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How accurate do you polar align?


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On 5/18/2014 at 17:03, frugal said:

The weather has finally cleared enough that I can actually use the scope the I bought in the FLO clearance sale a while ago. I spent a good while setting up the scope in the light. I collimated the polar scope and when it got dark I polar aligned as accurately as I could.

When I performed a two star alignment it reported a polar alignment error of about 1 arc minute in either axis. Seeking across to targets got them on screen first time every time.

My question is: Is a polar error of 1 arc minute considered accurate?

About half of the 150 second subs had nice round stars, however the other half had eggy stars (but I think that this had more to do with the balance being slightly camera end heavy)

Sent from my SM-P900 using Tapatalk

I just wrote a little calculator based on Barrett's paper.  It will let you calculate how accurate your PA needs to be given other parameters, including dec, guide star angle, and FL.

You can download it, and the source code in case you are a programmer, here:

https://github.com/133Parsecs/PolarAlignmentCalculator

There is a link to Barrett's paper at the bottom of the readme.

I hope it helps.

In general, 1 arc-min is very good. So good, in fact, I would question it if you just did a quick two star test.  My experience is that two star tests often do not give highly repeatable results.  I would try it a few times to be certain.  And be sure to use different stars.

Also, the required accuracy will change as you go higher in dec, or change your guide star. 

My routine is to try to nail PA as well as possible. Then I use a few minutes of drifiting in PhD2 to quantify it.  From there, I plug the parameters into the program and see what my max exposure time is for each target I'm thinking about imaging.

 

Hope that helps.

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Here are my thoughts on polar alignment. My polar alignment was about as slapdash as you can get. A simple app showing where on a circle Polaris is and then eyeballing it through the polar scope in 10 seconds max, and that was that. Alignment is completely ignored and I use platesolving to line up objects. I then rely totally on guiding to mask all sins. Field rotation was never a problem with the shortish focal length and smallish sensors I use. Recently I got hold of a PoleMaster and my PA accuracy is considerably higher. Guiding now has significantly less work to do, as the tracking has improved massively. However, the end images are not that noticeably different. They are better, and I have now started using a longer focal length instrument, but the difference was not that much for the shorter focal length refractor. The seeing wobbles my stars with my long exposures more than the tracking wobbles them.

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I would say that you should aim to get it as accurate as possible... then if it is causing problems you can tweak things by backing off a bit.

I use a PoleMaster (equivalent) and it makes polar alignment quick and easy and to a degree where I don't have to worry about it.

SharpCap now supports a polar alignment routine similar to PhotoPolarAlign and I will try that at some point.

My biggest issue was flexure...which is hopefully now sorted...

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  • 3 months later...

Hi just to add that Skywatcher say my AZ-EQ6GT mount should achieve a best of 1 arc minute PA error.  

To do this:  PA with polar scope, 2 or 3 star alignment, and then the additional HC Polar alignment routine (adjusting ALT and AZ manually on another target, one at a time), and repeating this 2-3 times.

Sounds like about 30-40 minutes worth of work.  it also says cone error will have a limiting factor on this as well, so check that first.

tbh I haven't tried the additional routine, the PA with scope achieves 5-20 arc min error on first pass.  this has been ok for 60-90 subs for me, but i am no expert..

 

Mike

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  • 2 weeks later...

I'm using the PoleMaster polar align camera. The first time I run the proceedure it took me about 10 to 15 minutes to align. Just for test purpose I took this 10 minute exposure. It's taken through a Megrez 90 with a focal length of 560 mm. This piece of sh-t is amazing :-)

M31 10min x1 unguided orginal.jpg

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  • 2 months later...

There is an interesting article by Simon White in Decembers BAA Journal that covers just this issue. He argues that, where the errors in polar alignment are relatively small, the declination of the target is of little practical importance (note that he's assuming PAE of the order of 1 arcminute or so). His little thought experiment is roughly as follows (it's based on the small angle of error meaning we can treat the sky as a flat rotating disk):

Imagine a perfectly polar aligned mount, with the scope focused dead centre on a star (any star will do). Now move the scope out of alignment by a small amount (say 1 arcmin). The star will move exactly this much in the field of view, so it is now offset from the centre by 1 arcminute. If we leave the mount to track for 1 sidereal day (and imagine we could watch the star the whole time) we would see the star describe a circle in the eyepiece, around the centre of the field of view, and return exactly to the point where it started. A distance of 2 x Pi x PAE, or 0.262 arcsecs per minute for a 1 minute error. We know that pixel scale in arcseconds is given by 206 x pixel size / focal length. That means the time to drift 1 pixel (in minutes) is given by:

