XnView MP is ridiculously well featured to the extent it opens .fits files! nice.
I do have a request to make concerning the rotate tool in the edit mode. It would be nice to have a live preview of the rotation when you adjust the image angle, having to input an angle and okay the change without seeing it first is a bit inconvenient. While still in the rotate tool, it would be nice to have an eyedropper in the background colour dialog to enable you to choose a custom colour based on the image you are trying to rotate.
I hope these suggestions are taken on board as they'll make an awesome tool better in my opinion.
Rotate tool enhancements
Moderators: helmut, XnTriq, xnview