Question

What are the advantages for "professional astronomers" of the Cassegrain design over the Newtonian design?

Answer 1

Most reflector telescopes will use a smaller secondary mirror in front of of the large primary mirror to reflect the light to a more convenient viewing spot. Isaac Newton used a flat secondary mirror at a 45° angle to reflect the light to an eyepiece at the side of the telescope tube near the top. Such an arrangement, called a newtonian design is used by many amateur telescopes.

Many reflector telescope use another light path design called the cassegrain design to reflect the light back through a hole in the primary mirror, so that detectors or the eyepiece can be conveniently placed behind the telescope. Most of the large telescopes used for research, including the Hubble Space Telescope, are of this design. Some of the largest telescopes like the Hale Telescope and the Keck Telescope have places to put detectors at the prime focus, where the light from the primary mirror first comes to a focus. The images in reflector telescopes do not have holes or shadows in them because the light rays from the unblocked parts of the primary mirror are all added together when they are focussed together. Even though part of the primary mirror is blocked or missing, there is still plenty of usable primary mirror space to gather the light.

Advantages:

The main reason is that Cassegrain telescopes are much shorter than Newtonians for the same focal length. This means that the overall cost of the observatory, which includes a mount, dome, and building, will be much lower.

secondary reason is that professional telescopes use heavy detectors at the focus of the telescope. Newtonian telescopes put the focus on the side of the telescope, so heavy instruments and detectors would need to be placed on the side of the telescope; in general, this can make things difficult, because the weight on the focus exerts a variable torque on the telescope's tube.