In Image>Transform>Rotate… and Edit>Selection>Rotate… I can’t change the angle from 15 to any other value. Nor can I delete this number. Nothing happens when I try to change it. How can I rotate my image by less than 90 degrees?
I would suggest using the TransformJ plugin: https://imagescience.org/meijering/software/transformj/
However be aware that when you rotate less or more than 90 Degrees the program needs to perform interpolation! Thus it needs to calculate from the existing pixels new pixels that did not exist before. Depending on the different algorithm this creates MORE or LESS apparent artifacts but it is NEVER error free. For visualization this is fine but might be an issue for a quantification.