Solid tube or truss

[Scooot]

Had a bit of aperture fever lately and been wondering about a larger scope. I don't want to use a ladder so I think a 16" is what I'll end up going for, maybe.

I really like the OO scopes (anyone know the eyepiece height at the zenith) although they're pricey, and I like the fact it's a solid tube. I don't really travel with it so portability from that viewpoint isn't that important.

I'd guess a solid tube is better for protection from dew and stray light, but are there other factors other than portability I should consider.

[russ.will]

If you're not planning to travel and are happy lifting the weight of a solid tube (OOUK tend to be lighter than average as their tubes are aluminium) then the only issue that springs to my mind, is that the open structure of a truss tends to be affected less by wind.

Of course, if light pollution is an issue, then you need a shroud and then you're no better off than a solid tube in a breeze.

Russell

[Scooot]

Duplicate post - Mods: please delete.

Russell

Many thanks, hadn't thought about the wind. So from my perspective a solid tube is probably better.

[ollypenrice]

Personally I don't think so. The advantage of a good truss is that it is stiffer, particularly in respect of keeping the deformations harmless. The Serrurier truss tends to keep the top and bottom planes of the scope parallel with each other and this is the main thing. Indeed it's the whole point of the design. Then there's cooldown. If you don't have a tube you won't get tube currents. (OK a cheap point but it contains, I think, some useful truth.) Cooling big tube scopes can be a royal pain. Our half metre Dob gets mighty hot in summer but is as good as gold for cooling down at night. I'm a bit of a fan of the Serrurier Truss. It is cleverer than it may appear. Worth a bit of Googling.

Olly

[Scooot]

Hmm I find it difficult.to grasp the point that the truss is stiffer and more stable than a solid tube but I accept what you say. Cool down, yep follow that. Decisions ?

[Caldwell14]

Another vote for a OO Newtonian, a VX10 or VX10L

[John]

On the cool down issue, I've often wondered if an open truss design is vulnerable to warm air currents getting into the light path from the observer. The normal human body temperature is 98.6F and, despite good insulating clothing, some of this could radiate into the light path as we stand right next to it when using a dob ?

[IanWatkins]

John,

Not in our climes. Body heat wouldn't affect an open truss as much the residual heat of a closed tube. Once temps get down in the minus 20 C, maybe. Well, anything at that point.

Cheers

Ian

[jetstream]

Rumour has it that a truss dob with no shroud is susceptible to strange thermals,from boundary layer to the situation described by John.Apparently leaving the boundary cooling fans on (after cooling) can help things if no vibrations and the mirror box is properly designed.

Personally I would choose a truss at 16" and I like the fact that the mirror can be cleaned while still in the mirror box

[faulksy]

eyepiece height is the same as my 14" richard, im 5-10" and its perfect would be good if your a couple of inchs smaller as well

[estwing]

Once you've used a truss dob you won't believe how easy they are to move/transport/assemble...and that's it, why wouldn't you want to take that 16" to nice dark skies! A solid tube dob get very heavy after 14" I can still get mine out the back of a car...on my own

[Beulah]

Portability is a huge factor in larger telescopes.

One of the disadvantages of a truss system is the collimation being quite out - with a solid tube of the same size, collimation can either hold or budge slightly. The cure to that problem is simply patience.

I can see why a truss system would be more rigid than a solid tube system; you would be surprised at how 'floppy' a larger solid tube is when disassembled - essentially the same sheet metal gauge than telescopes of smaller aperture but having to bear the weight of a larger focusser and secondary assembly.

As already mentioned, considering the storage of larger scopes is also important. It is amazing how much space a telescope can take up in a house, especially when wanting to store the tube in a horizontal position after a session (dew on the primary with change of temperature).

From reading this forum over many years, it seems apparent that scopes of 12" and smaller, solid optical tube assemblies are preferred; truss tube versions for larger apertures of 14" and above.

[Mike73]

As long as you don't have steps at home a large truss dob with wheel barrow handles is about as portable and practical as it gets, perfect example is Calvins 18".

At home you can just wheel it out of your shed, quick collimate and start observing but you can also break it down to make it easily manageable for when you do want to take it to a dark site and its those dark site trips that really let these large apertures shine.

When I first got my truss dob I had every intention to use it at home and dark sites but even though set up time is only 10 minutes it still became a bit of a chore just for quick sessions from home, a different design with wheel barrow handles would of made all the difference.

[Mike73]

Oh and one other thing Richard, if you have a 16" scope in mind and you want to carry on using your EQ platform then ideally you do need to be looking at f/4. I tried my 16" f/4.5 on my platform for the first time the other night and it just makes the EP height a little too high.

[estwing]

Sound advise from Mike73...

[Moonshane]

I have the 16" f4 oouk dob albeit on a home made base. in use at home it's easy to move with a sack truck (I can but would not recommend carrying it more than a few feet on flat ground). it's a superb scope and if you are not putting it in a vehicle it is well recommended. that said if I remove my front seat I can get it, a 6" f5 and all my camping gear (a lot of it) in my small hatchback (a Daewoo Lanos - like a Rover 200). that said again, I am in the process of converting it to a truss system as this will make life easier for travel. I may then dispose of the solid tube system assuming the truss works as well which it should. the 16" is a substantial step up in bulk from the 14" even though it's the same length.

[Paul73]

How about the Skywatcher Flextube 16" offering??

Best, or worst,of both worlds?

One thing for sure is that it will be no lightweight! I have the 10" version and love the quick set up and ridgedity of the whole thing.

