But the problem is that I should scale a lot the background image in order to have a large covering of the sky, and this brings a lower quality of it.

Seems you are using a wide angle FoV here.
Try setting the FoV to around 60 and move the camera back until you have a nice shot.

Note: you can move the camera further back to see the coordinates better. In fact you can move the camera anywhere on the x,y and z -axis, and your preview will still show exactly the same sector of the sky.



Do this again:



Look at the coordinates. Your image does not need to cover more than what's inside the coordinates, neither the width or the height.


Cec