Thanks all for comments and asnwers
and sorry if I answer after a bit of time.
Well, it's a special comet because its distance it doesn't have a tail again, so the outburst makes the actual coma.
@RocketBoy: I use to add a curve layer and then apply about 12-15 points and drag the points as you can see in the image attached here. Ihave many variants for mono and coloured versions. Anyway the LS filtering is the best way to emphatize details inside the coma.
Here you can find other two images that shows the difference in the coma diameter just in two days (about 2 times up):
Comet 17P with Canon Rebel XT (350D) Baader modified
Comet 17P two days comparing
Best regard, Anto