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Old 01-08-2021, 12:20 PM
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g__day (Matthew)
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Polar aligning and guiding on a rig that can correct for polar misalignment

Hi folks, quick question it has taken me years to think off asking!

With any three star model my old mount (Vixen Atlux and SkySensor2000-PC) can correct for polar misalignment of up to 20 degrees - it simply runs both motors at correction rates it solves mathematically for all over sky pointing - including atmospheric refraction - but what if it's a tiny bit wrong I just wondered...

I wonder now if I should turn off my mount's polar misalignment correction capabilities whenever I am:
  1. polar aligning my permanent rig by any drift or T-point method and
  2. when doing guided imaging - as seeing versus real drift vs mount corrections could all cause slight see saws in my tracking.
I only realised now there could be a small feedback loop causing some extra difficulty for the guiding - where the mount corrects in anticipation for expected polar misalignment and possibly at times being a bit ahead or behind optimal gear movement to keep the star centred it might be worsening the guiding slightly...

I was pondering why subsequent TSX 300 star models gave slightly different polar correction advice - normally adjustments less than 2 arc minutes on one or both axes. So I might do two subsequent runs - one straight after the other - only noting the correction advice - not implementing it - and see if between subsequent runs (with a new supermodel each time - complicating things possibly) whether the polar correction advice was consistent or not - it was slightly off - normally only in East / West corrections.

Given I have a permanently set up rig - I wonder if I might be best to only do a 1 or 2 star align in my hand controller - to avoid triggering the SS2K's polar misalignment function (or do a three star model - but just click accept on the third star - without any re-centring on the third star even if it is a few arc minutes of dead centre) - then use TSX closed loop slew or NINA or SGP to centre on any target and then use PHD2 to track.

Does that make sense to folk -any suggestions?

Last edited by g__day; 01-08-2021 at 12:42 PM.
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Old 02-08-2021, 07:30 AM
gregmc (Greg)
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Actually, I think you have missed the main reason for polar alignment. If it was just tracking the sky, you could also do that on an Alt/Az mount with motors.

The reason it's needed is to prevent field rotation in your images. The camera needs to turn with the sky. This issue increases with the larger the sensor and longer the focal length.

On an EQ mount, the RA motor already should be turning at a rate to match the earth rotation and the dec motor doing nothing unless needed. Guiding basically does two things on an EQ mount. It bumps the speed to slow down or speed up the RA motor to match the earth rotation or bump the dec up or down to allow for any drift away from the movement of stars rotating around the celestial pole.

What is best is to polar align the EQ mount so that the RA axis is accurately pointing to the celestial pole. If you can see NCP/SCP, then programs like SharpCap is one of the faster ways of doing it. If you can't see the SCP/NCP then an app that does drift alignment is a slow way but the overnight build of NINA now has plugins, and a plugin you can get is the 3 star polar alignment that does not need to see either SCP or NCP.

Also, some mounts have a 3 star polar alignment in the handset but they rely on manual centering of each star. It's also best not to use a handset and replace the computing done in the handset with applications on a computer.
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Old 02-08-2021, 08:37 AM
Startrek (Martin)
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Originally Posted by gregmc View Post
Actually, I think you have missed the main reason for polar alignment. If it was just tracking the sky, you could also do that on an Alt/Az mount with motors.

The reason it's needed is to prevent field rotation in your images. The camera needs to turn with the sky. This issue increases with the larger the sensor and longer the focal length.

On an EQ mount, the RA motor already should be turning at a rate to match the earth rotation and the dec motor doing nothing unless needed. Guiding basically does two things on an EQ mount. It bumps the speed to slow down or speed up the RA motor to match the earth rotation or bump the dec up or down to allow for any drift away from the movement of stars rotating around the celestial pole.

What is best is to polar align the EQ mount so that the RA axis is accurately pointing to the celestial pole. If you can see NCP/SCP, then programs like SharpCap is one of the faster ways of doing it. If you can't see the SCP/NCP then an app that does drift alignment is a slow way but the overnight build of NINA now has plugins, and a plugin you can get is the 3 star polar alignment that does not need to see either SCP or NCP.