206 x pixel size  x 24 x 60 / (2Pi x focal length x PAE(arcmin))

He uses this to work out a little table of time (in the article for a Nikon D90) for a one second drift with various lenses and focal length scopes (with an 805mm scope there is 1 pixel of drift every 322 seconds; while a 200mm lens will take 21 minutes for the same drift). The figure from that table can then be divided by the actual alignment error (assuming this is known) to calculate the approximate achievable exposure time. The figure is easy to adjust of you are happy to accommodate more than 1 pixel of drift.

Not mathematically exact, but a straightforward rule of thumb that (according to White) gives results that are very close to the original Hook formulae.

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On 20/05/2014 at 17:48, glowingturnip said:

i like the theory that a slight mis-align means that your guiding doesn't keep switching on dec - sounds like an excuse for me to be lazy  ;-)

It's very well known and certainly not my idea. I find it works, though.

In saying that if some subs are good and some not it cannot be PA, I should have added 'while imaging the same target.' 

Olly

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On 19/05/2014 at 07:56, ollypenrice said:

a slight misalignment allows you to run the autoguide Dec corrections in one direction only and this can remove the problem of Dec oscillation. I have to do this on one of our mounts routinely.

Hi Olly, sorry to butt in. How do you do this? If my guiding goes wrong, it's nearly always the Dec!

Alexxx

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1 hour ago, Astrosurf said:

Hi Olly, sorry to butt in. How do you do this? If my guiding goes wrong, it's nearly always the Dec!

Alexxx

Firstly you need to be slightly misaligned. I've never actually set out to misalign, I've just exploited the misalignment that was there already! (By not endlessly refining the PA but settling for a 'close enough' alignment.) You then disable the guiding in Dec on one axis and see if that's the axis you do or don't need to guide on. If the mount guides in Dec that's fine. If it drifts off target you need to re-activate the direction you disabled and disable the other one. Write down the result of this test (if you're anything like me...) and reverse the disabling after the meridian flip.

Olly

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  • 2 weeks later...
On 14/12/2016 at 09:13, ollypenrice said:

It's very well known and certainly not my idea. I find it works, though.

In saying that if some subs are good and some not it cannot be PA, I should have added 'while imaging the same target.' 

Olly

blimey, i'm being quoted from over two and a half years ago !  :-)

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  • 3 years later...
On 13/12/2016 at 21:23, billyharris72 said:

There is an interesting article by Simon White in Decembers BAA Journal that covers just this issue. He argues that, where the errors in polar alignment are relatively small, the declination of the target is of little practical importance (note that he's assuming PAE of the order of 1 arcminute or so). His little thought experiment is roughly as follows (it's based on the small angle of error meaning we can treat the sky as a flat rotating disk):

Imagine a perfectly polar aligned mount, with the scope focused dead centre on a star (any star will do). Now move the scope out of alignment by a small amount (say 1 arcmin). The star will move exactly this much in the field of view, so it is now offset from the centre by 1 arcminute. If we leave the mount to track for 1 sidereal day (and imagine we could watch the star the whole time) we would see the star describe a circle in the eyepiece, around the centre of the field of view, and return exactly to the point where it started. A distance of 2 x Pi x PAE, or 0.262 arcsecs per minute for a 1 minute error. We know that pixel scale in arcseconds is given by 206 x pixel size / focal length. That means the time to drift 1 pixel (in minutes) is given by:

206 x pixel size  x 24 x 60 / (2Pi x focal length x PAE(arcmin))

He uses this to work out a little table of time (in the article for a Nikon D90) for a one second drift with various lenses and focal length scopes (with an 805mm scope there is 1 pixel of drift every 322 seconds; while a 200mm lens will take 21 minutes for the same drift). The figure from that table can then be divided by the actual alignment error (assuming this is known) to calculate the approximate achievable exposure time. The figure is easy to adjust of you are happy to accommodate more than 1 pixel of drift.

Not mathematically exact, but a straightforward rule of thumb that (according to White) gives results that are very close to the original Hook formulae.

Such a useful post. Thank you. I've been looking for just this information. The numbers are very close to what I get in practice with a DSLR and 400mm Tele bolted on top of my scope when I look up the alignment error I've got. I'm limited by light pollution in the length of my subs. So if I can get 2mins unguided and keep things simple, I'm very happy. 

 

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