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Polar alignment - how accurate?


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Hi folks,

I'm thinking about a little project (just for the fun of it) to try and automate some of the initial setup required.

My plan is to us an electronic compass to find magnetic north and then offset this to find true north (I'm assuming that the centre of the circle that polaris describes is true north - is that a valid assumption?). 

I've seen devices which claim accuracies of +/- 1 degree, 0.5 degree and 0.1 degree. Obviously the more accurate the better, but given that I want to keep the cost down, how much accuracy do I need for:

a) observing and

B) imaging?

Thanks

Paul

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Not quite sure what you are aiming for but for imaging you aim for less than 30 arc seconds accuracy in your alignment. That's roughly 0.008 degrees. Sent from my iPhone using Tapatalk

Yep I'd agree with all that. Before I had a polar scope I tried all sort of tricks with a compass and inclinometer, and although I got close enough for visual it was miles off for imaging. If you have

http://www.365astronomy.com/amici-prism-for-polarscope-p-3500.html Try this. I meant 0.5 deg being 1 full moon width in my previous post...

Observing, no worries. For imaging, I think forget it. The magnetic lines of force are themselves not that accurate, and their direction varies on a local scale and could even be affected by your own metallic equipment. Regardless of the purported accuracy of a device, it can't surmount those obstacles.

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a standard half decent compass is more accurate than most digital / GPS units I've seen at a fraction of the cost
I have 3 GPS / compass apps and even with 10+ satellite fixes they can be off by 10*

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Not quite sure what you are aiming for but for imaging you aim for less than 30 arc seconds accuracy in your alignment. That's roughly 0.008 degrees.

Sent from my iPhone using Tapatalk

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Yep I'd agree with all that. Before I had a polar scope I tried all sort of tricks with a compass and inclinometer, and although I got close enough for visual it was miles off for imaging.

If you have a GoTo, cant you just do rough polar align, and then use goto for accurate reset of Alt/Az and go from there? Maybe you do that already?

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What methods give polar alignment accuracy of 30 arc seconds? And how do you know?

Chris

Synscan 3.35 does it all in my case. EQMOD and other planetarium software too as many others use.

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Thanks for all the replies.

Just to clarify, I wasn't thinking about a GPS solution, but something like this:-

https://www.sparkfun.com/products/7915 (though I'm guessing thats whats inside a gps..)

So, the general gist is that its not really worth doing (except maybe as a fun project) and I'm better off getting roughly aligned with a standard magnetic compass and then doing a multi-star alignment?

I know I could use a polar-scope, but unless anyone knows of one with a right-angled eyepiece, I was avoiding this as I don't want to be grovelling around on my hands and knees to look through it..

Cheers

Paul

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I use the ASPA routine in the Celestron mounts and AFAIK the Synscan routine is similar.  But 30 arc seconds?  I don't know about Synscan but the Celestron gives an unrealistic estimate of the error after ASPA because it assumes that everything is perfect. You need to do a new alignment to get an idea of the residual alignment error.

Chris

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I know I could use a polar-scope, but unless anyone knows of one with a right-angled eyepiece, I was avoiding this as I don't want to be grovelling around on my hands and knees to look through it..

Cheers

Paul

Amen to that - I spent last Monday night grovelling to get a decentpolar  alignment, and then more grovelling looking for the Pinwheel galaxy and came away from it with knees shot to bits. I'm hoping to get HEQ5 or similar with Synscan soon and use PA feature. Might not be perfect but better than physical disablement.

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I don't know what its like in the deep south of Englandshire but up here (Hebrides) there are so many magnetic anomalies that compass readings can be up to 12 degrees out and its not even consistent - it can be 12 degrees out one way then 12 the other way so up to 24 degrees swing !!!!

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So, the general gist is that its not really worth doing (except maybe as a fun project) and I'm better off getting roughly aligned with a standard magnetic compass and then doing a multi-star alignment?

Cheers

Paul

For imaging, there is no substitute for an accurate drift alignment. For visual, your mount's routines will be fine.

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Amen to that - I spent last Monday night grovelling to get a decentpolar  alignment, and then more grovelling looking for the Pinwheel galaxy and came away from it with knees shot to bits. I'm hoping to get HEQ5 or similar with Synscan soon and use PA feature. Might not be perfect but better than physical disablement.

http://www.365astronomy.com/amici-prism-for-polarscope-p-3500.html

Try this.

I meant 0.5 deg being 1 full moon width in my previous post...

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Amen to that - I spent last Monday night grovelling to get a decentpolar alignment, and then more grovelling looking for the Pinwheel galaxy and came away from it with knees shot to bits. I'm hoping to get HEQ5 or similar with Synscan soon and use PA feature. Might not be perfect but better than physical disablement.

Yes an HEQ5 with Synscan works perfectly or at least mine does. Don't understand the 'knees' bit?

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I have 3 GPS / compass apps and even with 10+ satellite fixes they can be off by 10*

A phone GPS app is nowhere near as accurate as a dedicated GPSr. Even the normal ones that you can buy in a shop still have an accuracy at the best of around 8 feet. For proper accuracy you are talking about military grade which then gets into 4 figures plus. 

The one thing that even a GPSr needs before it gives a proper heading too is that it requires movement. So unless you are going to walk around with a unit you will not be even anywhere near accuracy.

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http://www.365astronomy.com/amici-prism-for-polarscope-p-3500.html

Try this.

I meant 0.5 deg being 1 full moon width in my previous post...

Ah now thats a good idea - I wondered if there mght be such a thing. However - I also thought I might get my up and downs back to front if you see what I mean! Anyhow, Ive just this minute lashed out on on HEQ5, and will probably use the synscan PA routine. Also, even if I need to use polarscope I can use the mount set higher cos this mounts sturdier, so bye-bye knee problems...... (and bye-bye to a fistful of cash!)

Yes an HEQ5 with Synscan works perfectly or at least mine does. Don't understand the 'knees' bit?

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Well my current mount is EQ3-2 and a bit flimsy, so I dont extend the tripod legs at all - means the polar scope is only about 60 cms from the deck, so I have kneel and contort sideways to look through it.

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I have used the equivalent of one of thos right angle viewers on my my old Canon 400 D which did not have live view and I got fed up with doing human origami/limbo/..

It saved my neck.

I have neck injuries which are making the neck stiffer as I get older. No elses fault but mine from 30 years of doing kendo and wearing a helmet that weights in around 3 Kgs....

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you are right about your assumption of true north. here is a useful page if you want to experiment

http://www.threelittlemaids.co.uk/magdec/index1.html

But I fear that others are correct there are so many things that can affect a compass, underground power lines, overhead power lines even any steel in your scope and mount

that's quite true... i found that if i hold my tablet against one of the pillar legs of my mount that the north point indicated on the tablet compass moves significantly.

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Yep that was a very simple mod to make that I use all the time. Nowadays I have the mount controlled via eqascom so I just use the polar alignment tool in eqmod. Once the mount has slewed to the or text Polaris position I put the right angled viewer on and adjust the alt/az bolts and it is done :).

To reduce error further I also use alignmaster software too.

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for visual a rough polar alignment is fine. for planetary webcam imaging i still only do a rough polar alignment, although a better alignment does make life easier! For imaging, for DSO imaging accurate drift alignment is essential.

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Aaahhh, I see. I use my caravan step as a seat when using my polarscope on the HEQ5. It's the perfect height.

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So having spent all that loot on a HEQ5, I now need a caravan? :laugh:

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