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Polar align question relative to telescope

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#1 pscm

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Posted 22 July 2019 - 07:06 PM

I have a good understanding and setup process for polar alignment I believe.  I am about to get into guiding (even though I said I never would).  My question is around the telescope a d guidescope setup.   In the past I understood that it wasn't necessary that Polaris to be in view in the telescope.  I would think that this would make goto functions much more accurate and require less stars to get the mounts thinking correct.  I own an AVX and it wants you to start at the home position by setting RA and DEC  marks lined up.  I would think one would be better off manually moving the axis, then lock and call this the home.  I know this isn't critical for tracking but I am curious on my thinking.


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#2 OldManSky

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Posted 22 July 2019 - 07:25 PM

Whether the main scope can "see" Polaris or not doesn't matter.  What does matter is that the mount axes are "pointing" to the NCP.  Which means RA axis so that the counterweight shaft is straight up and down, and DEC axis at exactly a right angle to it (pointing North).  Set that as your zero/home position, and the first slew to an alignment star will be (usually) pretty darn close.

That's all it will affect, though -- the first slew to an alignment star.  It won't affect tracking, and it won't affect pointing or slewing after you've aligned with stars. :)


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#3 Stelios

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Posted 23 July 2019 - 03:04 AM

I think you may be mixing up polar alignment (which is to set the physical RA axis of the mount pointed at the NCP (North Celestial Pole)) and star alignment (which aims to tell the mount where it is, so it can accurately execute GoTo commands).

 

Setting the mount to index marks (as well as inputting correct RA/DEC and date/time info) as well as leveling, is *only* to allow the first slew for *star* alignment to be fairly accurate. You could mung all the above, and as long as you centered properly all the stars during star alignment, subsequent GoTo's would be very accurate. 

 

Accurate *polar* alignment can be done with the mount turned off. It's a mechanical adjustment process using the Alt and Az knobs. Celestron has the ASPA routine to facilitate that, but ASPA is less accurate than other methods, though more than enough for visual and planetary AP, and not bad in a pinch even for DSO AP with shorter F/L's.



#4 pscm

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Posted 23 July 2019 - 06:59 PM

Yes I do manual polar alignment.  Giving the scope a home position with weight straight up and down makes sense.  It would seem to me that if the scope was also centered that would make goto alignment that much easier.  I suppose that the OEM can factor this with the software.  Mine is close but I am also a perfectionist.



#5 OldManSky

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Posted 23 July 2019 - 07:21 PM

Yes I do manual polar alignment.  Giving the scope a home position with weight straight up and down makes sense.  It would seem to me that if the scope was also centered that would make goto alignment that much easier.  I suppose that the OEM can factor this with the software.  Mine is close but I am also a perfectionist.

Here's the thing about Polaris, though:  if you have a perfect zero position, your scope will be pointing at the NCP, not at Polaris.  If your FOV is wide enough, Polaris *might* be in the FOV.  Or it might not, if you've got a longer focal length.  So it isn't about Polaris.  Make sense?



#6 pscm

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Posted 23 July 2019 - 08:07 PM

Yes.  Never thought about that...duh




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