When installing an Elliptical Flat in a telescope , make sure you allign the Flat's Offset with the Main Mirror's Axis of Symmetry, not the Flat's Minor Axis which passes vertically through its geometric center.
Before I move on, there is a myth to dispell concerning the placement of a Flats. Some Amature Astronomers believe the light closer to the Main Mirror is brighter than the light further up the telescope's tube. They quote a badly misapplied Inverse Square Law, which properly applied requires one to move the flat closer to the focal point in order to get a brighter image.  Brightness or Intensity is measured as the amount of light per unit area of a plane cutting through (intersecting) a light cone perpendicular to the cone's central axis. No matter which end of of the cone the amount of light enters, the same amount of light will exit the other end.