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Do you use refracted or geometric pole for alignment with new tools

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#1 freestar8n

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Posted 27 June 2019 - 06:15 AM

There are many new software and optical tools out there for polar alignment - and I think people are comparing accuracy and ease of use.  When you are down in the arc-minute regime - the error will hardly matter since drift in RA and dec. will always be present to some extent due to refraction.  It is explained pretty well here and in associated pages:

 

http://canburytech.n...iftAlign_1.html

 

It makes sense to get the mount as aligned as possible in azimuth - but for altitude it will depend on your latitude and the star's alt/az.

 

So when people say they are 1' off in altitude - is that relative to the geometric pole?  What convention are people normally using?

 

This is mostly an academic issue because I don't think any particular altitude setting for the polar axis will work in general.  But it's important to know what the reference point is when measuring polar alignment.

 

Frank


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#2 t-ara-fan

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Posted 27 June 2019 - 06:44 AM

Good question. I use SharpCap Pro with a 480mm FL guide scope. It tells me the alignment is good to something like 0°00'04" give or take a little jumping around due to seeing.

I don't know if SharpCap factors in my latitude. PA is definitely good, but that good??

I don't mind if my GOTO is off a little, because I plate solve. But I do aspire to unguided exposures at my shorter focal lengths. Do I need a pointing model to make that happen?

Edited by t-ara-fan, 27 June 2019 - 06:56 AM.


#3 freestar8n

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Posted 27 June 2019 - 06:57 AM

Good question. I use SharpCap Pro with a 480mm FL guide scope. It tells me the alignment is good to something like 0°00'04" give or take a little jumping around due to seeing.

I don't know if SharpCap factors in my latitude. PA is definitely good, but that good??

4"?  I hear people talking about that level of accuracy.  Was that hard to achieve?

 

Below about 30" things like nutation become important - and it may or may not be factored into where the pole exactly is.  So that is another layer of ambiguity.

 

Polar misalignment causes 3 things:  RA drift, Dec. drift, and field rotation.  Field rotation will be very small once you are a few minutes away - and dec. drift will depend on where you are in the sky.  For RA drift you can align on the refracted pole and use King rate.  Are people using King rate?

 

RA errors are easy to guide out - but dec. errors are harder.  But I find that dec. errors tend to go both up and down anyway due to mount imperfections and other factors - and they can be guided out because they tend to be slow.

 

So I assume all these tools work pretty well unless there is a bug.  And it's probably best for most people to align on the refracted pole - in which case the polar alignment error should be relative to that.

 

Frank



#4 Dynan

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Posted 27 June 2019 - 07:02 AM

I use refracted correction in SharpCap PA and get good results. I am at 31°N, so the CP is a bit low, giving me plenty of atmosphere to fight. But I assume that the refraction correction is to adjust orthagonality to the geometric pole.

 

When I get close to PA, within seconds, the seeing obviously becomes the limiting factor. I then play the average...the better the seeing, the smaller the swing, equal amounts Up/Down - Right/Left. But I haven't verified SC PA versus drift align since my images are fine in regards to SC PA. It's the other ways that I ruin my images that become my priority.

 

But yes, I use refracted index. At 31°N the amount is, I believe, about 2', which I think is calculated in SharpCap.

 

I guess it gets down to the fact that we're all 'Lucky Imaging' to a certain extent, though it can reach minute proportions, unless you're riding Hubble hubble.jpg



#5 calypsob

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Posted 27 June 2019 - 05:20 PM

4"? I hear people talking about that level of accuracy. Was that hard to achieve?

Below about 30" things like nutation become important - and it may or may not be factored into where the pole exactly is. So that is another layer of ambiguity.

Polar misalignment causes 3 things: RA drift, Dec. drift, and field rotation. Field rotation will be very small once you are a few minutes away - and dec. drift will depend on where you are in the sky. For RA drift you can align on the refracted pole and use King rate. Are people using King rate?

RA errors are easy to guide out - but dec. errors are harder. But I find that dec. errors tend to go both up and down anyway due to mount imperfections and other factors - and they can be guided out because they tend to be slow.

So I assume all these tools work pretty well unless there is a bug. And it's probably best for most people to align on the refracted pole - in which case the polar alignment error should be relative to that.

Frank


Have you tried sharpcap polar align? I think its free if you dont own it. They also have pro. On a cge, it is possible to dial in 0 arc seconds if seeing is decent.
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#6 t-ara-fan

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Posted 27 June 2019 - 08:50 PM

Have you tried sharpcap polar align? I think its free if you dont own it. 

You need the PRO version to get PA. 

 

SharpCap PA is faster and easier than the PoleMaster software.  And with a longer FL scope you get more precision. 


Edited by t-ara-fan, 27 June 2019 - 08:51 PM.

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#7 roofkid

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Posted 28 June 2019 - 03:36 AM

I also use refracted correction. It's quite the difference if you compare both. It also has made an improvement in my guiding, even though this could also just be placebo - I have not done a scientific analysis of this ;)




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