Re: [ATM] String telescopes: what does it take?

["RodShea" <RodShea@comcast.net>]

Hughes,

I've been looking at a string scope for a while now, and a couple of things 
come to mind.  One of my problems is that I have no background in mechanical 
engineering, so my thinking may be fuzzy, but here goes.

So far as I know, most string scopes have been built with BCY 450 Plus, 
because of its very low stretch and creep (permanent elongation)  I have not 
been able to get good info on the stretchiness of the stuff.  I understand 
the Vectran helps with low stretch.

Did you take into account that there 2 strings that take the downward force, 
in either a 3 or 4 pole design?  Also, the poles themselves, all of them, 
can take up the force as well, if their attachment mechanism is rigid.  If 
this helps, I made a mockup of Dan's pole setup, and found I could generate 
500 lbs of force vertically on the jack pole, by twisting it until it was 
"tight" against a large beam in my house.  (The other end of the jack pole 
was, in effect, on a scale.)

I also think the decollimation is complicated.  If you mean 1/4" movement of 
a laser dot from the center of the mirror as the scope goes from vertical to 
horizontal, then, with the scope horizontal, this would represent the net 
sum of at least 2 things. 1.  The UTA moves vertically, at right angles to 
the primary's optical axis.  In other words, the UTA's optical axis and the 
primary's axis are no longer coincident, but parallel.  And 2.  The UTA 
"leans" so that the UTA's axis is no longer parallel to the mirror's.  Both 
contribute to the net movement of the laser dot.

Rod

----- Original Message ----- 
From: <Hugues.Laroche@ses-astra.com>
To: <atm@atmlist.net>
Sent: Thursday, October 27, 2005 5:57 AM
Subject: [ATM] String telescopes: what does it take?


> 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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> 


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