Also, some mounts have a 3 star polar alignment in the handset but they rely on manual centering of each star. It's also best not to use a handset and replace the computing done in the handset with applications on a computer.
Greg,
Iíve used my Handset on different Skymounts with Synscan PA for over 4 years now connected on my Lappy using BYEOS and a simple DSLR and its equal to if not more accurate than Pole Master , and you donít need a view of the Pole plus you only need 2 alignment stars not 3. Live view on BYEOS with the DSLR has no lag on your alignment Star when moved its instantaneous
Takes me 5 to 10 minutes to polar align
Even Dylan OíDonnell endorsed it on one of his YouTubeís ( not that Iím a fan )
Itís an popular method thatís not considered or even not known about in favour of Sharpcap and many others. Works treat for me
Cheers
Martin
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Old 02-08-2021, 09:27 AM
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g__day (Matthew)
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Let me restate - I am still trying to improve both my polar alignment and guiding. When one is within 2 arc minutes of the celestial pole do you really think frame rotation is a big thing - I am not doing 30 minute subs anymore since I switched to ZWO camera - I rarely to 6 minute subs even! Stacking takes out any small field rotation easily.

I do not have an unobstructed view of the celestial pole - and after long discussions between both the Bisque brothers and the author of PEMPro - the Bisques kind of won saying a large Tpoint model will always be superior for all points in the sky over prolonged drift alignment over only any two points in the sky... as it adjusts for all telescope and mount behaviours over all sky.

Now I ponder that the best polar alignment I might ever achieve is something like is between 30 arc second to 2 arc minutes from the refracted celestial pole with my gear and current skill level.

But that was never the point of my post. It was I have potentially competing tracking algorithms going that will compete - so how do I analyse them to achieve the best outcome?

The SS2K may also believe I am say 2 arc minutes off the SCP - so depending where in the sky I am pointing the SS2K controller can either be told to adjust RA speed and DEC or not. Given I normally have it on it could and probably would be creating periodic slight adjustments to my mounts position (I don't know the correction frequency or amount). The amount the SS2K may be changing the RA and correcting DEC is likely to be very small. It might be saying run the RA at 0.999998 sidereal and send a +0.0001 to the DEC motor ever 26.7 seconds for instance - picking numbers out of my head for an example when a star is on the equinox at say 30 degrees elevation above the horizon. Now that might perfectly adjust my guide star for polar misalignment - or it may not - but one thing for sure my guide program would see it and either say well that wasn't me - but it helped - thanks - or whoops - was that just seeing - ignore it for a few seconds and see if it goes away or opps I am not tracking correctly - issue a guide command to correct for drift. Either way I now have an extra feedback system trying to help - and it may or may not be succeeding - I simply don't know.

The easiest thing I could do would be to guide for a few hours and say every twenty minutes switch on and off the mounts polar misalignment correction and see if the guiding improves or worsens according to the guide camera.


My gut says that for visual or on a rig that isn't within say 30 arc seconds of the pole it will help - outside that it would only help if the adjustments were really, really fine tuned and the gear was perfectly balanced with no backlash - otherwise you have in effect two different people trying to steer the car so to speak and they are probably interfering with each other!


Does my thinking make sense to folk - or would you suggest a different approach?


Thanks, Matt
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Old 02-08-2021, 06:36 PM
Startrek (Martin)
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Originally Posted by g__day View Post
Let me restate - I am still trying to improve both my polar alignment and guiding. When one is within 2 arc minutes of the celestial pole do you really think frame rotation is a big thing - I am not doing 30 minute subs anymore since I switched to ZWO camera - I rarely to 6 minute subs even! Stacking takes out any small field rotation easily.

I do not have an unobstructed view of the celestial pole - and after long discussions between both the Bisque brothers and the author of PEMPro - the Bisques kind of won saying a large Tpoint model will always be superior for all points in the sky over prolonged drift alignment over only any two points in the sky... as it adjusts for all telescope and mount behaviours over all sky.

Now I ponder that the best polar alignment I might ever achieve is something like is between 30 arc second to 2 arc minutes from the refracted celestial pole with my gear and current skill level.

But that was never the point of my post. It was I have potentially competing tracking algorithms going that will compete - so how do I analyse them to achieve the best outcome?

