Solid tube or truss

[Moonshane]

great reply and thanks for the info on the websites of which I was blissfully unaware. all fair comment. just thought I should stick up for the hardworking builders trying to make a living and providing the world with excellent and innovative products. that said, as you correctly point out, they almost certainly don't have the capacity to supply 'the world'.

[michael.h.f.wilkinson]

https://sites.google.com/site/thedobsonfactory3/home

Has anyone over there checked this guy out?.

Looks really nice. Drooling already

[Scooot]

There certainly does seem to be a strong demand. I've just contacted the Dobson Factory by email with a couple of questions and due to the size of their order book the current delivery time  is estimated at 14 months!

[Mike73]

Not sure how long you'd have to wait for a Sumerian but I'm pretty sure you'd get it by the Autumn. 

I've just fitted Nexus to mine and what a nice little luxury that is!

[Charic]

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

Olly.......more studying for me! 

I seem to prefer a solid tube system ( never having tried a flex-tube) and was under the impression, that when we speak of tube currents, were specifically talking about the boundary layer across the face of the primary mirror!

Even with an open tube, surely this needs to be overcome as the mirror cools to ambient, therefore these currents still exist to some degree?

This could change my outlook for buying a flexi-tube

[ollypenrice]

Olly.......more studying for me! 

I seem to prefer a solid tube system ( never having tried a flex-tube) and was under the impression, that when we speak of tube currents, were specifically talking about the boundary layer across the face of the primary mirror!

Even with an open tube, surely this needs to be overcome as the mirror cools to ambient, therefore these currents still exist to some degree?

This could change my outlook for buying a flexi-tube

I don't think tube currents and the boundary layer are the same thing.

From what I've read (and I've also talked at length to optical guru Ralf Ottow about this) the boundary layer is a big problem and fairly easy to solve, but people are just reluctant to solve it. It involves blowing air across the top of the mirror from one side to the other. The assymetry of this arrangement seems to upset many people, when it shouldn't. It was proposed years ago in a Sky and Telescope article as well. Ralf attacked the problem by watercooling his primary mirror to a tad below ambient and then allowing it to warm up to ambient afterwards, after which it stays at ambient.

Let me stress that I've no hands on experience of testing any of this and I use a big Newt only at fairly low powers. (Well, I've observed with Ralf's 12.5 inch F6 watercooled Newt and, like most people who've looked through it, I think it's the best optic I've ever experienced.) It really is ridiculously good. I look forward to trying his home made SCT binoculars one day but they are too big to travel - seriously too big - so I'll have to go up to his place to do that!

Our own 20 inch 'Sir Isaac' has, for the last couple of years, passed into what seems like a spell of total structural stability. He lives outside in a drafty (good thing) roll off throughout the year. The truss is Obsession style, Ralf made the new secondary mirror and did an initial collimation, and now I hardly ever need to collimate at all. It's pretty weird. Night after night, month after month, I pop in the laser and it's right. That is, within the error bars of a laser collimator. Perfectionists can do a star test tweak but, honestly, for low power DS observing is it worth it? We do planetary in other scopes better suited.

Olly

[Charic]

I don't think tube currents and the boundary layer are the same thing.

Olly

.......I didn't think you could  have one without the other? I'd assumed  that any thermals in the OTA are all one and the same!  Any heat from the mirror would rise in the tube, 'tube current' and the OTA itself needs to cool, again, more  thermal currents in the tube,  but  like you said, if you had no tube, then effectively you should have  no currents. But there is still the boundary layer to contend with?

I have spoken about this in another thread, and I had the Draper ready, upgraded to a Dremel,  to create two doors!,  a fan door, and an exit door? I still want to keep the OTA sealed/solid for better internal protection.

I just don't wan't  cut the holes  yet!

I have my flexi-hose/fan/9V battery DIY gadget, working quite well when I need to speed things up a little.

[John]

Interesting article on newtonian thermal issues here which includes the boundary layer:

http://www.garyseronik.com/?q=node/55

[Charic]

...Cheers John. One of the many sites I've read.

When  the scope reaches  ambient,  or better still, stored at ambient,   there should not be too much of a problem.

I have sat and watched, the thermals dissipate, whilst my fan is running, a kaleidoscope in grey! but they do shift!

But back to Scooot's  question. I still favour a solid/closed OTA, although my other half, would prefer not to see any scope at all, fully assembled,  in the house, despite  being shorter than her, so a smaller (height) scope would be advantageous, or stick 'it' in the shed, she says......

[Moonshane]

In larger mirrors an issue re the boundary layer is that the mass of the mirror makes it hard to keep up unaided with temperature changes as the night goes on. This where active cooling really helps.

[Scooot]

.

[Scooot]

...

But back to Scooot's question. I still favour a solid/closed OTA, although my other half, would prefer not to see any scope at all, fully assembled, in the house, despite being shorter than her, so a smaller (height) scope would be advantageous, or stick 'it' in the shed, she says......

Having now had experience of both I prefer the truss for a larger scope, I wouldn't be able to budge the 16" if it was a solid tube. The air can get to the mirror whilst it's cooling, and I can look up to see the secondary without a tube being in the way whilst collimating. I can also adjust the primary from above. It stores into a smaller package and fits easily on the back seat of my car, A solid tube of this size or above needs to be in an observatory in my view.