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

["James Lerch" <jlerch1@tampabay.rr.com>]

----- Original Message ----- 
From: "Tom Krajci" <tom_krajci@tularosa.net>


> >From: "James Lerch" <jlerch1@tampabay.rr.com>
>
>>...here is my latest version of
>>the mirror support / collimation method I use on each primary of my 16"
>>bino-scope.  (If pointed at the horizon, left would be downward in this
> >image)
>
>>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)

>>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.

>>At each end of each bar is a 1/4" universal socket extension, which
>> provides the
>>mechanical hinges for tip and tilt of the three support points, while
>>constraining axial rotation of the support points.   In hind-site, a single
>>nylon bolt to replace the 1/4" universal socket extension would have made a
>> much
>>better hinge for the tip and tilt of the three support points.
>
> Could you have made the flotation part of this cell 'lower profile'?  (Does
> the higher profile required for the universal socket extension produce any
> problems in cell performance?)

As-is the cell over all height is about 1 1/2 inches, with the back of the 
mirror ~1/4" above the top-side of the black cell framing members.

The first incarnation of the mirror support was a 6 point configuration.  The 
three bars of the support system were attached to the top side of the black cell 
members.

When I "up-graded" to an 18 point cell, I wanted to keep the black cell member 
and mirror in the same physical positions with respect to the OTA.  The only way 
to achieve this was with the "Under and Over" approach where the 3 bars were 
located under the cell frame, and the six three point load spreaders were above 
the cell frame.

The "Under and Over" approach satisfied my desire to keep the cell frame and 
mirror in the same relative location with respect to the OTA, but did 
necessitate for the 1" long profile of the Tip/Tilt mechanism for each of the 
three point load spreaders.  The universal socket extension just happened to 
provide the needed function, have the desired length, and were on sale at 
Harbour Freight for less than a buck a piece..

Oh, and for the record, I put quotes around my comment of "up-graded" because in 
the end, I saw no discernable difference between the 6 and 18 point cell 
arrangement at 800x during the star test.  Nor was any difference noted under 
any other view conditions.

The only gain the "up-grade" provided was the end of fielding the question "Why 
a six point cell, wouldn't an 18 point cell be better?"   :-)

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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