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--- courses:ast201:6 [2023/11/08 21:07] – [1.2 Reflection and refraction] asad
+++ courses:ast201:6 [2023/11/25 23:43] (current) – asad
@@ Line 24: / Line 24: @@
 A **coherent** source emits all light in phase, and in this case the geometrical wavefronts also correspond to surfaces of constant phase called **phase fronts**.
-==== - Optical materials ====
 {{:courses:ast201:n-lambda.png?nolink|}}
-Surface coating can increase or decrease reflectance. A coating with 1/4 wavelength thick film will create two reflected waves that are exactly 1/2 wavelength out of phase with each other; they will destruct each other if they have the same amplitude. If the glass and the coating have indices $n_s$ and $n_c$, then the amplitude of the two reflected beams will be equal if
-$$n_c = \sqrt{n_s}$$.
 ==== - Reflection and refraction ====
@@ Line 91: / Line 86: @@
 where $P_{12}$ is the power in diopters. Positive $P$ tells you how strongly converging a lens is, and vice versa.
-===== - Mirrors, lenses and antennae =====
+===== - Mirrors and lenses =====
 {{:courses:ast201:thick-lens.png?nolink|}}
@@ Line 215: / Line 210: @@
 For reflecting systems, chromatic aberration does not occur. But even monochromatic light (light of just one wavelength) can have differing focus after getting reflected from a mirror.
-{{:courses:ast201:wavefront-aberrations.png?nolink&700|}}
+{{:courses:ast201:optical-planes.webp?nolink&800|}}
 The plane of the diagram (a) is called the **tangential** plane and the plane perpendicular to it is called the **sagittal** plane. The chief ray $V'VF$ and the ray $CFBFC$ are shown on the tangential plane. The test ray $P'PF$ could be on a different plane than the tangential and sagittal.
@@ Line 320: / 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|}}