Motorised Polar Alignment - Why Not?

[netwolf]

About 2 years back when i first got into this hobby i was discussing with my friend Apoman, the polar alignment routine. I recall noting to him why when you can have motors for RA and DEC can you do not have motors for the polar alignment Alt(Latitude)/Az(True south) adjustments. I have not seen a single mount that has this feature, but i have seen many mounts that cost enough money to warrant this feature. The answer i got from my mate was why dont i patent the idea and make it so.

The many recent threads on this subject got me thinking again about things like how Tpoint and similar software plus plate solving can indicate errors in polar alignment. But still we don't have any means for the software to act on this information on its own by fine tuning the polar alignment.

Does anyone know if such a mount exists that does have this feature. Is there some good reason for this task to remain manual?


Regards
Fahim

[OneOfOne]

In a similar vein, I have also wondered with GoTo scopes why they only move in one axis. I think an option to allow it to move in two axes for tracking objects would be fairly simple (two axis tracking?). Once the reference objects are centred and entered, the computer will have a model of the alignment. From this model it should be able to calculate where the centre of this model differs from where it should be and know that it needs to move the dec motor a certain amount to allow for this error.

If I goto to an object it will get it quite close to the centre, even if the alignment is out quite a bit (I just plonk the scope at the moment, rarely level it). Over a few minutes the object will drift out of view, but if I goto it again, it will recentre the object in much the same position as it did originally where it will drift again with time. So the goto must take into account the alignment error, but once it gets there it just moves the RA axis.

I would not expect this method of tracking would be as good as accurate alignment, but it would be good enough to keep an object in very good alignment for visual use, perhaps for an hour or more.

As a programmer myself, I don't see that such a routine would be rocket science and so could be added to the "next release" of the software.

As for alignment motors, perhaps a couple of geared motors with some sort of coupling to the adjustment screws would not be rocket science either. Altitude would be quite easy, azimuth may be a little more challenging but some sort of block for a motor driven screw to move inside would be suitable.

[gbeal]

My $0.02 worth.
First, in my case the mount is polar aligned, and each time I put it on the pier it goes back to the same spot, so motorization isn't required, not by me anyway.
Secondly, if you do use some form of motorization, it needs to be sufficiently strong, and accurate that it could overwhelm the whole idea in the first place. Moving the respective axes, and then locking them is different to making them moveable all the time, via a motor.
Good luck though, always something to tease the brain.

[joshman]

mmm, i'm with gbeal on this one, fantastic idea, but quite an expensive one to implement given the necessity of high torque motors. unless there was a way to polar align it with out the necessity of there being a scope on the mount (there may be a use for the polar scope yet!!!), then once it's polar aligned you simply lock off the gears of your alignment motors.

keep going with the idea, there's defintely merit behind it!

[Geoff45]

My Losmandy with Gemini will do this with the "closed loop tracking" option. There are 7 tracking options:
1. Terrestrial ie none.
2. Sidereal
3. Lunar
4. Solar
5. King (corrects for average differential atmospheric refraction)
6. Comet (give the starting and end coordinates of the comet over a time period and it will track the comet--good for photography)
7. Closed loop---moves in Dec and RA when the polar alignment is off (up to 8 degrees they say) and you have synched on several stars. Fine for visual, but will give field rotation, so no good for long exposure astropics.

I'd be surprised if other mounts like Takahashi didn't have similar options.
Geoff

[AndrewJ]

Gday Fahim

I've actually been looking at this for a year or so, but i'm slow
at turning it to reality.
I built a manual wedge and am now in the process of motorising it.
Expensive, high torque motors arent required if you do it right.
( In my finished alt system, the motor was $13 and the gears $5 )

However my wedge is not a normal design.
It folds down to allow the scope to be loaded and once in position, it is essentially held together by gravity.
As such motorisation is relatively simple
I have attached piccies of the wedge design, and the finished Alt adjustment motor below. One rev of the motor output shaft gives approx 15arcsec of wedge adjust, so its easily controllable.
I plan to write a simple app to capture a webcam stream, calc drift and adjust accordingly
ie do an automated drift align.
Shld be fun if it works.

Andrew

[Astroman]

EQMOD for the EQ6 is working on the above methods in a continuous upgrade process. Making the mount more useable as a cheaper alternative to a G11 or the like (although without the carrying capacity of course) It can already do Dual axis tracking and GOTOing and follow satellites etc...

