Question on guiding and alignment

[Starwiz]

How important is it to get polar alignment spot-on when using auto-guiding for imaging and what's the effect if it's not? 

John

 

[vlaiv]

Depends what you mean by spot on

In general it is advised not to have perfect PA when guiding - this helps with seeing induced oscillations of guide star. If you have perfect PA you should not be guiding in DEC at all, but if you don't have perfect PA then it is better that any movement of guide star is interpreted as actual movement and not seeing induced. This way you avoid problems with DEC backlash - you only need to nudge the mount to one side, and not switch direction. If you have just a bit of PA then you can't distinguish if guide star is just oscillating about it's position in DEC or you should actually do corrections, but if there is definite movement in DEC you can tell guide app to move to that side only and only in the case if guide star deflection is greater than seeing oscillation.

I forgot to mention: effect of non perfect PA on imaging would be limit to exposure length that you can take before you have problem with field rotation. Depending on how far off with PA you are, stars in the corners and the edge of the frame will start to trail - this could be really small amount for short exposures (less then fraction of pixel) and invisible but for long exposures it adds up.

[Ouroboros]

Suck it and see I suggest.  What you can get away with depends on the focal length of your scope. I find with my ed80 refractor (f=600mm) and 200p (f=1000mm) I only need to set the polar scope reticule to the right hour angle, get the Polaris in the little circle, and that's good enough for me personally as long as I guide - but not good enough if I don't. But then I'm not going for competition level photographs. Plus life is too short to spend too long drift aligning when I have to set up and tear down each time. I suspect an experienced astrophotographer would be more picky than me though. 

[Starwiz]

9 hours ago, vlaiv said:

Depends what you mean by spot on

In general it is advised not to have perfect PA when guiding - this helps with seeing induced oscillations of guide star. If you have perfect PA you should not be guiding in DEC at all, but if you don't have perfect PA then it is better that any movement of guide star is interpreted as actual movement and not seeing induced. This way you avoid problems with DEC backlash - you only need to nudge the mount to one side, and not switch direction. If you have just a bit of PA then you can't distinguish if guide star is just oscillating about it's position in DEC or you should actually do corrections, but if there is definite movement in DEC you can tell guide app to move to that side only and only in the case if guide star deflection is greater than seeing oscillation.

I forgot to mention: effect of non perfect PA on imaging would be limit to exposure length that you can take before you have problem with field rotation. Depending on how far off with PA you are, stars in the corners and the edge of the frame will start to trail - this could be really small amount for short exposures (less then fraction of pixel) and invisible but for long exposures it adds up.

You're right, nothing is ever exactly spot-on.  What I really meant was spot-on +/- a gnat's whisker   

I don't understand polar alignment well enough to to qualify it better than that at the moment.  

Thanks for the answer.  Useful information.  

John

[Starwiz]

9 hours ago, Ouroboros said:

Suck it and see I suggest.  What you can get away with depends on the focal length of your scope. I find with my ed80 refractor (f=600mm) and 200p (f=1000mm) I only need to set the polar scope reticule to the right hour angle, get the Polaris in the little circle, and that's good enough for me personally as long as I guide - but not good enough if I don't. But then I'm not going for competition level photographs. Plus life is too short to spend too long drift aligning when I have to set up and tear down each time. I suspect an experienced astrophotographer would be more picky than me though. 

Thanks for the answer.  I also have a 200p, so will see how it goes when I get started on DSOs.  

John