Truss mount question

[Brian28]

Hi Gents .. what are the advantages and disadvantages of Truss mount scopes .. over solid tube designs ..  

[NGC 1502]

Truss tube :-

Main plus point, makes a larger aperture scope more portable because it breaks down into smaller and lighter parts. 

Main minus  points, more complicated and expensive than a similar sized solid tube, longer set up time.

Solid tube :-

Main plus points, cheaper, simpler, less prone to losing collimation.

Main minus points, usually very heavy with larger sizes and takes up more room for transportation.

HTH Ed.

 

Edited November 22, 2019 by NGC 1502
correction

[Brian28]

thanks NGC .. 

[Louis D]

In addtion to what @NGC 1502 said above:

Truss tube primary mirrors can also cool more quickly because there's no large column of warm air above them as in a solid tube.

Truss tubes are more prone to stray light intrusion and "tube" currents induced by body heat unless a shroud is used.

10 inches (250mm) is about the break-even point between the two designs.

[niallk]

15" truss in my saloon car - takes 5mins to assemble, and an additional minute to collimate   I've a solid tube 10", and the 15" is more convenient to throw into the car.  The 10" is a doddle for just lifting out to the garden though. 

Several members here have some really immpressively compact designs - eg Sumerian scopes.

[domstar]

Sorry to jump in on this thread but what about keeping the mirror clean? I'm concerned I would kick up dust with my feet while observing. Do the truss owners here just plonk it on the ground or place it on a large sheet of plastic? 

[ollypenrice]

I think you're far better with a truss in large apertures because of tube currents and cooldown. I had one here in dry, dusty conditions, on a gravel observing site, without any issues at all. I can't agree that tube Dobs have a tube current advantage over truss designs because they trap the considerable thermal mass of the mirror in the tube. Although people are reluctant to do it I think research shows that fans blowing across the mirror are the most effective for breaking the boundary layer above the glass.

The proper Serrurier truss is also very ingenious because it is designed so as to keep any flexure in the least harmful plane. If it moves it does so while keeping the top and bottom sections parallel with each other.

Ours was never moved once we had it installed and once it had settled down it held collimation unbelievably well.

Olly

[tomato]

And truss tube scopes look really cool, until you put a light shroud on them.

[niallk]

1 hour ago, tomato said:

And truss tube scopes look really cool, until you put a light shroud on them.

...but the views through them are pretty cool 😁

10" vs 15" ~similar f-ratio (4.7 vs 4.5)

[niallk]

To the OP: one 'disadvantage' is that you must be very comfortable collimating your scope - in the dark.  My 10" hardly ever needed adjustment  - it retained collimation ridiculously well.  I collimate my 15" every time, but with the right tool - i use a barlowed laser - it's not an issue.

Edited November 23, 2019 by niallk

[estwing]

If a truss Dob is designed as a lightweight scope but still built with clever strengthening features you can get a lot of scope in a car.

Mapstars 22" truss Dob (seen here) fits into the back of an Audi A6.

[estwing]

my 18"...will fit in any hatchback. 

[DaveS]

A GSO 12" Truss RC comes in at 24 kg, A GSO CF tube 12" RC is 18 kg, and my ODK12 is 19 kg,  so there looks to be a weight advantage to a solid tube design. I'm not sure about the GSO, but the ODK has three fans for ventilation.

[westmarch]

10 minutes ago, DaveS said:

A GSO 12" Truss RC comes in at 24 kg, A GSO CF tube 12" RC is 18 kg, and my ODK12 is 19 kg,  so there looks to be a weight advantage to a solid tube design. I'm not sure about the GSO, but the ODK has three fans for ventilation.

The whole point about a truss Dob is that the individual components split the weight much more than a conventional Dob, as well as taking up less room. So you don’t end up having to do your back in.

John

[DaveS]

9 minutes ago, westmarch said:

The whole point about a truss Dob is that the individual components split the weight much more than a conventional Dob, as well as taking up less room. So you don’t end up having to do your back in.

John

I wasn't talking about dobs. I'm nearly 100% imaging, and looking at the OP's equipment list I'd say imaging was high on his priorities (Sorry if I've misunderstood this) so I was answering from an imaging perspective. The Serrurier Truss does have many advantages, but as Olly implied above, must be correctly designed, and not just to look "cool".

[Merlin66]

In my fifty years of astronomical experience I have never seen a proper amateur Serrurier Truss telescope. Never.

Truss designs are just that truss designs.

 

[Louis D]

5 hours ago, Merlin66 said:

In my fifty years of astronomical experience I have never seen a proper amateur Serrurier Truss telescope. Never.

Truss designs are just that truss designs.

 

True, but there are plenty of commercially made Serrurier truss Ritchey Chretien telescopes in the price range of amateurs.

[Merlin66]

Louis,

I don't believe even those commercial truss designs are actually Serrurier truss designs.

The truss sections need to vary with end load and all the joints to the central section etc. must be "pin joints" not bolted.

(My Bsc (Hons) was "The design and construction of the Issac Newton telescope". I was lucky enough to obtain copies of Mark Serrurier's original design papers and visited Grubb Parsons to discuss the INT design and construction. )

 

Edited November 24, 2019 by Merlin66

[Merlin66]

I see the GSO Ad says:

The trusses are also designed to flex equally, keeping the optics in collimation regardless of the position of the telescope.

I'd like to see the technical data to back that up......

[Whirlwind]

I've heard that for RCs a truss design makes it easier to centre the secondary because you can look directly at it whereas for a solid tube design this isn't possible (except with Superman vision).

[coatesg]

I have a double truss mounted on an Equatorial mount (14" f4.53) - I didn't design it to be a serrurier either, but it's fairly well over engineered! I had trouble with the original Orion aluminium tube - it was too thin and flexed at the focuser with the weight of the camera/focuser, and the tube rings were too weak too.

Here's the build thread: 

Edited November 24, 2019 by coatesg