'flat' field spectrograph and optcs focusing

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Flat-field spectrographs are named for their nearly flat image plane, which allows for the use of flat detector arrays. They typically employ concave gratings to minimize optical elements needed for spectrum imaging, though these gratings can introduce significant astigmatism and coma. Modifying the groove shape of the grating can help reduce these aberrations, but there is no one-size-fits-all solution for optimal design. The discussion emphasizes the importance of consulting resources like the 'Diffraction Grating Handbook' for in-depth understanding. Overall, the design choices in spectrograph construction significantly impact performance and image quality.
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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!
 
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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.
 
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Andy Resnick said:
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!
 
There is no single optimal solution to your question. I recommend reading the 'Diffraction Grating Handbook', freely available online, for a complete discussion.
 
So I know that electrons are fundamental, there's no 'material' that makes them up, it's like talking about a colour itself rather than a car or a flower. Now protons and neutrons and quarks and whatever other stuff is there fundamentally, I want someone to kind of teach me these, I have a lot of questions that books might not give the answer in the way I understand. Thanks
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