Polar alignment - should I see this?

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Luke Spacehopper
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Joined: Thu Jun 06, 2019 4:58 pm

Polar alignment - should I see this?

Post by Luke Spacehopper » Thu Jul 30, 2020 11:26 pm

Hi,

So I've been getting to grips with polar alignment.

Tonight, I got the best yet, really close to perfect.

Then, out of curiosity, I set my scope to the home position, took a shot, plate solved it, and sent the result to Stellarium. This is what I saw:
polar copy.jpg
polar copy.jpg (947.16 KiB) Viewed 94 times
Polaris is only just about in the frame (the red outline).

Does this say anything about my polar alignment being good/bad? Or is it more about how I've set the home position? Or doesn't it really matter as long as I align my stars properly?

Also, related to polar alignment: if I slew horizontally, and the star I'm looking at on my screen goes across at a slight, non-horizontal angle (same for vertical - slewing vertically and star deviates from the vertical line), does this imply anything about my alignment, or more about whether the mount is level?

Thanks.

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Re: Polar alignment - should I see this?

Post by admin » Fri Jul 31, 2020 7:10 pm

Hi,

it's not a problem – what it means is that your guide scope is offset by a degree or so from the RA axis when you're in the home position. The SharpCap polar alignment algorithm isn't sensitive to this misalignment and should still be giving you correct results.

The second thing you notice – that stars move at a slightly slanted angle when you move the telescope is all down to the rotation of your camera in the eyepiece holder. If you give the camera a tweak clockwise or anticlockwise by about 45° your see that they move at a completely different angle on screen but your polar alignment is completely unchanged.

Cheers, Robin

Luke Spacehopper
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Joined: Thu Jun 06, 2019 4:58 pm

Re: Polar alignment - should I see this?

Post by Luke Spacehopper » Fri Jul 31, 2020 10:36 pm

Thank you Robin - but I'm not actually guiding. This is from my camera. Does that make any difference?

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Re: Polar alignment - should I see this?

Post by admin » Sat Aug 01, 2020 4:15 pm

Hi,

you mean from your main camera attached to your main telescope on the mount? In that case there are two possible causes – one is that your home position isn't quite at 90° declination – maybe it's about 89° or so. The other possibility is that you have a degree or so of cone error which is a misalignment between the optical axis of the telescope and the RA axis when the scope is in the home position (basically the telescope is either pointing a bit away from the RA axis or a bit towards it). Again neither of these should affect the accuracy of the polar alignment results, but fixing them might make your GOTOs more accurate.

Cheers, Robin

Luke Spacehopper
Posts: 18
Joined: Thu Jun 06, 2019 4:58 pm

Re: Polar alignment - should I see this?

Post by Luke Spacehopper » Sat Aug 01, 2020 4:24 pm

Again, thank you so much Robin, you've been very helpful.

Time to crack open ConeSharp I think...!

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