Issue with stars moving along the imaging session

[SupernovaF1]

Hello all

I would like to know what your thoughts are on this. 

According to EKOS, I have what I think its an excellent Polar Alignment (4arcsecs).

The following video is a composites of all the exposures in ~1.5 hours of red, green and blue filters.
There is quite a lot of movement and I couldnt get more than 120" subs unguided as I lent my guiding camera to a colleague.

I dont recall much wind. The only suspicion I have is mount balance as in the DEC axis it isnt great. Has anyone thoughts on this?

Thanks

WizardNebula.avi
WizardNebulaZoom.avi

Edited September 20, 2020 by SupernovaF1

[vlaiv]

Most people think that elongated / moving stars come from polar alignment error only.

This is not true, as there are two different causes of star drift in the image:

1. polar alignment error

2. periodic error

Second tends to be larger with good polar alignment. This is "oscillatory" movement of mount in RA direction - happens because gears are not perfectly round, and repeats every RA worm period. Usually about 8-10 minutes for most mounts (each mount model has its own worm period).

You can distinguish the two by direction - Periodic Error happens in RA direction and Polar alignment error happens in DEC direction.

On your main movie (not zoomed in) - orientation is: RA direction up down, DEC direction - left right.

On your zoomed in video, you'll notice rather smooth movement from left to right - this is drift due to polar alignment - slow drift in DEC (left-right). There is jumpy / seesaw motion up and down - this is RA periodic error. There is general slow drift in RA direction and this can be either due to worm wheel shape not 100% round (period 24h - one full revolution) or slight tracking speed difference to sidereal. Second thing can sometimes happen if mount electronics does not have very precise quartz crystal to keep time accurate - so it makes full revolution in 23:55 or 24:05 or whatever.

You can see on your video that Periodic Error is much faster - really fast jiggling up and down on this sped up video. If you have star trails - this will be responsible rather than polar alignment error.

Cures?

To some extent : Periodic Error Correction - look up PEC in your mount manual, and of course - guiding.

[SupernovaF1]

Wow! Thanks so much for such a detailed analysis. It makes total sense. I wasnt expecting PE to be so visible.

I have to say that the scope also needs a field flattener adequate for its focal ratio but that shouldnt affect how the stars move, as you mention.

The mount has indeed PEC learning and will give it it a go. Thanks @vlaiv