If you want to reduce stars in PixInsight, you need a contour star mask. The StarMask process has this option, and for many images this works just fine. But occasionally I find it difficult to make this mask work for both large and medium sized stars. Here's an alternative to the standard method.
Use StarMask to create a standard star mask. Choose the number of layers large enough to include even large stars. To get this to work, you may need to decrease the noise threshold a bit. On most of my images, I start with 6 or 7 for the number of layers, and about 0.35 for the noise threshold. But really it depends entirely on your image and what size stars you have. Keep the structure parameters at their default values (Large = 2, Small = 1, Compensation = 2)
When you're satisfied with the mask, create another one, but decrease the number of layers by one, and set the structure parameters to 0.
Now, use PixelMath to subtract the second mask from the first. If you want to keep the original masks, create a new image. Otherwise use the expression '$T - star_mask2' and apply it to the first mask (with the larger stars). The star mask should now show donuts for stars. If the donuts don't open up (ie still gray or white in the center), you need to increase the intensity of the second mask by using the HistogramTransformation tool. Occasionaly, I have had to use the CloneStamp tool for a tricky star or two. Just take a sample from the black background and clone it in the white star that is reluctant to open up. Just make sure you put it in the right spot, otherwise you may end up with a lopsided star.
When you are satisfied with the contour shapes, you can increase the intensity of the mask with the HistogramTransformation tool, and blur the mask with the Convolution tool.
The advantage of this method is that it creates contour masks for large and small stars alike, while the standard method, sometimes fails to create contours for smaller stars.