As mentioned, when imaging over multiple nights you can use plate solving or sync based Goto to ensure your target is closely framed and aligned each night
Deep Sky Stacker ( DSS ) is an excellent popular stacking program ( free download ) to stack and align your deep sky images
Even if your frames are not aligned by a considerable margin ( eg: 20 to 30 pixels ) or your image is inverted due to a meridian flip, DSS will align and stack the data and produce a final image that just needs to be cropped slightly to remove perimeter stacking artefacts.
For Planetary imaging Autostakkert 3 is an excellent stacking program which is also free. The AS stacked file can then be processed using Registax 6.
Attached is some documents I put together for both DSS , AS3 and Registax 6
Cheers
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