Yes: with MorphologicalTransform.
You have two options, depending on the size of your stars. If your stars are big enough (I think they aren't; this happens when you have a very sharp image with very good seeing conditions), you can do a bit of erosion to the corners of the square. Select a diagonal structure ("X") and do the action through a morphological selection operator with a selection value of <0.50.
If you have tiny stars, you need to make them round from the square. In this case, use the same operator with a selection value of >0.50. I think a diamond structure will do the work nicely.
Always use this technique through a good star mask!
Good luck!
VIcent.
