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Ade Turner

What’s a useable polar alignment deviation?

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Hi Folks

I’ve been using a Celestron 8” Edge HD on an Evolution mount for a year or so. Mostly visual, but I’m starting to dip into imaging.
 

I recently bought a wedge (I can’t afford an EQ mount atm). I use StarSense to align and then an all star polar alignment. And all seems to work fine, but I realise the polar deviation should be as close to 0 as practicable. So what numbers in degree terms would be considered ‘good as it gets’ given my mount and method? I need something to aim at :) 
 

The pic is the deviation from one session.

Thanks.

90A88BB9-48EB-4DEC-BBD3-43433DB797A9.jpeg

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I'm not sure at what point alignment is 'good enough' for your particular mount.

I'd say 6' (0.1 degrees) is a good alignment for normal use, that should give you good enough except for very long exposures..

Closer is better, but at about 2'/120" you get into which particular error are you trying to minimise. The correction is different for minimising different types of drift or field rotation.

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Thanks very much 🙂

Pretty sure I can get both down to 0.1 deg or better with a bit of care when I do my ASPA. Just need another run of clear nights to practise 👍

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If you're guiding, the PHD2 Developers say 5 arcmins is good enough, pretty much what theastrodragon said.

But if you're not guiding, as good as you can get, but there comes a point where it's too difficult to get consistently better.

Michael

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I found the ASPA good enough for planetary/ visual but not good enough for imaging deep sky

Are you running the starsense first and then the ASPA? Surely polar aligning afterwards knocks your star alignment out?

Also because the handset shows you're out by x amount, don't mean that you are

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When drift aligning my mount, I aim for below 4' (that's about 0.07 degrees) however I have a permanent pier and wouldn't want to do that every night!

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On 01/05/2020 at 00:06, michael8554 said:

If you're guiding, the PHD2 Developers say 5 arcmins is good enough, pretty much what theastrodragon said.

But if you're not guiding, as good as you can get, but there comes a point where it's too difficult to get consistently better.

Michael

That’s encouraging, thanks Michael. I haven’t been guiding, but I’ve just ordered an oag (something else to learn and get to grips with... :)) Hopefully I’ll be able to tweak down to 5 arcmins with a bit of care.

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On 01/05/2020 at 10:40, newbie alert said:

I found the ASPA good enough for planetary/ visual but not good enough for imaging deep sky

Are you running the starsense first and then the ASPA? Surely polar aligning afterwards knocks your star alignment out?

Also because the handset shows you're out by x amount, don't mean that you are

Yes, StarSense first then ASPA, but then I repeat the StarSense alignment - a sequence I found suggested in a separate thread which also suggested rinsing and repeating until there’s no further improvement. I’m working through SkyPortal on my iPad Pro atm rather than the handset. 

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I try and get down under 120", preferably closer to 60", but then I'm imaging unguided.

This may be of interest

 

Capture2.JPG.5260c8012b41ac61475ae51bbea46bec.JPG

 

I try and get the azimuth as close to zero as possible, but as you can see the ideal elevation depends a lot on what you want to minimise. I usually try and make that err on the minus side, usually around -60". But your mileage may vary.

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On 01/05/2020 at 11:09, Shibby said:

When drift aligning my mount, I aim for below 4' (that's about 0.07 degrees) however I have a permanent pier and wouldn't want to do that every night!

Sounds complicated :) I’m really just starting out and although I’ve heard of drift alignment I’m not completely sure what it involves in practice.

I’m just hauling my tripod in and out atm, but a pier sounds handy - I have a spot in mind, but not sure whether I could get away with building one 😆

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On 02/05/2020 at 12:25, theastrodragon said:

I try and get down under 120", preferably closer to 60", but then I'm imaging unguided.

This may be of interest

 

Capture2.JPG.5260c8012b41ac61475ae51bbea46bec.JPG

 

I try and get the azimuth as close to zero as possible, but as you can see the ideal elevation depends a lot on what you want to minimise. I usually try and make that err on the minus side, usually around -60". But your mileage may vary.

Thanks for that, I didn’t realise the axis settings could be varied to accommodate particular corrections.

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I only really started to pay attention when I went to unguided exposures!

For guided work, +/- 5' in el and az is fine, the only error you'll be left with is field rotation, and unless your doing a really long sequence that's rarely an issue.

Unguided work does need a lot more care to be taken with your setup, but there are benefits - for me, the benefits outweigh the care/time needed

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