'flat' field spectrograph and optcs focusing

[einstein1921]

why call 'flat' field spectrograph ? what does the meaning of 'flat'. another question: if I what to focus the ray from a point source, how to do it?
anything help would be highly aprreciated.
best wishes and thank you!

 

[Andy Resnick]

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.

 

[einstein1921]

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.

THANK your for your answer! IF I use flat gratings and concave mirror,does this suffer from large amounts of astigmatism and coma? which is better between concave gratings. and how to modify the
groove shape, what kind of shape is better? THANK YOU again!
best wishes!

 

[Andy Resnick]

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