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Confusion about CEM40 "zero position"

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#1 bendgrampie

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Posted 25 September 2021 - 09:03 PM

I turn on my mount, level it, do a polar alignment, make sure "zero position" is set properly; the scope is now aimed at Polaris and I use it for focus.   Then, I (because I'm trying to understand all the gear), use the GOTO functionality to slew to Polaris.   I would think nothing would happen, but actually the mount spins on both axes, eventually ending up pointing toward Polaris, but from a different angle.   Is that right?   The hand controller shows the correct lat/long and time, so that's not an issue.   Am I missing a step?   Or just confused about how GOTO works.

 

TIA.



#2 jonnybravo0311

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Posted 25 September 2021 - 09:27 PM

When you're polar aligned, your mount is aligned with the celestial pole (either north or south depending upon which hemisphere you are). Polaris is _not_ exactly the north celestial pole. Therefore, when you're in the home position and tell your mount to slew to Polaris... well, it's going to do just that. Move in RA and Dec until it thinks it is centered on Polaris.


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#3 nicknacknock

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Posted 26 September 2021 - 07:57 AM

I’ll move this to the mounts forum - should get more help over there :)



#4 555aaa

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Posted 26 September 2021 - 10:31 AM

Slew to a star away from Polaris, ideally a star near zenith. Think of it this way. The pole is where all the lines on the map converge. Polaris is near that but not exactly at that point. The true celestial pole is an indeterminate point in the sky for an equatorial mount because all RA angles all become the same point, just like at the earth’s pole, all longitude lines converge and so there is no way to know your true longitude. If you want an accurate position you would go away from the pole a long way and then check your position. As mentioned above, your mount will make a very large move ( rotation) in the RA axis to get to Polaris even though it is a very small move across the sky because the mount has to move only along the RA (like longitude) and declination (like latitude) axes. It can’t just randomly scoot across the sky.

Edited by 555aaa, 26 September 2021 - 10:34 AM.



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