[ATM] String telescopes: what does it take?

[Hugues.Laroche@ses-astra.com]

Hi folks,

I have been reading with much interest a few web sites about 
string telescopes, in particular this of Dan Gray. <Dan are you there?>
http://www.siderealtechnology.com/28inch/
Interesting evolution towards lighter and ligther OTAs... 

I made an evaluation spreadsheet, but to make things short, here is
an estimation of orders of magnitude: the geometry is not forgiving for
string's stretch. Let us say that for a 28" f/5 you need cables that are
4x28" = 112". Given the angle, a decollimation happening when slewing
the scope from zenith to horizon of 1/4" would roughtly translate into
a maximum stretch 4-5 times smaller in the string, say 1/10". In relative
stretch, that is less than 1/1000, or 0.1%

Dan mentions that for his 28" he is using shooting bow string 
material for his strings.
>From what I read, Kevlar has now intesting alternatives such 
as Dyneema, Technora, Superaram etc... (I have no affiliation
 with any manufacturer/distributor of such materials). 
These enter into  sailing and power kites ropes among others.
Pound for pound they can be several times stronger than
steel, but can stretch a little bit more. For example, King Rope stretches
by about 1% when a force of 25% its breaking limit is applied, and in this
force range the stretch is roughly linear.

So the 0.1% max stretch obtained earlier translates in this case 
to 2.5% max force applied on the cable (always w.r.t. breaking limit).
Now the geometry plays a second time against us, since the force
applied along the cable is coarsely the UTA weight times 4-5.
 For a 28" scope, the
secondary mirror, focuser, 31 mm Nagler :-) and rest of the UTA
give you 5-10 kg. That translates into 20-50 kg along the cable, 
supposed to be less than 2.5% of its breaking force. So the cable
has to be able to withstand near 2 metric tons for this model.

Now what I am thinking is applying this to a 45" f/3.75... will it still
be intersting w.r.t. conventional aluminium truss tubing, or at least
steel cables? There the cable has to be able to withstand
 4-5 tons at least...  if my assumption of 1/4" UTA shift 
is still tolerable... What do you think?

Thanks for any tip/hint, and sorry for the rather long post...

Hugues

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