How to make a star field image equirectangular compatable?

So I have found an amazing tutorial on making a space scene (stars nebula planets ect) but now I’m stuck with how to convert the image into equirectangular format. Anyone have an idea how to do this in Photoshop or gimp?

If doing the equirectangular maps is not do-able in this way then can anyone point me into a decent skybox / sky sphere tutorial (one which involves slicing up the images and not just getting a pre-made image from Google)?

By the way here is the tutorial if anyone is interested. The results look amazing.

I think you can achieve great results with the “Polar Coordinates” effect on GIMP.

You can do it with Hugin.
Here is a tut:

Oooohhhh reading that now. Thanks Rich…you too Solar poke