Galilean, Keplerian, Newtonian and Cassegrain Telescopes

Galilean Telescope

The Galilean Telescope was named after its inventor, Galileo Galilei.
It is a simple refracting telescope design that uses a convex objective lens and a concave eyepiece.
This design provides an upright image, suitable for terrestrial viewing.
However, it only offers a narrow field of view, which makes it less suitable for astronomical observations.

The Keplerian Telescope, also known as an astronomical telescope, was invented by Johannes Kepler.
Unlike the Galilean telescope, it uses a convex objective lens and a convex eyepiece.
This set-up produces an inverted image, which doesn’t matter for astronomical use as directions are relative in space.
Keplerian telescopes have a wider field of view and better eye relief compared to Galilean telescopes.

The Newtonian Telescope is a type of reflecting telescope, invented by Sir Isaac Newton.
It uses a concave primary mirror (instead of a lens) to capture light and form an image.
A smaller secondary mirror is used to reflect the light towards the eyepiece.
Newtonian telescopes eliminate chromatic aberration, a problem in refracting telescopes, and are commonly used by amateur astronomers due to their cost-effectiveness.

The Cassegrain Telescope is a type of reflecting telescope that uses a concave primary mirror and a convex secondary mirror.
The light enters the telescope, hits the primary mirror, is reflected to the secondary mirror, and then is focused through a hole in the primary mirror to the eyepiece.
Cassegrain designs can achieve larger magnification and resolution in a smaller physical size compared to other telescope designs.
There are many variants of Cassegrain telescopes like the Ritchey-Chretien and Dall-Kirkham designs used in many professional observatories worldwide.