The SS2K may also believe I am say 2 arc minutes off the SCP - so depending where in the sky I am pointing the SS2K controller can either be told to adjust RA speed and DEC or not. Given I normally have it on it could and probably would be creating periodic slight adjustments to my mounts position (I don't know the correction frequency or amount). The amount the SS2K may be changing the RA and correcting DEC is likely to be very small. It might be saying run the RA at 0.999998 sidereal and send a +0.0001 to the DEC motor ever 26.7 seconds for instance - picking numbers out of my head for an example when a star is on the equinox at say 30 degrees elevation above the horizon. Now that might perfectly adjust my guide star for polar misalignment - or it may not - but one thing for sure my guide program would see it and either say well that wasn't me - but it helped - thanks - or whoops - was that just seeing - ignore it for a few seconds and see if it goes away or opps I am not tracking correctly - issue a guide command to correct for drift. Either way I now have an extra feedback system trying to help - and it may or may not be succeeding - I simply don't know.

The easiest thing I could do would be to guide for a few hours and say every twenty minutes switch on and off the mounts polar misalignment correction and see if the guiding improves or worsens according to the guide camera.


My gut says that for visual or on a rig that isn't within say 30 arc seconds of the pole it will help - outside that it would only help if the adjustments were really, really fine tuned and the gear was perfectly balanced with no backlash - otherwise you have in effect two different people trying to steer the car so to speak and they are probably interfering with each other!


Does my thinking make sense to folk - or would you suggest a different approach?


Thanks, Matt
Matt,
Who does 30 min subs ??
Thereís so much crap flying around the planet both in orbit and across the sky that makes long long subs too high risk
The longest subs I take are 5 min , mostly 3 min on nebula , galaxies etc...,
More shorter subs , less noise , less forgiving on your tracking and guiding and lower risk of satellite trails and other rubbish flying across your FOV
As long as your stars are round to edge of field after say a 5 min sub then you can achieve almost anything in AP , right ??
My PA is usually just under an arc minute ( maybe as low as 0.50 arc sec ) , I use low end Chinese mounts with newts , 15kg payload and my guiding on good nights is 0.60 to 0.70 arc sec error total, no problem with round stars at 5 min subs
Cheers
Martin
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Old 03-08-2021, 07:30 AM
gregmc (Greg)
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Quote:
Originally Posted by Startrek View Post
Greg,
Iíve used my Handset on different Skymounts with Synscan PA for over 4 years now connected on my Lappy using BYEOS and a simple DSLR and its equal to if not more accurate than Pole Master , and you donít need a view of the Pole plus you only need 2 alignment stars not 3. Live view on BYEOS with the DSLR has no lag on your alignment Star when moved its instantaneous
Takes me 5 to 10 minutes to polar align
Even Dylan OíDonnell endorsed it on one of his YouTubeís ( not that Iím a fan )
Itís an popular method thatís not considered or even not known about in favour of Sharpcap and many others. Works treat for me
Cheers
Martin
1) How do you know the handset method accuracy compared to pole master? I would have thought a comparison over multiple nights in different seeing conditions with drift alignment might have been the only way to know.
I only make this point as you raised the accuracy point where I was just talking about Polar Alignment and it's purpose to reduce field rotation and guiding can not fix that field rotation caused by bad polar alignment.

2) Have you tried the NINA version of the 3 star polar alignment? It works the similar way to the handset 3 star but automated. Search YouTube as there are now a few examples videoed.
I understood the handset 3 star alignment worked but mentioned NINA as it was a recent automated method if you wanted to use the latest software option.

3) "works a treat for me" that's fine and by all means, keep doing what works and what you know but it might not be the same for others which is why I mentioned it as a general comment. Accurate polar alignment particularly matters with bigger sensors like a full frame camera for those who want those stars around the edges not to have smears in a circular direction (so not talking about the effects of flat fields which is a different matter). But you can not correct field rotation caused by bad polar alignment by guiding. If you only have a small sensor and a short focal length, it might not matter.

I thought the topic was about a rig that can correct for polar misalignment. I presented options in broad categories and 3 point was one of them with NINA plugin being a recent release people might not know about.

