Converting a SW 16” Flextube Dobsonian to a Truss tube Dob.

[tomato]

Much as I like my 16" SW Flextube, it should be called "the Leviathan of Shrewsbury" given how much effort is required to get it out and observe with it. So armed with a copy of "The Dobsonian Telescope" and @Tomatobro's workshop, skills and toolset, we have embarked on the project to convert it to a light weight, compact and easily portable truss tube design.

The aim will be to utilise as much of the SW assembly as possible, with a view to being able to reassemble it to it's original configuration, if that's possible. To that end we will not be making a new primary mirror cell, the simple, but effective SW cell will be mounted in a new mirror box. TBH, the ribbed conical underside of the mirror would not easily lend itself to be mounted on a traditional 9 or 18 point cell anyway.

An 8' x 4' x 18mm sheet of quality marine plywood has been purchased,  I will try and make regular posts (with pics) as the project progresses. 

Edited May 19 by tomato
Title changed

[Piero]

Good luck with this project!

However, why don't you keep the skywatcher mount as it is so that you have a telescope while you build an obsession clone telescope? You would gain in everything. BTW, I made two Kriege-based mirror cells with no prior experience. I'd make one if I were you. 

[tomato]

I’m an imager first and foremost, and very much an infrequent, casual observer. I decided, maybe wrongly, that aperture is everything for viewing galaxies, my preferred targets, hence the 16” mirror.  I bought the first large second hand Dob that came up within my budget, and that was the flextube. It’s just too darn heavy and bulky to move round the house out of the garage and set up each time, so that, along with the cr*p UK weather means it seldom gets used, so not having the scope while converting it is no hardship. We would have no problem making a Kriege type mirror cell, but it wouldn’t work with the Skywatcher mirror design.

My other option would have been to try and sell the flextube and then purchase a light weight truss Dobsonian, but where is the fun in that?😉

[Piero]

Yeah, what I meant was to build the entire telescope, not just the mirror cell.

[tomato]

Work has started, secondary cage rings cut and brackets made to take the SW spider. It would appear that the secondary cage has been clouted at some point in the past but more on that later.

The base board to mount the existing SW mirror support has also been cut.

[Alan White]

Nice to see a started project.
I am sure you and tomatobro will produce a fine beast.

Your woodworking excels my own already, it looks good and no blood anywhere in sight.
Wood butchery and finger butchery are my trademark.

[tomato]

Thanks, we learnt long ago that having the proper tools for the job makes a huge difference. There was a well renowned Hot Rod builder in the US that claimed you could build  one of his incredible creations with just a belt sander but I think there was a bit of poetic licence in that statement.

[Tomatobro]

41 minutes ago, Alan White said:

no blood anywhere in sight

Getting the three cross head screws out of the support frame resulted in one skinned knuckle. They were so tight!.  Evidence of thread Loc.

[Alan White]

Its a proper job if it involves a skinned knuckle 😉

[NGC 1502]

Following with interest👍

Ed.

[tomato]

Secondary cage rings now bolted together, a nice sheet of black 1mm thick HIPS has been delivered from the Sheet Plastics Store to make the light baffle. We have spent sometime today making the focuser mounting plate, went for alloy rather wood, lighter and more precise to work with.

Soon be time to order the truss tubing...

 

This was the damage found to one of the SW spider fixing screws, looks like the original secondary cage has hit something in the past, denting it out of round and causing the damage. A new split screw has been made.

[Coco]

Plywood splinters are unpleasant 😡 great project I might have to copy your ideas with my solid tube 12 inch dob!

[tomato]

Progress has been delayed by Tomatobro’s pillar drill breaking, the return spring broke. After attempting to remake the end of the spring without success, the decision was to engineer another solution rather than incur a lengthy delay trying to source a replacement spring for such a vintage piece of equipment. A pulley and counter weight arrangement has been designed, fabricated and installed, the CW moves up and down inside a piece of plastic drainpipe under the work bench, Fred Dibnah would be proud.😊 

 

So back to the scope build, the original SW dual speed focuser has now been mounted on the secondary cage:

The extended collimation and locking screws have been fabricated and the SW mirror support frame mounted on the base board:

[Alan White]

Nice progress
I do like the inventiveness of the drill fix 👍👍

[tomato]

Thanks. I have been researching the pillar drill. The makers, Kerry Engineering are still in business, or at least the name is still being used. Tomatobro’s example, the Drill Master was their top of the range model, it is unusual in having a full 5 inches of quill travel, hence the hefty spring.

[Piero]

Nice to see this project moving forward.

However, I feel like the title of this thread is misleading because your project is really about transforming a SW dobson into a truss dobson. The design presented here is substantially different from the Obsession design (as well as any other professional telescope maker) from an engineering standpoint.