The idea of movable Alt-Az axis has been thrown around a fair bit, but upon looking at the costs involved in adding more moving parts ie.. worm drives etc... it just seemed too much for the average joe to be able to afford. Being able to write the corrections into software that could automatically fine tune the mount by moving each axis would be quite complicated also. Although it does sound like a good idea, the idea of moving around another 50kg of gears and motors attached to you mount doesn't.

[netwolf]

Thanks for all your replies, its good to know others have considered it. AndrewJ I like the wedge design and the fact that you have actually started to implement the idea and quiet cheaply too.

On a side not Do you think the scope is balanced enough for use in Alt/Az while the wedge is folded down? Your design could make it simple to switch between polar and alt/az mode.

Regards
Fahim

[ballaratdragons]

Fahim,
Apart from the Gemini system on the G11, the Vixen 2000 GoTo system has tracking in both axis as well. Allows for atmospheric diffraction, and comets etc.

Gears on polar aligning would not only bump the price up, but gives another source of PE. Imagine getting almost prefect and then one of those terrible gear slop jumps happens!!!! Frustrating.

[netwolf]

Ken, they should not be a source of periodic error. The polar alignment motors would only be used to do an initial alignment and then turned off.

[turbo_pascale]

I have seen earlier iterations of AJ's "wobbly wedge" as he likes to call it, and the alt-az mode is fine with his approach, at least the times I've seen it in action.
I've not seen the latest iteration in real life yet, but it holds great promise! I'm looking forward to the webcam polar align integration!

Turbo

[AndrewJ]

Gday Fahim

Re


No.
The basic wedge design is only folded down flat for loading/unloading the scope single handed, without doing in my back. It is stable enough to load, but not stable enough to use in that mode.
However, my wedge is designed such that it can be fitted in the field using only 4 small wingnuts and a split pin.
I have attached a piccy of the tripod top without wedge fitted to show the new top plate i made first.
This plate is 12mm thick and works like a scopesaver plate when Alt/Az, but also doubles as a foundation plate for the wedge.
The Wedge itself is made in two parts,
the rocking section and a base.
The second piccy shows how the wedge base connects to the tripod head
The rocking part is then "fixed" to the wedge base by the 1/2" axle bar.
As such, if i want to go Alt/Az, i remove the scope, unclip the wedge,
loosen/remove the four knobs and it all comes apart.
Its only a 2 minute exercise, with no tools required

Andrew

[rogerg]

I have often thought about this too. The problem I see is that when the scope is in use, to remove flexure problems, it needs to be bolted down well. My 12" LX has 4 bolts through the wedge. I notice a difference in pointing accuracy when I only have the centre one tight vs the 4 tight. I can see it would be possible to build a rotating base which has zero movement but allows rotation, but it'd be quite expensive I'd expect. It would be nice, I have often thought how easy it'd be to take things like PolarAlignMax and use the information it ends up with to adjust the polar alignment.

REgarding using the info to track in RA and DEC, as others have said, Gemini does this.. I've used it occasionally on my GM-8 but have to admit there's little benefit for me. I polar align quite accurately quite easily now, for photography, and when done, there's so little drift it doesn't bother me, and a 6 star pointing model in the Gemini will get objects anywhere in the sky centre FOV no problem. I also steer away from using it when doing photography because it introduce an unknown in to the autoguiding mix together with backlash concerns.

[montewilson]

Guys - Here is a thread I started a few months ago about a similar topic. It shows images of my fine-adjustment alt/az plate.

I have often thought about making a self aligning mount but the computing part is beyond my programing skills. There are some interesting engineering challenges too. But it wouldn't be impossible.

http://www.iceinspace.com.au/forum/showthread.php?t=22361

[g__day]

The Vixen SkySensor2000 will track and point in polar unaligned mode and run both motors accordingly.

My sense is to do this really well you need something like Tpoint or Maxpoint that model several tens to hundreds of stars and can determien alot off seeing and mechanical limits to your gear (e.g colimiation, axes mis-algnment, tube droop etc) as well as pointing alignmnet, backlash and other errors - quite hard to do.

To get long duration photography going I sense you need many of:

1) dark site
2) permanent pier
3) solid, high precision mount
4) excellent sky modelling software
5) feedback loops from imaging to pointing to racking and guiding analysis software


It would be great if it where all there and affordable!