PA Tracking or no?

Using SharpCap's Polar Alignment feature
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Jim_Ritscher
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PA Tracking or no?

Post by Jim_Ritscher » Fri Sep 20, 2019 4:45 pm

I was told that you should turn off tracking during polar alignment. Is this true? I'm using a ZWO 60 mm (focal length 280 mm) guide scope with a ASI 120 camera, which gives me a horizontal angle of view of about 1°. Without tracking, during a two second exposure, I can see the stars move. My mount is a CGEM II.

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admin
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Re: PA Tracking or no?

Post by admin » Fri Sep 20, 2019 6:45 pm

Hi,

It shouldn't matter at all. Remember that the tracking rate will rotate the mount once in 24 hours and that polar alignment it should take you a maximum of five minutes or so. In that five minutes the slow rotation of the tracking rate will really make no difference whatsoever.

Cheers, Robin

Jim_Ritscher
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Joined: Fri Sep 20, 2019 4:29 pm

Re: PA Tracking or no?

Post by Jim_Ritscher » Sun Sep 22, 2019 12:25 am

Robin,
Thanks for the response. Let me push back a little and see what you think. Perhaps my thinking (or calculation) is way off here.
• Say tracking is off.
• The earth moves 15° in one hour.
• That’s 15 arc minutes in one minute,
• and 15 arc seconds in one second.
For my guide scope, one pixel covers 2.76 arc seconds, so that means a star is moving through 5.43 pixels in a second (worst-case). For a two second exposure, that’s about 11 pixels. Won’t that cause some distortion in the plate solving for one plate? And won’t it cause some distortion when you compare one plate to another one taken earlier? What am I missing here?

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Re: PA Tracking or no?

Post by admin » Sun Sep 22, 2019 8:07 pm

Hi,

Those figures would apply if you are pointing at the celestial equator, but the movement rates are much much smaller near the pole. Within 5° of the pole the rate is about 11 times less and within 1° of the pole it is nearly 60 times less.

Hope this helps, Robin

Jim_Ritscher
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Re: PA Tracking or no?

Post by Jim_Ritscher » Tue Sep 24, 2019 9:16 pm

Thank you.
Jim

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