Edited May 18 by Piero

[tomato]

I grant you the primary mirror cell design is substantially different, due to the design of the SW primary mirror, but the secondary cage, truss tubes and clamps, altitude bearings, rocker box and mirror box are all following the designs described in “The Dobsonian Telescope” by David Kriege and Richard Berry. Hence I don’t agree that the thread title is misleading. 

[Piero]

I think you meant "the design of the primary mirror cell" because there isn't anything specific in the design of a primary mirror in terms of shape.

Your UTA is different because the secondary holder is assembled on the ring rather than the aluminium struts which can affect collimation at different altitudes. The same can be true for the installation of the upper truss attachments on the lower ring and in fact evolutions of the obsession telescope (still truss dobsonians though!) have departed from this design.

Besides, in your case one spider is right above the focuser light path. You also chose 18mm thickness plywood for the UTA - there is a lot of discussion in that book about this topic (wood selection, keeping the UTA as light as possible, number of plies, etc).

The mirror cell is a key component of the Kriege development, also because it's an entirely open design and there are reasons for that. There are a lot of thoughts behind that cell..

Talking about granting, I grant that you have been making a truss dobsonian from a skywatcher dob. However the fact that it is a truss, doesn't mean that it is an obsession-like telescope. There is far more than just using trusses in that book, but it looks like to me that you spent very little time reading it because otherwise you would have applied many and many of the engineering ideas reported in it. It is not my business how you built your telescope - in fact to me you can even add wings to it if that makes it more transportable to you. My point is that your pictures do not show an application of his design which includes his engineering choices and principles. You are simply building based on your own design choices.

Therefore the title here is misleading and quite a lot also. If you had a little modesty and most importantly consideration for the readers of this thread who do not have that book at hands, you would change it, because they could actually think that a truss dobsonian is like an obsession dobsonian and that isn't the case at all, in the same way as not every car is a BMW or a Jaguar just because it has 4 wheels.

---

For what is worth a visible improvement to your "skywatcher"-based truss dobsonian is to replace the original springs with sturdy ones and remove the locking screws. This is a well known and quick improvement that can be found in almost any astronomy forums.

[Stu]

Gents, can we please play nicely and be courteous. There is clearly a difference of opinion here, but that is all it is. No need for an argument so please let’s drop the ‘is it an obsession copy or not’ debate and just enjoy seeing the build, in whatever form it takes. Members can see themselves how it is put together.

Cheers,

Stu

[Carbon Brush]

Another one here following with interest.
I was thinking about parting with my (rather bulky) 12" solid tube. But I see an interesting alternative.

At risk of incurring wrath....
the CW moves up and down inside a piece of plastic drainpipe under the work bench, Fred Dibnah would be proud.😊
Sorry I disagree. Fred would have used a sash window lead weight inside a cast iron downpipe🤣
The (smaller Meddings) pillar drill we have at work had a broken return spring last year.
As it is only about 45 years old, Messrs Meddings provided a replacement!
But next time it breaks, I will know what to do😃
 

[tomato]

1 hour ago, Stu said:

Gents, can we please play nicely and be courteous. There is clearly a difference of opinion here, but that is all it is. No need for an argument so please let’s drop the ‘is it an obsession copy or not’ debate and just enjoy seeing the build, in whatever form it takes. Members can see themselves how it is put together.

Cheers,

Stu

Yes, Apologies if it was getting contentious, that’s definitely not in the spirit of the Sidewalk Astronomer. I’ve  removed the offending word from the title, we’ll  just keep on building it the way we want.😎

[tomato]

43 minutes ago, Carbon Brush said:

Another one here following with interest.
I was thinking about parting with my (rather bulky) 12" solid tube. But I see an interesting alternative.

At risk of incurring wrath....
the CW moves up and down inside a piece of plastic drainpipe under the work bench, Fred Dibnah would be proud.😊
Sorry I disagree. Fred would have used a sash window lead weight inside a cast iron downpipe🤣
The (smaller Meddings) pillar drill we have at work had a broken return spring last year.
As it is only about 45 years old, Messrs Meddings provided a replacement!
But next time it breaks, I will know what to do😃
 

I think the CW might be cast iron…

[Tomatobro]

I purchased the bench drill second hand about 35 years ago and this was the first quill return spring failure. I tried to find one to replace it but they were all too small so I took the spring out of the frame and gently heated the centre strip to take out the temper and bend a new centre tang. Getting the spring back was a challenge in itself. I installed it, put some tension on and would down the quill and the spring broke.

And so the pulley and counterweight system was  born. I guess that has invalidated the warranty.

[tomato]

Another update, the mirror box has been made, the tube clamping arrangements designed and made. Some keenly priced titanium threaded rod has been sourced for the top tube clamps, to keep the top end weight down.