Re: [ATM] Can a Trilateral Dob be computer Operated? (longish reply)

[Dave Smith <smithersscope@yahoo.com>]

Hi Kevin, All,
I'm currently using (and tweaking) a 16" tri-dob based
on Mel's design. I'm also planning the GO-TO
mechanical parts based on Mel's stepper control
version and software. I've bounced several questions
off Mel and the group and from the responses, it
appears that the tri-dob platform can be motorized.
Based on my perspective and research the altitude will
have to be driven via the central altitude fin (the
center alt bearing). Issues related to pointing
accuracy and imprecision resulting from non-concentric
alt bearings can be remedied in the software,
according to Mel. I don't know how "easy" that fix
will be, but given Mel's reputation, I have no doubts
that problems would be solved.
I know I've read on the internet (and for the life of
me can't find it again! arrggh.) that a gentleman
built a binocular based on the tridob platform and
motorized it. In his summary, which was an overview of
the scope's performance, he discussed an issue he ran
into trying to drive altitude from one of the side
alt. bearings. The take home lesson was that the whole
OTA would shift (by approx. one FOV) after slewing--a
result of the OTA shifting in the flex rocker and then
resettling after the slew. In fact, at first light, I
think he had to forego the GOTO function and switch to
manual use.(Can anyone supply the link? I can see the
web images of the alt trunions in my mind's eye--I'm
sure I didn't imagine it!). So he recommended driving
the altitude from the center alt fin. 
I foresee problems with weight or pressure on the worm
at the alt. drive contact unless there is a "trolley"
or pivot bearing installed very near the worm/bearing
contact point. Mel suggested the "trolley" idea...it's
in the archives under "weight on the worm".
Alternatively, you could widen the center alt
bearing/fin by reinforcing with an additional section
of ply, modify the existing contact point to a roller
bearing and add teeth to the nascent ply sector. If
you make the bearing "adjustable" with respect to
vertical, you wouldn't need to build "tensioning" into
this worm drive; you would only need rigid worm
support. 

Driving the azimuth appears straightforward: 
Generate the teeth on the outside of the ground
ring.(JB Weld, if you like.) Implement some sort of
bearing or guide system on the inside of the ground
ring (to maintain even contact between ground ring and
flex rocker). I've done this already on mine...(dang!
when's that website going to be finished!?)...I
mounted three bearings (another tridobism!) to the
underside of the flex rocker and they ride against the
inside perimeter/surface of the ground ring. I need to
figure out how to make one of them adjustable.
Install the stepper directly to the flex rocker and
aim it tangential to the outside of the ground ring.
Find a way to apply tension to the worm and you're
golden.

Ultimately, I think both steppers have to be mounted
to the flex rocker. I could be way off, though. Early
on I kept telling myself that my flex rocker was too
flexible to pull this off. But after thinking about
it, the flex rocker doesn't have any real restrictions
with respect to how thick or "solid" it needs to be at
the two positions where stepper motors will be
mounted. You could reinforce and build off of the flex
rocker as much as you need 'at these two locations'
and still maintain the flexure required for the OTA to
sit down on the rocker and adjust to a stable
three-point stance. 
And another thought I had was that I could actually
just build a whole 'nuther flex rocker and permanently
attach the stepper motors to it. Then, when my battery
or pc dies, all I need to do is swap out the stepper
flex rocker with my original teflon bearing flex
rocker, flip or drop out the roller bearings and I'm
back to manual steering. That would be pretty nifty
and spiffy.
As with all ATM projects, she's a work in progress.
But I gotta tell ya, there are few things more
enjoyable than waltzing a 16" scope out the back door,
setting up and collimating in 15 minutes and be
staring at the planets and stars just like that! 
Sweet!
And I owe it all to you fine folks on the ATM list!

Whooohhoooo!
Dumpster Dave


--- "Dowd(s)" <kd14349@alltel.net> wrote:

> I was wondering if the Trilateral Dob design by Mel
> Bartel's could be 
> adapted for computer operation.
> 
> Thoughts?
> 
> Thanks in advance,
> 
> Kevin.
> 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


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