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Old 17-03-2017, 08:27 PM
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thegableguy (Chris)
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Why does guiding require near-perfect alignment?

Hopefully there's a simple answer to this, but... Why does a guided mount require anything beyond vaguely good polar alignment?

If the guidescope is constantly sending movement data that the mount is compensating for, and if the mount is capable of moving in any direction at high speed, then what's the damn problem?? I can understand field rotation being an issue beyond a certain degree of bad alignment, and I understand that bad alignment will make the mount's database useless, and I understand that there's potential for (very very small) issues if the imaging OTA and guidescope aren't well aligned with one another... but all those things should make no noticeable difference as far as 2-3 min subs are concerned.

People constantly say that guiding can't correct bad polar alignment. Well, why not?? I don't get it!!

Clearly I'm missing something. There has to be a reason, likely a very obvious reason, that I've overlooked. I'd love to know what it is. Something fundamental about the design of the mount and the speeds they move?
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Old 17-03-2017, 08:46 PM
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RickS (Rick)
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Guiding with a poor polar alignment will keep the guide star in the same place but the field will rotate around it, and you'll get noticeable trailing of stars and details which will get worse the further you get from the guide star.

Cheers,
Rick.
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Old 19-03-2017, 10:59 AM
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billdan (Bill)
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Gooday Chris,

This is my understanding of what is happening. Keep in mind that guiding is reactive, the error has already happened and the corrections are compensating for it. Essentially the dog chasing its tail.

When guiding in RA all the guide corrections are speeding up or slowing down the RA motor to compensate for worm/gear PE.

In DEC however the motor is stopped, so when a DEC correction is issued there is inertia, stiction and backlash to overcome. So it may take 3 or more guide corrections before the worm gets the message and actually moves and then the motor stops again. With good PA this doesn't happen very often, with bad PA its continuous.

You get the classic sawtooth pattern looking at the DEC graph in PHD. /l/l/l/l/l/l/

In some mounts there is always the risk that the DEC motor will overshoot and the same situation will happen in the reverse direction.

Bad PA can also effect RA as well, assume the star is drifitng S and the DEC motor is getting a lot of N corrections to move it back. Because of the bad PA the star may actually be moving in a NE (or NW) direction so now the RA corrections have to compensate for this as well.

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Old 19-03-2017, 11:37 AM
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RickS (Rick)
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Bill: you are correct that bad PA can make the guiding work harder, but field rotation is the more fundamental problem. It would be an issue even if you were able to guide perfectly.

Cheers,
Rick.
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Old 19-03-2017, 10:06 PM
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thegableguy (Chris)
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Thanks guys.

RickS, I know field rotation will be an issue, however if the alignment is pointed at least vaguely in the right direction it won't be anywhere near enough to make any difference for subs under less than 5 mins. Not an issue I'm worried about.

Billdan, your answer makes a lot of sense... although I can't help but feel that it's a fairly large failing on the part of the mount if it can't make small enough adjustments to deal with such things. An NEQ6 costs a fairly respectable amount but it is, when you get down to it, a fairly simple mechanical device. Mildly surprised & slightly disappointed that it can't make subtle adjustments without slewing all over the place. RA affecting DEC should just not be an issue if the adjustments are small. I know there are a lot of variables, like wind / vibration / backlash etc, but I would have expected a precision instrument being instructed by a high-speed processor (such as those found in literally any laptop made in the past 10 years) to be more than up to the task if the mount was designed better. I dunno. I'm the furthest thing from a mechanical engineer so I'm not at all qualified to assert such a thing. Just disappointed, because in principle it seems like a pretty easy thing to program: if the star moves, counter it.

However, the bigger issue for me personally has been answered; my mount SHOULD be able to track a star all night regardless of alignment, whatever other backlash / adjustment issues might affect the subs, yet it keeps losing the guide star. Not every time, but often enough. So that's not just a matter of slightly bad alignment? Because I didn't think it could be. I've suspected for a while that there's something fundamentally broken about my guiding setup, and this kind of suggests strongly I was right.

Sigh...
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Old 19-03-2017, 10:10 PM
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thegableguy (Chris)
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I should perhaps add that I'm using a 600mm "guidescope" with a tiny camera using only the very center of the image, so the image sent to the laptop is smaller than a crop-sensor DSLR on my 1000mm Newt. PHD can detect movement of less than a pixel. It SHOULD work, if the mount is sound. That's my take on it, anyway.
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Old 20-03-2017, 04:20 PM
Mosc_007 (Charles)
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What problems do you have other than Field Rotation ?

I have been using rough polar alignment with just a compass for years. Lucky to be within 2 Degrees of alignment. My Guiding seemed to work fine. NEQ6 Pro. Subs mostly around 5 Mins. No noticable star trails. Field rotation was shown when stacking. But not enough to worry me.
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Old 22-03-2017, 12:09 PM
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ZeroID (Brent)
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It is best to minimise guiding corrections where possible. Most guiding software does not cater well for large excursions, they take time and introduce errors and overshoots leading to the dreaded sawtooth.
The less corrections needed and the smaller they are then the target will stay where it's put in the FOV.
That is based on my experiences when the scope was 'close' but not close enough to SCP alignment. Since I did a proper alignment it settles within a couple of minutes and stays on target for the rest of the night. The steadier your scope is the better the image..
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