Generally, 20 arc seconds is more than sufficient. Some of the fine points are:
1) 20 arcseconds in the altitude will cause an RA drift of 0.26 arcseconds for a 3 minute exposure at 0º declination.
2) The same altitude error is halved (to 0.13 arcseconds) at +60º declination. Depending on plate scales, these are negligible since "seeing" will bloat a star to more than 2". You willl not be able to see any elongation.
3) 30 arcseconds of altitude error increases the above two cases to 0.4" and 0.2" respectively. You may start to see elongated stars for a 3 minute exposure near the celestial equator. So you probably want to polar align to better than 30", but still not waste time getting it below 15".
4) However, if you are using ASIAIR for polar alignment, the reported error is never very precise. When it tells you that you are aligned to better than 1", the true error is much larger than that.
The accuracy depends a lot on how far in declination you are when you take that first image for plate solving.
If the initial declination is +90º declination, then the 60º slew that the ASIAIR makes simply rotate the FOV around the pole (i.e., you are already polar aligned).
However, if the initial declination is off by a few degrees (for example with the initially declination set to +88º when you plopped down the tripod), the ASIAIR will make a 60º arc that is 2º in radius, centered on the pole -- in this case, the error is of the order of 10" (I will need to dig out the post that I made to this forum a couple of years ago, when I measured it).
So, if you really want sub-10" type polar alignment with the ASIAIR, you will need to repeat the polar alignement process a couple of times. Each time, the initial declination offset will be less.
Do not take the reported number from ASIAIR polar alignment as being correct. In fact, you can repeat the polar alignement and you will discover that each time, the initial error (right after the 60º slew) is never the same as when you finished the previous polar alignment. Just because ASIAIR puts out the fireworks and tell you that your polar alignment is 0", you may indeed have worse error than someone else whose measurement came in at 10".
I don;t know why ZWO keeps showing that childish fireworks, when they kow that it is a lie anyway.
For people who do not have a semi-permanent or permanent pier, where the initial altitude and azimuth errors can be more than a degree off, I would recommend doing a second polar alignment to confirm that the plate solve right after the 60º slew tells you that you are close enough. Espcially if the first plate solve after the 60º select of the first polar aligment tells you that you are off by more than a dozen arcminutes -- the result from that first alignment can be quite far off, and a second polar lignment will be neccessary, just to get a true 20" alignment.
Chen
