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Old 13-02-2019, 12:56 PM
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The_bluester (Paul)
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Serrurier truss designs

I must have too much time on my hands, I keep idly looking at hardware that my AP journey might bring in to my hands next. I am without a doubt up against some design compromises with my C925, which was bought as a purely visual scope and which I have been cajoling into service for AP.

I keep looking at the CF truss GSO RC scopes and thinking they do not look right to me and would have potential flexure issues. The theory behind a Serrurier truss being a central structural section where the scope is mounted (Dovetail for instance) and then the truss tubes supporting structures to mount the primary and secondary mirrors and other hardware.

Are the GSO truss tube scopes really not a Serrurier design, just a truss tube design with the primary assembly basically being reinforced by the truss? They won't have equal flexure on the primary and secondary mirror assemblies will they?
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Old 13-02-2019, 01:10 PM
Wavytone (Nick)
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The principle behind the serrurier truss is that the poles have to be designed such that as the telescope tilts at odd angles, the top-end and bottom-end both sag by the same amount with respect to the middle section supporting them, so that the alignment of the telescope is maintained and there is no lateral displacement of the top-end with respect to the bottom-end.

On anything less than a 20" scope however the truss is invariably so rigid that any sag/flexure is non-existent, and the truss is more about saving weight (compared to a continuous rigid metal tube) and portability (large scope disassembles to fit in car).

So in the GSO efforts (and most amateur efforts) are just "trusses" - not really Serrurier trusses - because they are made from stock tubes, with no real mechanical engineering analysis regarding sag/flexure/lateral displacement.
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Old 13-02-2019, 01:13 PM
N1 (Mirko)
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G'day Paul, where does it say that these scopes were serrurier truss designs?
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Old 13-02-2019, 01:16 PM
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AstroJunk (Jonathan)
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The Serrurier truss was designed to allow for lighter tube assemblies to be utilised with very large professional telescopes where the weight of materials required to make a fully rigid truss or tube would have been too onerous.

With piddling little amateur scopes, the physics of flexure are much less demanding - even with my 20" Dob, there is no discernible deflection in the position of the OTA at any position. I think the CF trusses on the RC will be more than adequate.
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Old 13-02-2019, 01:28 PM
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Quote:
Originally Posted by N1 View Post
G'day Paul, where does it say that these scopes were serrurier truss designs?
Pretty well everywhere they are advertised. Presumably taken direct from the GSO blurb about them, to quote from one page (which I am not going to identify as I am not having a crack at the sellers here, just curious to confirm my own understanding)

Quote:
A series of CNC-machined heaxagonal aluminium support rings form the basic structure of the optical tube. Carbon fibre tubes connect the support rings in a Serrurier truss design.
I think posts prior have answered my question, they are not really a Serrurier design, just a truss design used as a standard tube would be getting unwieldy and heavy in scopes in the 10/12/16" range.

I asked the question given the reputation of the RC design for being really touchy about collimation being "just so" where it looks to me like these designs could mean collimation differences depending on where in the sky they are pointed. Happy to be told that in scopes this size the rigidity versus scope weight means it is not significant.
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Old 13-02-2019, 02:23 PM
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Merlin66 (Ken)
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As suggested, these are not Serrurier truss designs...

I did my Hons B.Sc at Strathcylde Uni and was lucky enough to be allowed to do my final year paper on "The design and construction of the Issac Newton Telescope"
This involved visiting Grubb Parsons at Newcastle and talking to various staff as well as spending time in their design and manufacturing areas.

Most of their larger telescopes at the time were Serrurier truss designs and I was impressed with the extent of the calculations used in the design. I'm sure nowadays it's much easier with computers.
I also have copies of papers by Mark Serrurier discussing the truss design as well as Sir Barnes Wallis' "limp dick" alternative.
Those were the days......
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Old 13-02-2019, 05:34 PM
Wavytone (Nick)
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Ken, for amateur scopes I'd have to say trusses do have a role - both as a significant weight reduction compared to a rigid old-school tube, and as a means to make a large scope both portable and very compact.

The big downside is baffling. An old-school newtonian with a full-length solid tube has a lot going for it, in this respect.
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Old 13-02-2019, 07:55 PM
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Quote:
Originally Posted by Merlin66 View Post
This involved visiting Grubb Parsons at Newcastle and talking to various staff as well as spending time in their design and manufacturing areas.
And here's the chief optician at his favourite scope near Newcastle. David Sinden was my astronomical mentor as a kid and ran a mirror making workshop every Saturday! He was also the boss of a young upstart called Fred Watson
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Old 13-02-2019, 10:05 PM
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I do love the tangents threads can go off on sometimes. I got my answer, but it is interesting to see some of the history of this stuff popping up.
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