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Servo vs Stepper

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#76 WadeH237

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Posted 02 October 2012 - 12:51 AM

If I understand things correctly, polar alignment is achieved when the RA axis is perfectly parallel to Earth's rotation axis. And once polar alignment is achieved, the mount will track (periodic and non-periodic errors in the tracking gear train notwithstanding).

Since orientation of the dec axis is not part of the polar alignment or tracking equation, orthogonality errors do not impede polar alignment or tracking.

The important thing about orthogonality errors is that they will have a noticeable effect on pointing accuracy. This can indirectly affect polar alignment a couple of ways. First, if you are trying to polar align using Polaris, it is possible that a large enough orthogonality error may make it impossible to point directly at Polaris. Second, the all star polar alignment routine depends on an accurate pointing model (with some tolerance because it does a sync on the target star).

Saying it's impossible to polar align the mount is just flat wrong. A drift alignment does not depend on the pointing model at all. Thus a drift alignment will work no matter how much orthogonality error exists or whether you've modeled it.

Even on high end mounts that achieve close to perfect orthogonality errors, it is still possible (likely?) for the OTA to introduce it's own orthogonality errors. As you'd expect from my statements above, this can and does introduce pointing errors for mounts whose pointing models don't correct for it. But it does not harm their tracking performance in any way.

Now the 8/3 error is something that can effect the mount's imaging performance, but as has been said, it's effect can be mitigated through guiding. It will be interesting to see if Celestron fixes the issue. The only way that I can think of for them to do that would be to increase the number of PEC data points so that it can store 3 turns of the worm.

Just my two cents on the comments here,
-Wade

#77 freestar8n

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Posted 02 October 2012 - 01:16 AM

Will non-orthogonality affect tracking? of course it will! but that's why we guide in both axes!


Non-orthogonality will not affect tracking or polar alignment at all because the dec. axis isn't involved in either. I would hate for people to come away from this thread with the misconception that the claim had any merit. It would affect goto accuracy in a 2+4 alignment, but it would have no effect on tracking and minimal effect on autoguiding.

Frank

#78 Kolenka

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Posted 02 October 2012 - 01:32 AM

Ed, Derick also mentioned that the hard-wired ratio is scattered all over the code, hence it is impractical to have something like the Autostar where you can change the gear ratios any time.

It occurred to me though that it might be possible to modify the hard-coded ratios simply by doing a binary edit on the firmware binary before it is uploaded. That would allow the use of any gear ratio and thus much better servos would become available.

Not something I have the time to poke around with right now though, I've got enough leisure time tied up in the AP stepper mount...


If we could use a different ratio, that could make things very interesting. Unfortunately, that kind of coding is over my head. I would certainly be interested to hear if there is someone out there that could do it.


My concern is that the ratio is scattered all over the code. That means you can't just find "the bit that controls the ratios" and tweak it. It is possible, but to do it right means a rather lengthy analysis of the firmware. That could take quite a bit of work since you practically need to reverse engineer the whole thing to find all the references and the various ways it winds up in the final machine code.

The upside is that a person could probably ignore the handbox firmware and focus entirely on the motor control firmware. This assumes that the ratios are part of the motor control, and the handbox doesn't know them.

It can be done, but I wouldn't consider it a good use of someone's spare time unless they really want to reverse engineer the firmware. May be able to get a head start from the NexSXD folks, if they did more work than just reverse engineering the protocol between the handbox and the motor control firmware.

#79 orlyandico

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Posted 02 October 2012 - 03:16 AM

but Frank - nobody is "perfectly" polar aligned.

There would always be some activity on the DEC axis, even if minimal. Since everybody needs to correct in DEC anyway DEC guiding would also compensate for non-orthogonality.

#80 freestar8n

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Posted 02 October 2012 - 04:09 AM

but Frank - nobody is "perfectly" polar aligned.

There would always be some activity on the DEC axis, even if minimal. Since everybody needs to correct in DEC anyway DEC guiding would also compensate for non-orthogonality.



But that means "tracking" is unaffected - because it only involves the RA axis turning, and polar alignment is unaffected - because it only involves the direction of the polar axis.

