Preprocessing: Aligning

If your imaging is almost entirely overhead and you have excellent polar alignment you can align using translation based on one star.

If your polar alignment results in noticeable field rotation over the course of light frame collection, you'll want to use translation and rotation. This will require at least two alignment stars. There's no harm in using three stars other than the increased execution time.

If your target has changed altitude considerably during your light frame collection, or you've refocused during frame collection then you may need to use translation, rotation and scaling. Scaling can help deal with atmospheric refraction. In this case you should use three alignment stars.

All calibrated frames can be aligned together, but resize all the frames to match the size of the least binned frames before aligning unless your software handles a mix of image sizes automatically.

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