size of other components and also the balance point for the mirror box. Therefore you should always start with this element.

I am using traditional methods with two rings of 12mm plywood joined with short aluminium bars and a 12mm plywood focuser/finder boards. Things will develop as always based on what works best and hopefully others will learn as I go.

Here's some pics of the rings which is all I can do today (possibly this week). The materials are great to work with and very solid. They are 13.25" outside diameter and 10.5" inside diameter.

The boxes will be made with 12mm and 18mm thicknesses and joined with dovetail joints throughout.

[John]

I wish I had the skills and facilities to produce stuff like that.

Nice work Shane and an interesting project - I'll look forward to developments

[Beulah]

Hi Shane,

Good luck with this project. I made the same thing for the mirror set you sold me sometime back. It's sitting in the workshop waiting to be varnished! Tons of other projects to complete though...

That ply looks like a good quality one with hardly any voids - where did you get it from?

The 'telescope in a box' design inspired by some other folks on this forum was going to be adapted to that I can put it on my back and walk to a dark sky site - some of the best ones are inaccessible by car!! (Poles to be carried by hand or by a willing victim *cough* volunteer... )

[8kids]

Dovetails in ply.....(sucks through teeth) you'll have to be careful of bits chippy off if you use a router, if your painting it though it won't matter you can fill it.

[Moonshane]

cheers John, Sam and 8kids.

I know what you mean about lots of projects Sam - this is why I often end up using rough prototypes for ever, like my chair just thrown together from what I had spare in the shed to see that it would work. must make a better one soon.

the ply is a good quality Birch ply from my local independent joinery shop. they supply it only to order and it's more expensive than exterior hardwood ply but it's worth every penny.

I am lucky that my father in law has a good sized workshop and I can always go up there when he's about to use his many tools. these rings were actually done in the garden on the patio this morning though just before the rain started.

I know what you mean about ply/dovetails but my father in law has a good dovetail jig and I'll probably have a board behind the finished piece which will hopefully allow a clean edge. if not then I'll consider finger joints or even rebates. I am hopeful that the quality of the materials will allow dovetails but watch this space.

[Moonshane]

I should add that the mirror is a 10" f5 set from an explorer OTA.

[nightfisher]

If this turns out any thing like the 6" you made, it will be a super scope, having viewed Jupiter with the 6" i was very impressed, and thats praise from a refractor man

[Qualia]

Looks amazing, Moonshane and I look forward to further updates. It sounds and looks like you have quite a mission on your hands, and one quite tricky I imagine looking at the compromise of design requirements. I wish you the best of luck with the project.

dark sky site - some of the best ones are inaccessible by car!!

I've found exactly the same. I go to some arid lands near where I live and there are no 'roads' - if we understand that term as the concrete variety - entering or leaving them. Makes a blissful place to view from and one relatively free of nightly fear simply because no one enters and no one seems to live in these lands.

[Moonshane]

cheers Jules / Rob

the whole idea of this sort of system is to make it small and transportable and therefore able to reach the parts other scopes cannot reach. from a dark site, 10" would be a very usable aperture and provide great (visual) images of the fainter targets.

[asteria]

Hi there - I'll be looking forward to this new project. Thanks for keeping us up to date. Ed

[Moonshane]

Thanks Ed. I am looking forward to getting stuck in!

[Matthew]

Good start Shane having followed your other projects i know this will be a excellent scope.I will be taking notes i plan to start my own scope build shortly

[swamp thing]

The dovetails will cut fine Shane. Just go slowly with the router, and obviously make sure it's a good sharp bit.

Look forward to seeing this progress. The birch ply is a lovely material to work with eh?

[Moonshane]

cheers Steve. Ken has a Leigh 24" dovetail jig and router so hopefully using good methods and tools such as this will help. I have to say that birch ply is almost in another world to hardwood ply which although strong is not as strong or pretty and therefore more suited to painting. looking forward to working on it next week. might fix the focuser board with biscuits.

[swamp thing]

Yeh you'll need a jig to work dovetails with a router. The Leigh one looks good mine is a woodrat one, very similar ideas.

It's pretty straightforward once you've set the jig up TBH. Biscuit jointing the focuser board into position is a very neat way of doing it IMO. You may wish to add a second board for a Telrad/finder of some kind.

[Moonshane]

Dovetails in ply.....(sucks through teeth) you'll have to be careful of bits chippy off if you use a router, if your painting it though it won't matter you can fill it.

well I have to say you were spot on with this prediction. even though I was using a good quality jig, a new trend router bit and a careful hand this was the result! yummy.

