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Old 30-11-2011, 12:22 AM
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Deliberate inaccurate polar alignment?

Saw this post on sbig yahoo group recently


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Precise polar alignment can actually cause more problems than it solves. A small polar error results in a slight declination drift that with proper balance can peg the dec drive to one side of the worm and thus potentially avoid bouncing and occilations. This is similar to the common practice of weight-balancing RA to keep the RA worm on one side.

Stan
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Have not heard of doing this. Sound like all this energy I have invested In the past to get perfect alignment may be the wrong thing to do.

Any thoughts on this ? Anyone here actualy doing this ?
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Old 30-11-2011, 12:41 AM
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I do it all the time Stan. Oh wait, you said deliberate...

I guess it seems reasonable, but the question instead should be "how from the pole should I be to avoid oscillation in dec?"

Cheers,
Cam
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Old 30-11-2011, 10:28 AM
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In the same post someone said they miss align by 2 arc mins.

The poster from below was Stan not me. i am chris :-)

Last edited by cventer; 30-11-2011 at 10:39 AM.
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Old 30-11-2011, 10:32 AM
adman (Adam)
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it really depends on how much field rotation you can cope with over the course of your imaging session.
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Old 30-11-2011, 11:02 AM
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Good point. Although again someone in the same thread posted a 20 min exposure at 950mm focal length with FOV or 54 x 36 arc min and it shows no visible field rotation.
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Old 30-11-2011, 11:05 AM
adman (Adam)
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individual subs should be ok - but over the course of a few hours....
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Old 30-11-2011, 11:36 AM
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Exact polar alignment and imbalance both axes slightly sound a lot better to me.
Does the same but will have no field rotation.
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Old 30-11-2011, 12:08 PM
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I think even if I tried, my adjustment wouldn't get me better than 2 mins off except by luck!
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Old 30-11-2011, 01:12 PM
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+1 for what Adam and Jennifer said. Depends on your focal length and sub length as to how much rotation may affect you. Unless you have an observatory, you'll be doing well to get alignment under that reliably for a set up and pull down rig.

Certainly gives a good excuse not to be obsessive about it too!
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Old 30-11-2011, 04:31 PM
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Sorry, I think aiming for an intentional error is silly.

Sure, some mounts *might* give you some hysteresis in Dec. Most don't (particularly the Bisque variety )

The *less* corrections you have to do the better. Always correcting in a single direction is a recipe for eggy stars.

Sadly, perfect polar alignment is also a myth. The (refracted) pole goes up and down with atmospheric density (i.e. temp & pressure) so even if you nail it geometrically, it moves from night to night...no user error required.
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Old 30-11-2011, 05:10 PM
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Its an odd post in a way as if the mount was so accurate that such a perfect polar alignment were achieved then it would likely be largely free of backlash etc.

T-point model shoots for the refracted pole and you input current altitude and atmospheric pressure. But as Peter says if it shifts somewhat with pressure and temp I guess we are really only after very very close in the order of 30 arc seconds or so.

I can only say I have never seen the effect mentioned but I have only been that perfect in my polar alignment with the PME which doesn't bounce around anyway.

But if a mount does exhibit bounce and oscillations like this guys apparently has then perhaps it is good advice.

Greg.
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Old 30-11-2011, 05:34 PM
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Out of polar alignment

Actually Mike BJ's done the maths on this and also thorough experimentation on offsetting polar alignment. Here's an excerpt of a mail he sent me a while ago.

Quote:
Iíve had a good look at the effects of being out of polar alignment, both with rigorous mathematics and experimentally. Imagine plotting the T-point calculated position of your polar axis on a star chart. Suppose itís on the same hour angle as the object you are imaging. Thatís when it will produce maximal field rotation but minimal drift. The result is that your dec guider can get ďlost in the backlashĒ, and for me, this is the very worst situation possible. It will produce an image with every single star being a long N-S line. Conversely, if your polar axis is 6 hours ahead or behind where you are imaging, the reverse happens: you get very consistent drift in declination, easy to guide out so long as your guide shots are short enough (or your software smart enough to adjust motor speed rather than doing a mighty lurch once every guide exposure, and most arenít smart enough), but there will be NO field rotation at all, even if youíre a long way off polar alignment. My technique has been to put the mount intentionally out by about a tenth of a degree, at a position on the star chart 6 hours west of where Iím photographing.
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Old 30-11-2011, 05:39 PM
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It sounds like he is trying to solve a problem with poor backlash in his mount. I doubt a Tak mount or better would suffer from this issue. I never saw backlash in any of the mounts I used regularly. But if your mount does suffer from this it could be worth checking out. Its certainly a new piece of info.

Greg

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Originally Posted by multiweb View Post
Actually Mike BJ's done the maths on this and also thorough experimentation on offsetting polar alignment. Here's an excerpt of a mail he sent me a while ago.
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Old 30-11-2011, 08:40 PM
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Where could I read up on effects of temp and pressure affecting refracted pole (really refractive index of air mass I guess we're saying?). What would the limits on this be (Greg referred to 30 arcsecs?)

Rob
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