For a telescope focal ratio of f/2 (e.g. Celestron RASA), the filter centre wavelength (CWL) is required to be shifted (in wavelength) in order to compensate for the large angle of the incoming wavefront. The devil in this game is – even if one were to do the coating layer design for a huge incoming angle, at the same time the image is also coming in at zero-degree-angle – respectively at all angles in between. Which essentially would mean that no shift ought to be allowed. A filter made to cover that wide an angle must cope with these contradicting requirements. Hence no one else has succeeded in making f/2-filters that would not suffer from severe image contrast fall off and vignetting effects, either in the corners of the field or in the middle. Producing these filter coating runs in an appropriate and high quality way is about the hardest task in filter coating technology today.