So it reduces it to issues of autoguiding - and that would indeed be affected due to the non-orthogonality - but the effect would be miniscule - scaled by the sine and cosine of 0.5 degrees. If you made a 1" correction in dec., then it would be off by 0.008" in RA, and 3.8E-5" in dec. This would never be noticed while guiding.

And even this assumes the guy's mount really is 0.5 degrees off - which is not clear since both his TPoint and laser measurements raise some questions on methodology and interpretation.

Frank

#81 orlyandico

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Posted 02 October 2012 - 04:19 AM

Ah ok wrong terminology on my part. When I said "tracking" I really meant.. guided tracking, since I had assumed this is what all normal folks with normal mounts do :D

And you're right, the contribution of non-orthogonality would be tiny.

#82 Stew57

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Posted 02 October 2012 - 03:56 PM

Drift alignment takes care of the tracking issue. Does the 4 calibration stars (well actually one as they are averaged) compnstae enough to take care of pointing accuracy enough for ASPA to be accurate? Perhaps that (orthogonality) is a variable in the wide discrepancy in user reported success when using ASPA.

#83 jaddbd

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Posted 02 October 2012 - 07:02 PM

If you use a DRIFT alignment method (computer assisted or otherwise), you should be able to achieve accurate enough polar alignment regardless of orthogonality. The goal of DRIFT is to get the RA plane of the MOUNT perpendicular to the pole. Since you adjust the MOUNT to bring the DEC drift back to parallel when shooting East and at then at the meridian South, it seems to me that any angle of error on the RA plane would be irrelevant (since it is parallel to the plane you are trying to achieve) and any angle created by shooting a little high or low off the DEC plane would have to be very large to be of any measurable relevance.

That being said, I have 3 different scopes which I use (one at a time) on my permanent (drift aligned) set up, which all point slightly different when mounted, but all “track” and “guide” the same.

Also (responding to some banter earlier in the thread), my experience in thinking that you can just point and shoot a high end mount “unguided” is a bit of a misnomer. Even if you have excellent polar alignment, and very little periodic error, you still have to account for the fact that objects in different parts of the sky move at different rates due to refraction, and that rate may change position of the said object changes. There are software tools provided to adjust for this, but it is way easier and more accurate just to guide IMHO.

John D
Maryland

#84 rmollise

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Posted 03 October 2012 - 12:46 PM

If the users in this group can't, or refuse to, acknowledge the effects RA-DEC orthogonality has on the ability to polar align and ultimately its effects on tracking then I'm definitely in the wrong place.


It doesn't and won't. But stick around and you'll learn the basics before taking the bit in your teeth again... :cool:

#85 EFT

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Posted 03 October 2012 - 10:48 PM

I took a look at the possibility of putting the Atlas system into the CGEM. While maybe not impossible, it would not be easy. The first issue would be the motor mounts. I would be necessary to reposition the mounting holes on the Atlas steppers to fit the CGEM locations. That could probably be done and there is probably enough space for the motors.

The biggest challenge would be the motor board. The Atlas motor board is longer than the CGEM board but not as tall. Getting the board in the hole and attached to a face plate might be challenging. Worse yet, the Atlas motor cables attach to the board on end creating a further space issue. If it can all be fit, then a custom face plate would need to be made to attach the motor board to.

Perhaps not impossible, but definitely difficult. Considering the cost of a Synscan conversion kit (if you can find one anymore), it's hard to say whether or not it would be worth the effort.

#86 orlyandico

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Posted 04 October 2012 - 04:42 AM

Thanks Ed. I suspected as much regarding the motor board.

#87 andysea

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Posted 06 November 2012 - 08:19 PM

Also (responding to some banter earlier in the thread), my experience in thinking that you can just point and shoot a high end mount “unguided” is a bit of a misnomer. Even if you have excellent polar alignment, and very little periodic error, you still have to account for the fact that objects in different parts of the sky move at different rates due to refraction, and that rate may change position of the said object changes. There are software tools provided to adjust for this, but it is way easier and more accurate just to guide IMHO.

John D
Maryland


I fully agree with you John!
Guiding seems to be the best way to achieve good tracking even with excellent mounts; and guiding with an OAG in my opinion is the way to go.
As long as the mount as a smooth and repeatable periodic error it will be easily guided out along with any drift generated by not perfect polar alignment.

Andy






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