We don't have a finger joint jig but in truth I felt that without a really excellent set-up you'd struggle with ply. the traditional method of dovetailing uses the end grain and a test piece on a length of normal timber in the end grain created lovely dovetails. we therefore decided on rebated butt joints. these have good strength with two staggered gluing surfaces and where structure really mattered we added the odd brad with a nail gun. this leaves very small neat holes which are easily filled later.

other than that it's been an enjoyable process so far and over the last week I have made good progress.

the secondary cage has been created but just put together temporarily to check the fit of everything. I need to drill for the secondary spider and it can then be fitted together. I included a small board for finder shoes/bases. debating whether to use a black plastic liner or 1.5mm bendy birch ply for the insides of the secondary cage. will cover the ali bars in black heat shrink. the bars have a wooden dowel inserted (to be glued in with a threaded insert) to allow fixings.

also (almost) finished the mirror box. it has holes drilled for ventilation plus the 80mm fan to be fitted. just need to confirm the height of the filets and corner braces (shaped off cuts from the secondary rings) which also act as a support for the mirror cover. the secondary cage will sit on the cover inside the mirror box for safe transport. the mirror box is about 12" high and this all should help the final balancing.

today I also made the side bearings and rocker box. the bearings are around 15" diameter and should allow good balance with the full size mirror box. despite my comments above about setting the size of the mirror box and everything else on the secondary cage weight (fully loaded) I actually take a different tack for the travel dob as everything needs to fit inside (effectively) one box.

the rocker was constructed again with rebated joints and is high enough to allow 3-4 threaded hole positions for fixing the alt bearings. this will allow different balancing positions without counterweights. it's also big enough for the mirror box to sit inside.

finally, here's a pic with it sitting together pretty much how it will be. I'll make a lid for it and sort out a method to fix it all together in transit. maybe even some wheels on on the end of handles fixed to the rocker box to allow it to be wheeled about like a trolley. you'll see what I mean eventually. in this case, I may have to bolt the alt bearings to the lid rather than sit them inside but it will still be compact at about 14"x16"x16"

that's it for now. will start on the metalwork soon after sorting out the ground board (easy) and tidying up/bit of filling etc. and finishing the secondary cage.

[John]

Thats looking excellent Shane

I wish I could produce stuff like that - I can just about make a bird box or two and even then the birds didn't oblige this year

I can see that David Lukehurst could have some competition soon

[Moonshane]

not sure about that John, but thanks - nice of you to say! hard but rewarding work.

[John]

BTW Shane, I don't know if you have ever read the book "Unsual Telescopes" by Peter L Manly ?

If not, I'd highly recommend you try and get hold of a copy, it's a fascinating book for those interested in telescope design and especially those who like to design and construct their own.

[Moonshane]

cheers John - cool. I'll definitely be on the lookout for a copy - sounds excellent

[Qualia]

By any level of standard or comparison, it looks like you have a first-rate, top-shelf, beautifully executed telescope in making here, Shane. Thanks for sharing the pics and process of an artisan at work here

[8kids]

Sorry about your dovetails, I'm a cabinet maker by trade and they are difficult to achieve in ply, unless you cut them by hand. Rebated butt joints work very well, If I use fixings, screws or some such I usually leave them showing, adds a bit of interest.

It 's nice to see photos of dob builds, it gives the rest of us a little push to do our own, I've already made my second dob base which has similarities ( I stole your design) to one of yours.

[Moonshane]

By any level of standard or comparison, it looks like you have a first-rate, top-shelf, beautifully executed telescope in making here, Shane. Thanks for sharing the pics and process of an artisan at work here

cheers Rob - how nice of you to say so

[Moonshane]

Sorry about your dovetails, I'm a cabinet maker by trade and they are difficult to achieve in ply, unless you cut them by hand. Rebated butt joints work very well, If I use fixings, screws or some such I usually leave them showing, adds a bit of interest.

It 's nice to see photos of dob builds, it gives the rest of us a little push to do our own, I've already made my second dob base which has similarities ( I stole your design) to one of yours.

cheers for the compliment both there and by nicking my design

hope yours works as well as it should. it looks great! I definitely think that rebated butt joints are best with ply and will be using these all the time from now on I think.

[swamp thing]

Looking good Shane. The dovetails are not easy to do. Keeping the dovetails large and the pins small is the way I've found to work best. I also used sacrificial off cuts of gash ply clamped to the ply to be cut. It looks like you've made your way around it anyway. Nice work.