Polar alignment routine and platesolve discrepancy

Using SharpCap's Polar Alignment feature
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longinthetooth
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Joined: Wed Oct 03, 2018 3:49 pm

Polar alignment routine and platesolve discrepancy

#1

Post by longinthetooth »

Hi there - I have a Skywatcher azeq6GT mount, AA102EDT with AA 60mm finder attached and am using Sharpcap 3.2 with AA GPcam 3 178c and ASI1600mc. The polar align procedure solves without difficulty and claims excellent alignment. However, when I slew then use the Sharpcap platesolve routine I am always just over 2 degrees out and Sharpcap makes the adjustment, though the target is never exactly in the centre of the fov. Is there a simple explanation for the 2 degree discrepancy? My view to the north polar area is slightly reduced by next door's trees so will this affect the accuracy of the polar alignment feature?
John
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admin
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Re: Polar alignment routine and platesolve discrepancy

#2

Post by admin »

Hi,

If your mount is working as expected and the polar alignment is correct then the most likely cause for inaccurate go to is cone error. This is when the scope mounting is such that the front of the scope is either pointing away from the RA axis or towards the RA axis a little (with the scope in the home position). If you have cone error, and spun the mount in RA while the scope was in the home position, the place in the sky pointed to by the scope would trace out a small circle around the north celestial pole. If you imagine a line running along the centre of the scope then that would trace out a cone, hence the name cone error.

You can find lots of more detailed descriptions of cone error online (and hints on how to fix it). There is also a living tool I wrote called cone sharp which helps measuring and correcting cone error.

cheers, Robin
longinthetooth
Posts: 2
Joined: Wed Oct 03, 2018 3:49 pm

Re: Polar alignment routine and platesolve discrepancy

#3

Post by longinthetooth »

Thanks for that advice Robin I'll give cone sharp a try.
JB
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Re: Polar alignment routine and platesolve discrepancy

#4

Post by admin »

Hi John,

The other thing that occurred to me is that you may be getting incorrect results for the polar alignment if you have any flecks in the mounting of your guides or cables that are not properly restrained. Even the weight of the hanging cable on the camera in the guide scope can be enough to shift the guide scope slightly as you rotate the mount and will reduce the accuracy of the polar alignment routine.

Cheers, Robin
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