Subtract...
You should have an image already opened. A small window named Add is opened. It enables to subtract a constant value (e.g. 25) to each pixel flux of an image (in the intensity unit, usually in Analogic Digital Unit (ADU)).
This function is typically useful if you want to perform an operation on an image (e.g. to subtract a background). See also the other operation proposed in the menu Process/Single Image Operators.
It is not used in the actual series of EU-HOU exercises.
