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Has anyone here built a 3-axis mount?

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#1 obin robinson

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Posted 30 May 2013 - 07:25 AM

I came across this link and it looks pretty cool:

http://www.vtwilson.net/telescope.html

I like the advantages of the design:

"A few advantages of my design are:
No need to level.
No Dobs hole (unrestricted zenith).
No gem flip.
No latitude adjustment (works everywhere).
No polar aligning.
Fast and simple two star alignment.
Eyepiece can be rotated into any position.
Choice of:
Single axis (satellite!) tracking.
Alt Az tracking.
Alt Alt tracking.
Three Axes tracking."

It looks like a pretty cool challenge to build but the tricky part seems to be the mount control software. I'm finding nothing but dead links on how to control it.

obin :question:

#2 dhaval

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Posted 30 May 2013 - 08:42 AM

Wonder how good would this be for imaging?

Dhaval

#3 Starhawk

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Posted 30 May 2013 - 09:04 AM

This would be a lousy mount at best for imaging. An equatorial is vastly better for imaging- you only have to get one drive moving at the right rate.

Basically this is the usual Dob, but with roll rings like the ones Parks sells on the tube. I have seen and used these on an equatorially mounted newtonian, and they help like a rotating focuser does on a refractor. However, since a Dob already has the eyepiece at a comfortable position, I don't know what the point of this is as implemented.

-Rich

#4 Midnight Dan

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Posted 30 May 2013 - 09:31 AM

The part where this is very different from the dob design is in that U-shaped base component that swivels the axis that the OTA is mounted on. This avoids the dob hole at zenith, or the Polaris hole on an equatorial mount.

I wouldn't say an equatorial mount is vastly better because it uses only one motor. Controlling the motors on this mount would certainly be more complicated. But it has the distinct advantage of not needing a meridian flip. It also appears that field rotation is controlled via the base spin so it's coupled to the other motions as opposed to using a separate derotater as you would on a fork-mounted SCT for example.

Certainly seems like an interesting design. iOptron has be coming out with some unique mount designs like the ZEQ25. I'd love to see them take on a design like this!

-Dan

#5 Starhawk

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Posted 30 May 2013 - 09:39 AM

The base swivel doesn't avoid the meridian hole, actually. You need to tilt the axis to do that. In this case, it will show that problem along any horizon instead of the zenith (which is where I'd choose it to be).

I should clarify- pointing for visual GOTO, for example, is clearly feasible for this mount. Photographic tracking, where all three axes will need to be precisely controlled at constantly changing rates, that's going to be a problem.

#6 Starhawk

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Posted 30 May 2013 - 09:43 AM

Thinking on it, if this just has one axis equatorially aligned, it really becomes interesting, indeed.

-Rich

#7 cn register 5

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Posted 30 May 2013 - 01:21 PM

This seems to be interesting, at present maybe as a DIY project but I can see there could be a lot of potential. Moving three motors at a continuously variable rate should not be a challenge for modern computer control.

People seem to have got a good hand on the matrix arithmetic needed to transform between the mount axes and the sky coordinates of Ra Dec and field rotation.

Aligning would probably need a minimum of three stars because there are rotations about three axes to nail down.

Guiding might even be done by determining two offsets and a rotation so that all three axes could be controlled.

Not sure what the best mechanical arrangement would be.

I don't see how there can be any singularities in any axis direction because there are always two other axes that can rotate.

Chris

#8 TxStars

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Posted 30 May 2013 - 01:37 PM

As long as you are imaging this should work for any optical tube.
For visual I only see this working for a newt.

#9 gdd

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Posted 30 May 2013 - 04:23 PM

Isn't the reason for the 3rd axis to take care of field rotation, mainly needed for AP? Otherwise a 2 axis alt/az mount can goto any point in the sky and track with 2 axis but with uncorrected field rotation.

Gale

#10 Lightning

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Posted 31 May 2013 - 06:26 AM

The 3rd axis is designed to take the required simultaneous 2-axis movement out of either the zenith (Dobsonian) or polar region (fork/GEM). I think it's quite ingenious. I agree that optimising this for automated imaging could be extremely tricky. For visual use though this is a really clever idea.

Has anyone looked at this anti-backlash worm? I've been thinking of a similar idea, with a different implementation for some time now. This guy is very inventive.

Cam

#11 SkipW

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Posted 31 May 2013 - 10:09 PM

http://en.wikipedia....iki/Gimbal_lock

Gimbal lock is the loss of one degree of freedom in a three-dimensional space that occurs when the axes of two of the three gimbals are driven into a parallel configuration, "locking" the system into rotation in a degenerate two-dimensional space.

The word lock is misleading: no gimbal is restrained. All three gimbals can still rotate freely about their respective axes of suspension. Nevertheless, because of the parallel orientation of two of the gimbals axes there is no gimbal available to accommodate rotation along one axis.

...

Gimbal lock can occur in gimbal systems with two degrees of freedom such as a theodolite with rotations about an azimuth and elevation in two dimensions. These systems can gimbal lock at zenith and nadir, because at those points azimuth is not well-defined, and rotation in the azimuth direction does not change the direction the theodolite is pointing.


The addition of the third degree of freedom allows you to avoid gimbal lock in all cases in a 2-D coordinate system such as Alt-Az or RA-Dec.






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