Polar alignment and levelling

[Ken82]

So I have a semi permanent set up with the scope permanently polar aligned . If I wanted to view something closer the horizon one evening can I just raise the tripod assuming the polar align will still be good so long as I get the tripod level again . Or would a tiny fraction out in levelling cause the polar align to be out ? 

[belfieldi]

Hi Ken,

I polar aligned the other night and then raised the tripod a little. Even though the tripod was still level, the mount was no where near polar aligned anymore.

Clear skies

Ian

[cfpendock]

Yes.  The polar alignment will be out.  Certainly for imaging you need to be correct to within a few minutes of arc, and preferably much less.  I can't see that you can judge that accuracy with a level.  having said that, assuming you have an equatorial out, levelling is irrelevant, so long as you don't change anything once you have your polar alignment set.

Chris

[Tinker1947]

For viewing you may get away with it, but imagine will need a new PA done.....

[Ken82]

Thanks guys that's what I thought ! 

[Peje]

20 minutes ago, cfpendock said:

Yes.  The polar alignment will be out.  Certainly for imaging you need to be correct to within a few minutes of arc, and preferably much less.  I can't see that you can judge that accuracy with a level.  having said that, assuming you have an equatorial out, levelling is irrelevant, so long as you don't change anything once you have your polar alignment set.

Chris

Are there any hard rules for this? I use the PHD 'guiding assistant' to measure mine and the measurement typically jump about between 0.3 & 3.

Less importantly, why doesn't levelling matter with EQ mounts? Even a link to an explanation would do

[cfpendock]

1 hour ago, Peje said:

Are there any hard rules for this? I use the PHD 'guiding assistant' to measure mine and the measurement typically jump about between 0.3 & 3.

Less importantly, why doesn't levelling matter with EQ mounts? Even a link to an explanation would do

That is a good question.  For guiding, the accuracy of PA depends on where in the sky (how close to the celestial equator) you are imaging.  Personally, I try to get closer than 2 arc minutes for the PA, and this seems to work with my set-up.  

I'm not sure what you mean by jumping between 0.3 & 3.  Is this the error in PA in arc minutes?  If so I would have thought it ought to be OK.

For actual guiding, I normally aim to guide to within one arc second, although seeing, wind, and other factors unknown to myself sometimes frustrate this.  Of course, imaging at a shorter focal length (i.e. wider field of view) will generally make less accuracy acceptable.  The sampling rate, i.e. how many arc seconds per pixel, can also give a feel as to how accurate you need to be.  

You are aligning the mount with the celestial pole, irrespective of whether you are in the north of Norway or the south of Greece.  How your mount is fixed to the surface of the Earth doesn't matter.  You could say that you are effectively changing the levelling to match your latitude when you adjust the altitude during the polar alignment.

Hope this helps.

Chris

 

[kens]

1 hour ago, Peje said:

Less importantly, why doesn't levelling matter with EQ mounts? Even a link to an explanation would do

You can break down any out-of-level into two orthogonal components. One about an E-W axis which can be corrected by the alt adjustment bolt. The other is about the RA axis which can be corrected by simply rotating the mount in RA.

Levelling does help with the adjustments when aligning because when you adjust the alt bolt it only affects alt and likewise the az bolt. But it doesn't prevent getting polar alignment

[Peje]

42 minutes ago, cfpendock said:

That is a good question.  For guiding, the accuracy of PA depends on where in the sky (how close to the celestial equator) you are imaging.  Personally, I try to get closer than 2 arc minutes for the PA, and this seems to work with my set-up.  

I'm not sure what you mean by jumping between 0.3 & 3.  Is this the error in PA in arc minutes?  If so I would have thought it ought to be OK.

For actual guiding, I normally aim to guide to within one arc second, although seeing, wind, and other factors unknown to myself sometimes frustrate this.  Of course, imaging at a shorter focal length (i.e. wider field of view) will generally make less accuracy acceptable.  The sampling rate, i.e. how many arc seconds per pixel, can also give a feel as to how accurate you need to be.  

You are aligning the mount with the celestial pole, irrespective of whether you are in the north of Norway or the south of Greece.  How your mount is fixed to the surface of the Earth doesn't matter.  You could say that you are effectively changing the levelling to match your latitude when you adjust the altitude during the polar alignment.

Hope this helps.

Chris

 

Yes in PHD the PA reading fluctuates about, I'd assume if you left it long enough it would stabilise but who has the time!! Yes when I'm actually guiding I always aim to stay between +1 & -1, as you say this is the aim but sometimes it doesn't work out so neat. Thanks for the leveling info, it makes sense.

27 minutes ago, kens said:

You can break down any out-of-level into two orthogonal components. One about an E-W axis which can be corrected by the alt adjustment bolt. The other is about the RA axis which can be corrected by simply rotating the mount in RA.

Levelling does help with the adjustments when aligning because when you adjust the alt bolt it only affects alt and likewise the az bolt. But it doesn't prevent getting polar alignment

Thanks for the info, it's good to know I don't need to worry about it so much as it was the one thing making me avoid moving the scope a few more feet up the concrete to get around an obstacle that sometimes gets in my way.

[Leveye]

Leveling is important. Accuracy is important. Always strive for perfection.