[ATM] Wire Spider Vibrations

[Jack Swaton @ Starry Host]

Chris,
This is very helpful and you described it well. I'll think carefully about
this. It sounds like the two design goals you found successful are 1) spread
the upper and lower secondary attachment points as far apart as the upper
OTA will allow (up to the dust cover and down to the top of the secondary)
and 2) make the secondary attachment points as far from the optical access
as possible while still being hidden by the secondary.

~Jack

-----Original Message-----
From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf Of
Chris Dalla Piazza
Sent: Tuesday, December 05, 2006 11:09 AM
To: atm at atmlist.net
Subject: Re: [ATM] Wire Spider Vibrations


Jack,

The way that I attacked this problem was to make the wire attachment points
at the secondary as far off the optical axis as possible while still being
hidden by the secondary.

I too started with a screw roughly at and parallel to the optical axis just
like I saw on the Internet.  When I ran into torsional vibrations, I
realized placing the attachment points farther from the axis would create
more rotational inertia to resist the vibrations.

It's sort of like placing your finger on the string to damp the vibration.

My final design was to select a PVC pipe the next size smaller than my
secondary and then fasten the wire to the inside of the pipe, attaching the
secondary to the end of the pipe.  My attachment points were at the ends of
the pipe and one attachment at the telescope tube wall.

In contrast to your design, my triangle was made as wide as I could to also
resist lateral in and out, left and right vibrations.  I used the maximum
length of PVC pipe that I could so that the attachment points on the front
end of the PVC pipe were right at the dust cover and the attachment points
at the rear end of the PVC pipe were right at the secondary mirror.

Before I did this, just touching the scope, refocusing with the motor, or
even a breeze would induce vibrations.  After I did this, I've had none of
that.  I have to physically pluck the wires to get them to induce vibrations
which damp out after a few seconds.

-----Original Message-----
From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf Of
Jack Swaton @ Starry Host
Sent: Tuesday, December 05, 2006 1:37 PM
To: 'ATM List'
Subject: [ATM] Wire Spider Vibrations

My scope is nearing completion but my wire spider produces very rapid
micro-vibrations for minutes. I can't post photos at the moment, but the
basic design is 3-vane with two support wires. That is, for each of the
three wires, I use a piece of wire that starts around a screw on the
secondary mount and then travels to the OTA where it feeds around a cup hook
and then back to the same screw it started from. There is about an inch
between the top wire and the return bottom wire on this screw. The whole
route ends up making a long, narrow triangle between the spider hub and the
OTA. The upper and lower portions of the wire is separated by a piece of
plywood which is actually part of the secondary mount.  Each of the three
wires go out at 120 degree angles to the OTA. 
 
In an attempt to dampen the vibrations, I connected a single wire from the
secondary mount to the OTA. These two wires form a 180 degree line across
the OTA.
 
So I'm really asking what the basic design parameters are for a wire spider.
I've studied the images I've found on the Internet, but I must be missing
the engineering for stability and dampening.
 
Thanks,
Jack
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