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

["Tom Krajci" <tom_krajci@tularosa.net>]

>From: "James Lerch" <jlerch1@tampabay.rr.com>

>> Have I done the math to see how much 16 inches of allthread stretches
when
>> under 10, 20, or 30 pounds of force?  No.

>Neither have I, but I did use this stainless steel cable stretch
calculator:

>http://www.spaceagecontrol.com/calcstre.htm

>Given:
>The mirror weighs 26 pounds
>The cell weighs 10 pounds (rough guess)

>Weight per piece of all thread = 12 lbs
>The length of the all thread = 16 inch
>The 1/4-20 all thread has a solid diameter of 0.2 inch

>Assuming the solid all thread will stretch at least as much as a  7x7
stranded
>rope (probably much less)

>All the above predicts a stretch of 0.0006 inches, adding a 300% safety
margin,
>lets call this 0.002 inch of stretch.

OK, I've done some math on a 1/4 inch steel shaft - using the same
load/length as you did above:
stretch is 0.00015 inches.  Sounds fine by me.

>...B) the length of all thread is already under tension before any weight
of the
>mirror / cell is applied.

That does not change how much the all thread will stretch if additional load
is applied to it.

>C) I can not imagine a flexural hinge that would allow for +/- a 1/4 inch
of
>translation travel, with out altering significantly the centering of the
cell
>structure, while still fitting in the space allotted by the design.

I can!  A large version of the Clement focuser!
http://www.clementfocuser.com/pages/2/index.htm
...or this!
http://overton2.tamu.edu/aset/krajci/compliant-sct-mirror-focuser/compliant-
focuser-square-base.htm

>In use, at 800x with a 60 degree Afov eyepiece, 1/4 turn of any collimation
>screw moves a star from the center of the field of view to the edge.  At
this
>power, no discernable amount of histeresis or backlash was detectable when
fine
>tuning collimation with second person operating the collimation knobs.

I'll buy that.

Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/




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