Mosaic question, what's going in here

[cmarcus]

I'm collecting data for a two panel mosaic of the Soul nebula and being one of those people who can't tell if the pieces of a mosaic do actually fit I did a quick trial with two subs to see that they aligned using Register Union Mosaic in Star alignment.
I see they do fit, but are at an angle. This has been shot using the Sky mosaic feature. I've attached a screen shot view of the issue here and was wondering if anyone knows what's happened? Is it a PI issue? (Forgive my innocence but I can't see how the actual equipment has moved, I don't have a rotator...).
TIA for any input and apols in advance if this is NOT a p/i issue....

[NGC7789]

In my experience rotation between frames of a mosaic is normal.

One possible explanation is that if your polar alignment is not dead on then there will be rotation over time. It may not be sufficient to cause trails in your subs but it may be sufficient between two image sets. How much time elapsed between the two?

[cmarcus]

Thanks for your answer. They are about 2.5 hours apart.

[cmarcus]

Just to add, that the effect is still the same for two consecutive frames, 20 minutes apart (these are 15 min frames), i.e. the last of "A1", the first panel, and the first frame of "A2" the second one.

[oldwexi]

Hi Cmarcus!
That has to be that way and is s pretty normal if you dont rotate the camera!!!
It has nothing to do with PI!!!

If you produce a mosaic at DEC 0 the will be no Rotation at all. There it will work the way you think.
The more far away from DEC 0 to the South or to the North you expose the more
ROTATION YOU GET !!!

The smaller the circle in DEC the more Rotation you get.

Gerald

[Andres.Pozo]

Hi Cmarcus!

you could try to plan your frames using the script MosaicPlanner. It has an option to calculate the rotation of each tile so they are orthogonal when the mosaic is merged.

[cmarcus]

Thanks, Andres, I was not necessarily worried per se, in fact was glad enough of this allignment as I have been having teething problems for some time. Just wanted to check that nothing else was out of line.
But I shall look into that.

[chris.bailey]

The take away from this (and a lesson I have learnt from bitter experience) is to allow plenty of overlap between panels and not to frame objects too tightly. In SkyX I routinely allow a 20% overlap in the mosaic grid and give objects a similar amount of free space around them to the border.

Chris

ps Use Gradient Merge on those two frames and you should still have a very nice result.

[cmarcus]

Chris, thanks for that, I am hoping the mosaic to turn out ok, I have done them in the past, http://www.astrobin.com/136805/ and http://www.astrobin.com/136637/  and these were just two HA subframes that I wanted to check the mosaic alignment on.
It's just that having had a few teething problems in recent months I was worried that maybe there was another issue for me to worry about.