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

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

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

>>>http://lerch.no-ip.com/atm/Bino_Cell_Sm.jpg  (640*480  @ 108KB)
>>>http://lerch.no-ip.com/atm/Bino_Cell_Md.jpg (1280*960 @ 370KB)
>>>http://lerch.no-ip.com/atm/Bino_Cell_Lrg.jpg (2560*1920 @ 1.2MB)
>>
>> Does this mean the length of 1/4-20 all thread that is under tension (and
>> defining the collimation position) is from the purple frame members to
the
>> black cell members...approximately 10-12 inches?  Does this reduce
stiffness
>> of the collimation screw?

>The all thread is closer to 16 inches long, as it passes thru both the
upper and
>lower purple frame members, while threading into the black cell member in
>between the two frame members.   The tension is provided by the collimation
knob
>on the top frame member, and a second lock nut on the bottom side of the
lower
>frame member (plus some tension from the weight of the mirror and cell)

My concern is that this long length of allthread is the part that constrains
the cell for collimation tip/tilt.  Imagine this scenario.  Take an
allthread that's 100 yards long.  Pull on one end.  You'll probably notice
that it stretches a bit.  It acts like a spring.

Long pieces in tension/compression are more like springs than you want them
to be.  Short pieces that support mirror cells for collimation tip/tilt are
better than long pieces...short pieces are more rigid.

I know you want collimation knobs in a convenient location, but you could
use another approach that puts knobs close to your hands, yet also uses
short lengths of allthread to support/collimate the mirror.  Perhaps you
could transmit/transfer the knob's rotary motion to the allthread with
timing belts/toothed pulleys?  This way the knobs do what knobs do
best...transmit rotary motion, and not try and support/collimate the mirror
with long lengths of allthread.

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

>>>Two L shaped delrin blocks (on the left of the image), constrain lateral
>>> motion
>>>of the cell, and also apply lateral pressure on the threaded collimation
>>> rods.
>>
>> I can see how the two L blocks constrain motion up/down (up/down as seen
in
>> the image), and they constrain motion to the left of the image.  How is
cell
>> motion to the right constrained?

>By two methods,
>A) The bushings the threaded rods pass thru in the upper and lower purple
frame
>members  are offset ~1/8" to the left as seen in the above image.   The two
L
>blocks "Push" the black cell members to the right (again with respect to
the
>image above), so as to center the cell in the OTA frame.  Since the
threaded
>rods are offset an 1/8 to the left, and the L blocks push the cell 1/8" to
the
>right, the result is the threaded rods work like much like a leaf spring in
an
>automobile.

>B) At positions other than zenith, gravity also assists in keeping the L
blocks
>up against the purple guide rails / frame members.

>Of course, if the scope was pointed at zenith, and you kicked the OTA hard
>enough, it would be possible to overcome the spring effect of the threaded
rods.
>So long as the kick wasn't hard enough to perminently bend the threaded
rods,
>the cell would return to its starting point.

Delrin being pushed against frame members...that produces friction.
Friction implies hysteresis/sticking in your collimation motion.  Flexural
hinges would improve that situation be eliminating friction/sticking.

Tom Krajci
Cloudcroft, New Mexico



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