Modifying a large SW Flextube Dobsonian into a mirror box truss tube style Dob.

[tomato]

When I decided to get a scope for visual astronomy I knew I wanted plenty of aperture so it was always going to be a Dobsonian. I had a hankering for a 16" Explore Scientific Ultra Light but with a budget of around £1K I was going to wait forever for one to come up second hand. However I did acquire a used 16" SW Goto Flextube with a broken Syncscan (subsequently fixed, thanks @malc-c) and although the views through it are all that I expected, it is just so darn heavy and bulky to move from the garage and  around the house and into the back garden. It's mounted on a 4 wheel trolley but I still have to dismantle the side gate to get it through!

So I'm thinking it would be a nice project to take the key components (optics, mirror cell, top assembly?) and make a light weight mirror box/truss tube Obsession/David Lukehurst style scope. I am just wondering, has this been tried before?

 

[Mr H in Yorkshire]

In the sense that most DIY scopes are made from individual optical components, yes it's been done. I can recommend reading a lot first, in particular getting a copy of Albert Highe's book, Engineering, Design and Construction of Portable Newtonian Telescopes. If you can't find one let me know and I'll loan you mine.  

[tomato]

Thanks for the offer. I have located a copy of “The Dobsonian Telescope, a practical manual for building large aperture telescopes” by David Kriege and Richard Berry which looks to be a similarly comprehensive guide.

I was just wondering if someone had specifically pulled a flextube apart and gone this route but I suppose that if you are buying new you will pick the design of scope that suits.

Having used the 16” flextube my personal view is that maybe SW should have stopped at the 12” and moved to their Stargate design for the larger apertures.

[tomato]

No feedback on this one so it looks like it will be a pioneering project to turn this:

 

Into this:

I know, I could try and sell the flextube and source a used 16”  ES UL but where is the fun in that? I’ll try and keep the thread updated.😊

[Mr H in Yorkshire]

Hi again, I have both books concerned, my only comment is that while both are well written and detailed, the Albert Highe book is more modern in the sense of ultra low base units. The Kriege and Berry book is still in the era of large boxy base units. There is even a lighter design called a flex-rocker, which may appeal as you are going to want to shed as much weight as you can. https://www.cloudynights.com/topic/268199-my-flex-rockeruc-16/

All the best with your endeavour.

[tomato]

Thanks for the link, that’s a real minimalist design, I was shocked to see the accidental sun scorch damage.😳

I’ll check out the Albert Highe  book, as as you point out the other book is a bit dated.

The idea will be to use as much of the SW hardware as possible, but you are right, it will need to go on a serious weight loss programme.

[Mr H in Yorkshire]

I just idly googled Albert's book, a mere £690 on Abe books, or a real bargain on Amazon, £397. I thought it was steep at £40 and that was second hand! Maybe I'll rent it rather than loan it 😕 I reckon I've got an heirloom for the grandchildren.  PS The offer still stands.

[Captain Scarlet]

This has reminded me of my own (non-) project. I have a 20” mirror-set sitting in a box taunting me about not having embarked on it yet.

The beauty of designing a truss OTA is that if you get it wrong and cannot focus, it’s no disaster: you only need adjust or replace the poles.

M

[Mr H in Yorkshire]

Hi Tomato, here's another one off CN.  https://www.cloudynights.com/topic/743145-roels-16-f4-ultralight-traveldob-propdob-mega/#entry10698873

[tomato]

Thanks for the posts guys, great to see this won’t be the first go at this. 
At the moment I’m proposing to cut the base tube right down, keep the mirror cell and frame it into a box for some altitude cradle bearings. Then make a rocker box out of box tube and alloy sheet a la the ES UL Dob or make one out of the SW boards. I’m  thinking a 6 tube truss to use the existing alloy rings, and replace the steel sheet on the secondary housing with struts to make a lighter cage.

The key objectives are make it lighter and more manoeuvrable to get it into the back garden in one piece while still making it easy to disassemble and transport in the back of the car to a dark site.

 

[tomato]

Even though I said I wouldn't I briefly toyed with the idea of selling the SW 16" and putting the funds towards an ES UL 16". But... I would probably need to put upwards of £1K into the fund to make it happen, and after absorbing the available info it looks like the SW could be put on a diet for less than half that so that's the plan, hopefully this will now become a build progress thread.