I'm trying to create a small poster of a 2D animation character — irrelevant for this matter — which doesn't exist in a bigger image for manipulation available anywhere, so I captured a frame of said animation at 720p but when I opened the PNG with Photoshop I noticed the sky behind the character had a not-so-good gradient:
Opening the image helps to see the problem
I don't know the proper terms, but it's very noticeable that in the first 1/3 there is something like a curved gradient and around 3/3 another one more straight and, in both cases, the colours aren't mixing very well. And when applying the Levels and Curves I usually do for some basic enhancement these... banding becomes really evident and ugly.
The first attempt to solve or at least improve was with a Spatter (Filter Gallery > Brush Strokes) with a Darken Blending Mode:
While the transitions after the first 1/3 and before the 3/3 became smoother, not only I had a "spray effect" but I also lost the small dots representing the stars.
Unable to bring decent results from my searches, I deleted the main character in a duplicated layer and added a Gradient Overlay:
Because I didn't know where the stop points were, in my ignorance I computed the rule of three of all points — to get the approximate X% required by the overlay settings — whenever I could notice a significant difference of colours.
The result was better than the original screencap, but not quite yet. And I still lost the stars.
And I would like to know if, maybe, there's a better way I don't know to smooth this gradient (first picture), especially in a "fail-safe" way when applying Levels (Mid-tones Darker) and/or Curves (Linear Contrast) as these are like my "go-to" settings for this set of pictures.
I felt the need to update the question with the current progress as I couldn't follow any of the suggestions in here so far in their totality.
Well, although blurring the problems was a possibility, the results weren't too good and it became even worse with the basic enhancements, so I indeed had to recreate the sky as much as possible. If you care to take a look, I've hosted a sample on Dropbox.
I simplified the process with fewer colour stops of a Gradient Overlay instead of directly with the Gradient Tool because, for some reason, the Layer Style resulted in better quality. I even managed to fake some stars and the curved lighting on top (although the latter didn't blend that nicely).
But if you notice, I've included copies of my usual "go-to" adjustment layers, specifically the Levels set for "Midtones Darker" and the Curves set for "Linear Contrast". Along with Brightness/Contrast and Hue/Saturation, this gang worked like a charm 90% of the times so far, but this time just by enabling them and all the relative success went down the drain. :(
What should/could I do to enhance this background a little bit, perhaps darkening it, in a way it doesn't destroy the gradient and/or to end up out of balance with the main character that has been treated only with layers like those added to this sample?