What's happened here?

[Atacamallama]

I'm collecting data for a two panel mosaic of the Soul Nebula and thought I'd check to see if the subs do align. Unfortunately I can never tell just by eye where mosaic panels go, so I did a trial align on two subs (in HA, FWIW). Using register union/mosaic in Star Alignment in Pixinsight.

They do fit and I assume would be useable when cropped, but they are off by a few degrees, and I can't understand why - or if this is a PI issue or a data collection one.

I used the SKY mosaic function to collect the subs.

I've taken a screen shot of the sky mosaic screen, and one of the two "aligned" subs.

Anyone have an idea what could be going on?

TIA for any input.

[MattJenko]

If your polar alignment is not accurate, guiding keeps the guide star bang on, but the image will slowly rotate around the guide star. I don't know what duration of subs you had here, but it may be the difference in PA misalignment rotation between the 2 mosaic subs after stacking them together.

[Atacamallama]

thanks for that, they are 900 sec subs, about 2.5 hours apart.

[julianr]

This is normal I think of it as the curvature of space

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[Atacamallama]

Double take here... then surely the same would happen between subs on the same target that much time apart...???? i.e. they would have rotated that much?

[MattJenko]

Correct. Bad PA will result in significant field rotation around a guide star. Alignment routines during stacking will in effect reduce the size of the image by only including the image intersects. Not saying that has happened here though, just thinking out loud. Another possibility is that if the camera is not orthogonal to RA/Dec, then a slew will cause a change of perspective when moved across the sky

[Atacamallama]

Ok, I have now aligned two files closer in time, in fact two consecutive frames, the last of A1 to the first of A2, there is about 20 minutes between their starting times (remembering they are 15 minutes subs). As we can see they still show the effect. Does that have implications for the polar alignment/rotation suggestion?

[julianr]

As matt jenko said it could be that you are not quite orthogonal to the ra/Dec axis - I have noticed this in all my mosaics

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