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Polar Alignment Question

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#1 borr

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Posted 23 September 2020 - 04:56 PM

I currently use an ipolar to PA and have just gotten into guiding and I've seen all the options to do your PA with the guide scope and software like sharpcap or APT or whatever. My ipolar is mounted center axis on the mount but my guidescope is mounted off-axis to the side of the telescope where the finder would normally go. 

 

Since the guidescope is off axis, if I use the guidescope for PA, why isn't the PA going to be all over the place once the mount starts tracking and the guidescope is moving? I am assuming the software somehow corrects for this offset? And if so, is using the off-center guidescope just as accurate as using the on axis ipolar? 

 

 

 

 



#2 Michael Covington

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Posted 23 September 2020 - 06:04 PM

OK, first of all, the iPolar is as accurate as anything; no need to use another technique.

 

Second, the various guidescope-based techniques find the point around which the mount is rotating.  They do not have to aim straight at it in order to do so.  

 

The drift method, for instance, checks that the telescope is moving in the right direction to track any star.  Because the celestial sphere moves as a whole, if it's correct for two stars that are relatively far apart, it's correct for all of them.

The plate-solving-based methods in both iPolar and SharpCap have you rotate the mount around its axis in order to find out the axis around which it rotates.  (iPolar has you do this just once, the "Confirm Position 1" and "Confirm Position 2" step; other software has you do it every time, on the spot.)  Once the point around which the mount rotates is known, its RA and declination are determined by plate solving (= comparison to a built-in star map).  If it's the pole, you're good to go.  Otherwise you are told how to move the polar axis in order to reach the pole.


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#3 borr

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Posted 23 September 2020 - 07:51 PM

Ok. I figured it was calculating the axis when you rotate the RA around but wasn't exactly sure how that worked.  I'm happy using the ipolar so I'll more than likely continue with that, although I could eliminate another cable if using the guidescope. But, the ipolar is pretty simple though. Thanks! 


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