Paul

[swamp thing]

One of the disadvantages of a truss system is the collimation being quite out - with a solid tube of the same size, collimation can either hold or budge slightly. The cure to that problem is simply patience..

Depends on circumstance more than design Sam. A solid tube bounced about in the back of a pickup on route to a dark site will be further out than a truss Dob wheeled out of a shed. We also have to consider the quality of the mirror cell. Most budget truss Dobs will not hold collimation as well as an expensive OO solid tube, but buy a premium truss Dob......you'll find they hold collimation just as well as any OO offerings.

Eg my 20" home built scope holds collimation way better than my old 16" LB. This shouldn't be possible a 20" f/4 is way bigger, heavier and more likely to move than a 16" f/4.5 surely. It's down to the primary cell.

[John]

.... my 20" home built scope holds collimation way better than my old 16" LB. This shouldn't be possible a 20" f/4 is way bigger, heavier and more likely to move than a 16" f/4.5 surely. It's down to the primary cell.

The mirror cell on my 12" Meade Lightbridge was built from quite thin metal. I'm sure the elements of the cell used to flex as I moved the scope around. Using the locking screws certainly caused it to flex so I didn't use those.

The cell on my 12" Orion Optics is much more sturdy despite being a simpler design. When I bought the scope the 60 miles or so to the SGL star party this year it was laying on it's side in the back of the car and I must have hit dozens of pot holes during the journey, and then there are the speed ramps on the entry road to the camp site, but when I put the tube on the mount the collimation was still spot on. Hats off to Orion Optics for that design   

[ollypenrice]

Why is a truss stiff? Because it uses a system of four triangles and triangles are astonishing structures with formidable integrity.

Then there's the way it deforms. The Serrurier truss was designed because its inventor realized that making a totally non deformable structure was impossible. So if it is inevitably going to deform, what is the least harmful deformation from the optical point of view? When a Serrurier deforms (imagine it at an altitude of 0 degrees, ie parallel with the ground) the secondary support will 'droop' but, critically, will remain parallel with the primary. That's the point of the design.

Any old truss won't do. It should be a Serrurier. It was invented by Mark Serrurier for the 200 inch Palomar. No wonder it is popular! http://en.wikipedia.org/wiki/Serrurier_truss  I seem to think that I read somewhere about the engineer ending his days in some kind of run down trailer park, but I can't be sure.

Olly

[Beulah]

Depends on circumstance more than design Sam. A solid tube bounced about in the back of a pickup on route to a dark site will be further out than a truss Dob wheeled out of a shed. We also have to consider the quality of the mirror cell. Most budget truss Dobs will not hold collimation as well as an expensive OO solid tube, but buy a premium truss Dob......you'll find they hold collimation just as well as any OO offerings.

Eg my 20" home built scope holds collimation way better than my old 16" LB. This shouldn't be possible a 20" f/4 is way bigger, heavier and more likely to move than a 16" f/4.5 surely. It's down to the primary cell.

Thank you for you post, Steve, it highlights apparent issues with the Skywatcher Flextube.

The secondary seems to be out of collimation by a lot but the primary often tends to be out by only a smidge as the mirror cell seems to be very well made (collimation holds very well with the 300p).  I think this is due to the sliding trusses as there is a wee bit of play before locking them into position. I have been seriously thinking about rebuilding the dob according to the Serrurier design for a long time now. I must search the archives and take a look at your 20" project. 

[scarp15]

Depends on circumstance more than design Sam. A solid tube bounced about in the back of a pickup on route to a dark site will be further out than a truss Dob wheeled out of a shed. We also have to consider the quality of the mirror cell. Most budget truss Dobs will not hold collimation as well as an expensive OO solid tube, but buy a premium truss Dob......you'll find they hold collimation just as well as any OO offerings.

That can certainly be true. I get round this by transporting my 14" solid tube scope nestled into two chunky molded package foam from a former scope. This helps to absorb road bumps and thus retain collimation (which is checked and tweaked if possible anyhow on arrival).

There is a lot of talk at the moment concerning the Skywatcher truss 18". Information as to when this is available in the UK may become clearer by contacting Optical Vision Limited. This being Skywatcher, I would expect that a third party dedicated shroud will in due course be available to.

[Scooot]

Thanks everyone, lots of very interesting comments.

I've had my scope for about 2.5 years and in that time I think I've put it in the car twice, which is why I didn't think portability from that point of view is much of an issue for me. I'd also intend to store it in either my garage or shed which are both very close to where I normally view, and use wheelbarrow handles to move it. So even if I bought a truss I'd store it made up. I follow the collimation issues but as above it wouldn't be much of an issue for me, and I now understand why the truss is stronger (thank you Olly).

I think OOUK scopes are well made so perhaps the strength of the design comparison isn't quite so important in this case so for me it comes down to cool down time and the dew factor. As I'm storing it in a shed or garage presumably that will aid cool down. I had the opportunity to use a 20" truss dob for a few days whilst on holiday in September last year. We seemed to be constantly using a hair dryer to combat the dew throughout the night, and this is what's mainly put me off the truss design.

I also wonder why OOUK have made their 16" as a solid tube if a truss would have been better. Presumably a truss is cheaper to make, could be wrong about this. In fact why aren't smaller scopes made in the truss design?

Anyway lots of things to consider for when the temptation gets the better of me. -

[swamp thing]

I also wonder why OOUK have made their 16" as a solid tube if a truss would have been better. Presumably a truss is cheaper to make, could be wrong about this.

A truss is more expensive to produce than a solid tube. That's why OO use solid tubes.

[Scooot]

Think I'm swaying towards a truss now. -