----- Original Message -----
From: "Tom Krajci" <tom_krajci@tularosa.net>
> 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.
>
> 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.
Assuming a worse case scenario (only one of the three pieces of all thread
stretches), will result in an angular change of the primary's optical axis of:
invtan(0.002/14.8)=28 arc seconds
0.002 = 3x predicted stretch in inches
14.8 = perpendicular distance in inches between any 1 collimation point and a
line draw between the remaining two.
In addition, there are several factors that make the actual result much better
than the 3x predicted result:
A) the actual length of threaded rod under tension from the weight of the mirror
/ cell is the distance between the top frame member to mirror cell (actual
length closer to 10 inches)
B) the length of all thread is already under tension before any weight of the
mirror / cell is applied.
C) I doubt a solid piece of stainless steel will stretch as much as a 7x7
stranded rope of the same diameter and material
D) ** at any angle of altitude, all three pieces of all thread will equally
support the same amount of weight, and thus stretch the same amount. This will
produce a shift in focus, but not an angular collimation change.
(** changing angle of altitude WILL slightly change the weight seen by each
collimation rod, the Delrin support blocks are slightly offset from the center
of gravity of the cell / mirror when pointed at the horizon)
> 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.
Indeed, but:
A) Delrin on glossy powder coated aluminum results in low friction
B) The amount of hysteresis is limited by the rigidity of the welded 3/4"
aluminum cell structure and the amount of stretch in each collimation point's
threaded rod.
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.
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 was actually surprised by the above result, so much so I am giving serious
thought to motorizing the three collimation knobs and using them as a fine focus
control, with the side benefit of at the eyepiece collimation while alone in the
observing field.
Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)
http://lerch.no-ip.com/ChangFa_Gen (My 15KW generator project)
"Anything that can happen, will happen" -Stephen Pollock from:
"Particle Physics for Non-Physicists: A Tour of the Microcosmos"
" Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent. "
Calvin Coolidge
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