Re: [ATM] Compensating sag of the truss frame by flexibility in the mirror cell

["Roy Diffrient" <diffrent@bcpl.net>]

Robert,

Don't forget that the Serrurier truss is based on parallelogram action at 
both ends, so the optics stay in alignment as both ends of the structure sag 
by an equal amount.  Looks like your idea is based on a slight tilt of the 
primary to compensate top-end flexure -- But tilt of the optics can cause 
decollimation.  If the truss supporting the secondary sags, and the primary 
is also tilted, how can the telescope still be in perfect collimation?  Or 
have I missed something?

Enjoyed the web site.

Roy Diffrient
Monkton, MD
USA

______________________________________________________________________________

Date: Tue, 27 Feb 2007 13:23:50 +0100
From: "Robert Houdart" <robert.houdart@cruxis.be>
Subject: [ATM] Compensating sag of the truss frame by flexibility in
the mirror cell
To: <atm@atmlist.net>
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While designing some mechanical aspects of the future 110 cm scope, I've
stumbled on an interesting concept.

Consider the following two elements:

1) A reasonably flexible truss frame
By "reasonably" flexible I mean that the upper cage will sag about 0.3
mm (0.012 inch) when pointing at the horizon.
This sag that by itself should be perfectly acceptable (the upper cage
does not rotate), and it's hard to make a 3.7 m long single-stage truss
frame a lot stiffer without increasing its weight beyond portability.

2) A mirror cell on reasonably flexible support beams
Once again, "reasonably" means that we're talking about deformations of
about 0.13 mm (0.005 inch).
An interesting element is that the two support beams (one carrying a
single support point in its center, the other carrying two support
points off-center) do not behave exactly in the same way. Because of
this different support stiffness, the mirror will be tilted very
slightly when moving the telescope from zenith to horizon.

Now here's the "trick":
By appropriately choosing the properties of the mirror cell support
beams, it is perfectly possible to compensate the 0.3 mm sag of the
upper cage by tilting the mirror on its support beams. In other words,
the tiny deformations of the mirror cell will make the focal point
follow the sag of the upper cage.

This is basically the same concept as used in a true Serrurier truss
design: have the flexibility of two elements in the OTA compensate each
other.

You can find the computational details on the following pages:

Mirror Cell: http://www.cruxis.com/scope/scope1100_mirrorcell.htm
Truss frame: http://www.cruxis.com/scope/scope1100_trusstubes.htm

Cheers,
Robert Houdart


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