Simple mosaic results in swirled areas  

I tried dabbling with creating a Mosaic following a video tutorial provided on your site. Excellent tutorial but results were very far off. 

So I took a different route, processed each of the two frames separately into their own respective rgb images without any stretching, just created and saved.

Then I used these two files as the lights for a Mosaic, setting all the recommended settings in the video. They actually got assembled opposite of target center! So I started trying different settings. Changing one thing at a time till at least I got the center of the object in the center! My files were of the M31 Andromeda galaxy, shot during a gibbous moon to test the mosaic function. 

While I did get the center correct, the outer edges of the galactic star disk were rendered as swirls! And this was not only significant but would not stop happening regardless of any other changes I made. 

Rather comical actually.... and probably completely noobish in some error. 

I did do some searching here in the forums for swirl, didn't find much, so asking now. What could be causing this. I am attaching a simple screenshot of the affect to keep file size down.