Disadvantages Of Keplerian Telescope

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The Keplerian Telescope: image source
The Keplerian telescope has two lenses, both the objective and eyepiece lenses are convex (converging). When light parallel to the principal axis passes through the objective lens, the rays refract and converge until they hit the focal point. After they pass the focal point, the rays start to diverge. The rays are then intercepted by the eyepiece lens where the light refracts as it passes through the lens and causes the light rays to become parallel again as discussed in Converging Lens. The image produced by a Keplerian Telescope is magnified, inverted and virtual.
To calculate the focal length for both lenses, the Descartes’ Method (thin lens method) is used, which is the same method used
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The image that the Keplerian telescope produces is inverted, which means it needs to be erected.

The Galilean telescope has a small field of view, it typically has a 15-18 arcminutes view (an arc min is 1/60 of a degree), to put that into perspective, the moon has a diameter of approximately 30 arcminutes. When increasing the magnification for a Galilean telescope, it reduces the the field of view which is already quite small.

image source
Spherical Aberration occurs in both types of refractor telescopes as their objective lens are both spherical. Spherical Aberration is when light rays refract through the lens and focus at different points, as shown on the diagram where the light rays (in red) do not converge at the same point. The diagram shows that the light that travels and refracts through the lens near the center of the lens is less refracted than the light travelling near the edges of the lens. In other words, light that travels parallel to the principal axis do not converge at the same point, which then affects the resolution and the clarity of the image (the image is not

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