Differences

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--- courses:ast201:6 [2023/11/13 23:47] – [5.1 Monochromatic wavefront aberrations] asad
+++ courses:ast201:6 [2023/11/25 23:43] (current) – asad
@@ Line 315: / Line 315: @@
 {{:courses:ast201:field-curvature.png?nolink&500|}}
-Off-axis images fall on the Petzval surface.
+If the previous aberrations are absent, image should be formed at the focal plane at a distance $f=R/2$ from the vertex. However, the actual imaging surface is not a //plane// but a //curved surface// called the **Petzval surface**. If the object is on-axis, there is no problem, but the off-axis sources are necessarily out-of-focus because they are not imaged on the focal plane, but on the curved Petzval surface. This is field curvature.
+The detector is usually placed on the Petzval surface. If the detector is small, the curvature can be ignored. But for large detectors, this cannot be ignored. In the past, glass photographic plates used to be bent to match the curvature of the Petzval surface. However, modern CCDs cannot be bent. So corrector plates or lenses are used.
+==== - Distortion ====
+{{:courses:ast201:distortion.png?nolink|}}
+Straight lines on the sky become curved lines in the focal plane. The **pincushion** and **barrel** distortions are shown above.
+Below you see the visual representation of all the distortions for both on-axis and off-axis objects.
 {{:courses:ast201:spot-diagram.png?nolink|}}