strange pattern on my image  

This is called "walking noise" as far as i am aware. The solution to this is dithering but this depends on your setup and if it is able to do it automatically. It can also be done manually but that would require you to stay with your setup all the time. At least you know the name now 🙂 I do not tihnk this is easy to remove with software in post processing.

Aha!

So that's Walking Noise? Helpful to me as well, since I got the same pattern in a stack of Markarian's Chain I took a few nights ago (52 x 4 minute exposures). I thought it was my camera sensor (DSLR) getting to hot and not taking enough dark frames to subtract.

Need to look into dithering myself apparently. Thank you.

Yes, that's a classic example of walking noise. Dithering is the best solution for that, I tend to do pretty big dithering steps like 5-10 pixels. If I have a lot of subs I dither every 3 frames, otherwise every frame (which might be a bit over the top, depends on the result).

@vincent-mod @wheeljack And if you increase the dithering step to 20 pixels for example, the noise level really goes down to the point where dark frames are useful for amp glow and the bad pixel map. Dithering, once you add it to your guiding routine, really does help and I think you will be surprised how good your images are underneath all that walking noise once you take new subs. 

Perfect. Thank you so much for your replies.

Yes, if you have bigger artefacts, bigger dithering steps will help. It's an old saying.. dither or die (I would rephrase that to something more gentle, but I didn't invent it). 😉