How important is PA when guiding?

[Phil-lost!]

I've finally managed to make it out for a night of imaging but Im not having much luck with the synguider. I only did a rough polar alignment as I assumed the guider would keep things on track, would this be the cause of the problem? The synguider locked onto a star and said guiding but I still got trailing. Oh well, all good practice I suppose.

[Aeseir]

Guider is meant to remove any worm gear errors out of equation, i.e. make ever slight adjustments to ensure your image is dead center.

What makes the image stays dead center is the polar alignment. Without polar alignment (proper one), you are fighting an uphill battle as your guider is constantly having to chase the image and not correct for it.

So without PA may be a issue you facing

[silios]

What mount are you using?

Polar alignment is a must for any long exposure astrophotography, guiding or not.

If you have a pretty good PA, then the guiding will be minimised to correcting the periodic error (imperfections in gears etc.) of the mount.

If you don't have a good PA then the guider will be trying to compensate for movements in both RA and DEC axis in quite large steps, which could lead to fail guiding (star trails).

You could try to drift align the mount just to be sure you are dead on PA.

There are some pretty good youtube videos about PA and drift alignment to check out!

[Leveye]

Here's the difference. With a rough PA and autoguiding I can get just over five minutes. Most are keepers but not all. With a perfect perfect PA I can go almost 20 minutes and nearly every one has perfect stars. Take the time to get the best PA possible and then either do a drift alignment using PHD2 which i can't recommend enough or if you have a polar scope use that and the HC to get your pole star in the right place and then do a 2 star align at least.

[Scorpius]

I use a polar scope and the manual Alt/Az adjustment knobs on the AVX to get Polaris in the right spot on the reticle and that seems to work pretty well. After a while you can sort of tell if your PA is a little off and you can also tell when it seems real close. One night you may only be able to get 3-5 min. subs before star trailing occurs (more likely if you rush the alignment procedure including PA) but another night you may get 10 -15 mins (or more) if you took your time and/or got lucky. I think making an effort to get well polar aligned at the start of an imaging session is essential and should always be first on the to-do list once you're set-up. After you PA using your chosen method, and have done your star alignment - just center a star in the eyepiece (illuminated crossshair reticle is best) and watch it over the course of 10 or 15 mins. If it starts drifting away from the crosshair in 5 mins - your polar alignment likely needs some tweaking. This is the basic principle behind drift alignment except with that you are making manual adjustments to the mount as you observe the star's position and direction of drift in the eyepiece or on the computer screen. I agree with Aeseir that guiding is for very fine adjustments to the mount's tracking and should not be expected to compensate for bad PA...

[D4N]

There is probably a maximum correction the synguider can send to the mount, if the PA is to far out to be corrected by this then you will get trails.

In PHD it is possible to increase this limit, I don't know if you can on a synguider though.

Sent from my iPad using Tapatalk

[Ant]

I have guided for a long time (30 minute subs) without properly polar aligning.

It was something I forgot to do. PHD was more than happy to guide on a star and correct for whatever drift there was in the system - which that night was quite a bit.

But the graph looked ok and on initial inspection the subs looked good too... But whilst PHD was guiding on the star, and guide it did very well, it only guided on that one star. Over the course of many hours the real effect of bad polar alignment become apparent.

Each of the subs were slowly rotating around the guide star - so the bottom line is the better PA you have the less field rotation you'll get in the subs.

If your subs are fairly short, maybe 5 minutes, the stacking software will rotate each sub to correct for it. The rotation in each sub should be barely noticeable. If there is any rotation in the single subs the stacking software cannot do anything about it.

Cheers

Ant

[Phil-lost!]

Just me being lazy then! I just plonked the mount down roughly facing north while looking at the sun in the afternoon and left it where it was for the evening, lesson learned! thanks for all the replys. 

[MarsG76]

Hi Phil,

Accurate PA is very important, when I was rushing my PA I too was getting drift in subs that were 5-10 minutes even if they were autoguided because , as others wrote, you get field rotation around the guide star which looks like "drift".

When I setup early and spend a good amount of time on the PA as soon as stars are visible and got it aligned as accurate as possible I can constantly get low ISO 30 minute+ NB subs through out the night and all have round stars and are usable.

The guider is meant only to iron out the PE. 

[Thalestris24]

Hi

I think the bottom line is, that when guiding, the effect of PA error depends on focal length, sub length and the relative position of the guide star. I think there is a formula for calculating the amount of field rotation - a google search should find it!

Louise

[Scott]

Many people have said here that the best guider is the one that has to work the least. I have a pier so anything other than optimum pa would be silly but when I'm at a dark site i still try for the best pa possible (stopping short of drift alignment) At the end of the day, if all is done right, the end results will show