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Tunable Truss Flex Compensation

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#1 Roy Diffrient

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Posted 18 November 2019 - 01:04 PM

Hello All,

 

First post here -- be gentle, still figuring this out.  I'm new to CN but not really a newbie.  Just finally catching up and finding time to do this after spending lots of time on my 28" f/3.6 Newtonian.  Some of you may have previously seen or heard of this telescope -- it was at Stellafane in 2018 and won several awards, including one for the subject capability ( https://stellafane.o...018-scopes.html ).  There is also an article on its truss flex compensation in the current (Fall 2019) issue of Amateur Astronomy magazine ( http://amateurastronomy.com/ ).  I'd like to take the opportunity to briefly describe it and how it works.  The attached drawing will help.

 

Basically, it does the same thing as a Serrurier truss -- that is, compensate for truss flexure while keeping the optics aligned,  But as implemented it only works for alt-az mounts -- the Serrurier truss of course also works for equatorial mounts.  It uses components that may already be present -- a separate primary mirror cell frame, and the collimation bolts, plus a rectangular aluminum tube in the mirror box -- to generate the compensation.  So it's a simple mechanism with no electronics.  Also, it's there and functioning all the time, without switching it on, just as is truss flexure.

 

Here's how it works: at low altitude angles, most of the weight of the primary mirror and cell rests on part of the bottom of the mirror box, which functions as a flexural beam.  The beam flexure allows the cell to move slightly.  The amount of flex exactly equals, and thus compensates for, (typically) the calculated amount of truss flexure.  At higher altitude angles, most of the cell weight hangs on the collimation bolts.  These bolts extend between brackets on the front of the mirror box and the cell frame, and function as a parallelogram linkage, similar to the action of the Serrurier truss.  In between high and low, support of the cell weight automatically divides between the beam and the collimation bolts, with the result that the beam flex inherently tracks the changing value of truss flex.

 

Also, my telescope is unique, so far as I know, in being able to easily measure and adjust the amount of truss flex compensation.  This is easily done because the entire mechanism is within the bottom end of the telescope.  I use a feeler gauge to measure the flex, as shown on the drawing.  Adjustment is accomplished by changing where on the beam the cell weight is applied.  I use two small rollers, clamped to the cell, to do this.  Move the rollers closer to the box corners to reduce beam flex or move them toward the center to increase it.

 

The graph included shows the calculated flex of a simply-supported beam versus weight position, using the standard mechanical engineering formula shown.  My welded-in beam is somewhat stiffer -- its flexure is 0.010" at 5.6" from the corners.  My designed range of adjustment is 0.005" to 0.020", and this range requires about 4" of adjustment of each roller.

 

This has been working well and I have not had to readjust it for over a year now.  I keep checking, but it is essentially becoming "set and forget".  The only relevant changes I've made have been for transport in my cargo trailer (with the truss disassembled of course).  To keep things adjusted, assembled and undamaged I added scope tie-downs, clamps on the mirror cell, and cushioned stops above the primary.  I'm still looking but so far I have seen no truss flexure effects.

 

Note that this method does not do perfect collimation for you -- that's still up to you.  It just prevents added decollimation from the truss during use at varying altitude angles.

 

Roy Diffrient

 

 

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#2 Roy Diffrient

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Posted 18 November 2019 - 01:28 PM

Here's my telescope, with my very understanding wife doing the modeling.

 

Roy Diffrient

 

 

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#3 macdonjh

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Posted 18 November 2019 - 01:46 PM

Roy Diffrient, it's cool you've getten so detailed with your design.  Have you done the calculations to determine if changing 1-1/4" orthoscopics for 31mm Naglers will change the deflection of your truss enough to warrant changing the adjustment of your mirror cell?



#4 Roy Diffrient

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Posted 18 November 2019 - 02:05 PM

Good question, Aurora.  Let's see, that might change the truss flex by a thousandth of an inch or less.  I'm sure that would be invisible.  And my seeing certainly doesn't get that good.

 

Roy Diffrient



#5 macdonjh

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Posted 18 November 2019 - 02:12 PM

Good question, Aurora.  Let's see, that might change the truss flex by a thousandth of an inch or less.  I'm sure that would be invisible.  And my seeing certainly doesn't get that good.

 

Roy Diffrient

That's what I think, too, but then, I haven't done the math.  My only real reason for asking is the change in moment between a small ortho and a 31mm Nagler would be pretty big, since the truss is so long.  My guess is the deflection wouldn't be enough to move your laser beam outside your primary center spot.

 

I hope I haven't given you something to obsess about.  I'll be in trouble with your wife.

 

Between your adjustable mirror cell and Oberon's hexapod collimation adjustment, Dobsonians are getting pretty sophisticated.


Edited by macdonjh, 18 November 2019 - 02:13 PM.


#6 mark cowan

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Posted 18 November 2019 - 06:20 PM

Interesting - thanks for posting!



#7 MitchAlsup

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Posted 18 November 2019 - 06:32 PM

I explained how to use the edge supports to compensate for trus flexure in::

 

https://www.cloudyni...l architecture

 

The 13" does not use flexure compensation.

The 20" uses this flexure compensation.

The 30" incorporates the compensation, but goes further in also compensating for thermal changes.


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