'flat' field spectrograph and optcs focusing

Flat-field spectrographs are so-named because the 'image' plane (the surface conjugate to the entrance slit) is flat, or nearly flat, enabling use of a flat detector array. Recall, spectrographs generally use concave gratings (that is, a reflective grating in the shape of a concave spherical surface) to reduce the number of optical elements required to image the spectrum. Concave gratings suffer from large amounts of astigmatism and coma. By modifying the groove shape, aberrations are reduced.

I don't understand your second question: you cannot focus a single ray.

 

There is no single optimal solution to your question. I recommend reading the 'Diffraction Grating Handbook', freely available online, for a complete discussion.