If you want a rig that is tolerant to polar alignment, perhaps a rotator with some derotation software is a better path but I'm guessing that might be easier to get going on an Alt/Az mount so you don't have to do any Polar Alignment.
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Old 03-08-2021, 07:39 AM
gregmc (Greg)
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Originally Posted by g__day View Post
Let me restate - I am still trying to improve both my polar alignment and guiding. When one is within 2 arc minutes of the celestial pole do you really think frame rotation is a big thing - I am not doing 30 minute subs anymore since I switched to ZWO camera - I rarely to 6 minute subs even! Stacking takes out any small field rotation easily.

I do not have an unobstructed view of the celestial pole - and after long discussions between both the Bisque brothers and the author of PEMPro - the Bisques kind of won saying a large Tpoint model will always be superior for all points in the sky over prolonged drift alignment over only any two points in the sky... as it adjusts for all telescope and mount behaviours over all sky.

Now I ponder that the best polar alignment I might ever achieve is something like is between 30 arc second to 2 arc minutes from the refracted celestial pole with my gear and current skill level.

But that was never the point of my post. It was I have potentially competing tracking algorithms going that will compete - so how do I analyse them to achieve the best outcome?

The SS2K may also believe I am say 2 arc minutes off the SCP - so depending where in the sky I am pointing the SS2K controller can either be told to adjust RA speed and DEC or not. Given I normally have it on it could and probably would be creating periodic slight adjustments to my mounts position (I don't know the correction frequency or amount). The amount the SS2K may be changing the RA and correcting DEC is likely to be very small. It might be saying run the RA at 0.999998 sidereal and send a +0.0001 to the DEC motor ever 26.7 seconds for instance - picking numbers out of my head for an example when a star is on the equinox at say 30 degrees elevation above the horizon. Now that might perfectly adjust my guide star for polar misalignment - or it may not - but one thing for sure my guide program would see it and either say well that wasn't me - but it helped - thanks - or whoops - was that just seeing - ignore it for a few seconds and see if it goes away or opps I am not tracking correctly - issue a guide command to correct for drift. Either way I now have an extra feedback system trying to help - and it may or may not be succeeding - I simply don't know.

The easiest thing I could do would be to guide for a few hours and say every twenty minutes switch on and off the mounts polar misalignment correction and see if the guiding improves or worsens according to the guide camera.


My gut says that for visual or on a rig that isn't within say 30 arc seconds of the pole it will help - outside that it would only help if the adjustments were really, really fine tuned and the gear was perfectly balanced with no backlash - otherwise you have in effect two different people trying to steer the car so to speak and they are probably interfering with each other!


Does my thinking make sense to folk - or would you suggest a different approach?


Thanks, Matt
Stacking can not remove field rotation. It can align images that have been rotated which is different.

Field rotation is where during the exposure, the camera does not rotate exactly with the earth during the actual exposure.
The stars in one part of the image will move at the guided rate but the stars at the other end of the sensor may not in a noticeable manner.

If a short focal length, a smaller sensor or lower resolution, shorter exposures (etc?), it may not be noticeable so may not be a concern.
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Old 04-08-2021, 05:52 PM
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g__day (Matthew)
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The 30 minute subs were at F10 - 2.3 metre focal length to see what my mount could achieve - stars were round but a tad bloated to say the least - but it gave me an idea of what Vixen Atlux could do carrying three scopes and five cameras.

With more sensitive and cooled ZWO cameras generally 3 mins to 6 mins is fine. But the key question is how to best tune my gear - and I think as ever experiment and collect data!
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Old 05-08-2021, 07:07 AM
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Depending on how far you go down which particular rabbit hole in imaging, I think that accurate polar alignment, no PA related tracking corrections in the mount and guiding is the best way to go, otherwise as you say, you may well have the mount issuing its own corrections and complicating the guiding.

If you do happen to do long exposures you won't want to spoil them with field rotation, and if you send the mount around the sky with imaging software and plate solving, any PA correction benefit to basic pointing in a multi star alignment of your mount will be lost as well. Even with my Orion AZEQ6 clone I don't bother with a multi star alignment any more, or any initial alignment in fact. I make sure the PA is good then simply start from the park position and it is close enough to it's assumed position that plate solving takes care of the rest. And I know that the mount is tracking only in RA so no guiding complications to worry about.
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