Re: [ATM] Mirror Cells - Springs vs. Push/Pull

["Donald Good" <donald.good@comcast.net>]

Assume a scope with a secondary mounted 4 ft. from the primary and that the
scope is constructed well enough that after the preliminary collimation
steps prior to primary tilt adjustment (which is at zero tilt relative to
the bolts) are completed, the laser beam is off target by 2 in. at the
secondary.  This ratio of 2/48 or 1/24 should be within the capability of
average care in mounting the cell support (what I might consider worst case,
Vladimir) which represents an angle of 2.4 deg.  Superior care may cut this
angle in half.  Whatever the angle, the collimation adjustment to the
primary is 1/2 that angle or 1.2 deg. in this case.  That is the same angle
that occurs at the compliant joint in the bolt and is well within its
capacity.

But that fixed angle is not the source of the possible fatigue failure I
referred to previously.  When the scope is pointed straight up, the
compliant joints are loaded in compression from the weight of the cell and
mirror.  When pointed toward the horizon, the compliant joints are loaded in
sheer.  This repeated transfer of load from compression to sheer and back
during normal operation of the scope will be the source of the fatigue
failure of the compliant joints, eventually, even though the angle change in
the compliant joints is VERY small during this operation.  We can only delay
that failure as long as possible and removing the stress risers is one
method of significantly extending the life of these compliant joints.

When the scope is being constructed, measure the deflection of the bolt tip
when the tip is loaded with a specific load.  Make a note of all the test
parameters so that it can be repeated later.  Maybe attach the note to the
scope near the bolts.  Later when you have the mirror out for resurfacing,
do the test again.  Any increase in deflection means the bolts should be
replaced.

Don

> -----Original Message-----
> From: atm-bounces@atmlist.net 
> [mailto:atm-bounces@atmlist.net] On Behalf Of Tom Krajci
> Sent: Saturday, December 03, 2005 9:16 PM
> To: atm@atmlist.net
> Subject: Re: [ATM] Mirror Cells - Springs vs. Push/Pull
> 
> 
> Thank you for the warning.  However if I use this approach in 
> collimation bolts...and I subject the collimation bolts to 
> relatively little bending (a couple degrees?)...is this 
> fatigue failure less likely to happen?
> 
> Tom Krajci
> Cloudcroft, New Mexico
> http://overton2.tamu.edu/aset/krajci/
> 

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