What I do if I want a better starfeild effect is make the HSV noise but then go over it with the sparkle filter.
My methods for making a starfield is as follows...
There are basically four types of stars you can see: blue, red, yellow and bright. The dimmest stars usually just appear "bright", meaning they have no distinguishable colour. Brightests stars like Sirius, Arcturus, Antares or Betelgeuze have clear colour onto them, although they too are primarily just bright... So I make some plasma, then make every other colour but blue, red and yellow transparent (gets rid of cyan, green, magenta, white and black) to form a random colouration map for the brightest stars.
Then I make some grayscale noise, apply brightness/contrast at about 60/110, duplicate the layer, apply brightness/contrast at about -60/40 to only get the brightest stars visible. Gaussian blur with value 2. Normalize, desaturate if necessary. Apply the colour map to this layer at about 50% intensity, and the normal starfield layer on top of that. Should look about like this:
Aardwolf, make the differently sized stars on different layers. Same brightness/size goes to same layer. Then apply the diffraction filter on each layer separately, length varying based on the star brightness, and it should work quite nicely. Of course, if you did something else than a filter, then this might not work...
You could also try and kinda blur the combined star layer before diffraction, and apply that to increase the brightness of the nebula. Right now the brightness of the nebula seems only adjusted by the diffraction filter around the stars, which is not necessarily good thing.
Another thing to keep in mind is that while that kind of diffraction might be accurate for a photograph taken with a telescope that has four support beams for the secondary mirror, human eyes see diffraction very much differently (and diffraction wouldn't be static anyways, so baking it onto the background imagery wouldn't necessarily be a bright idea)... For FreeSpace background use, keep that in mind; for art, you can do whatever pleases the eye most.
