\section{V. Different Radio Observatories -}

One of the most famous radio observatories is the Arecibo Observatory in
Puerto Rico. This observatory was the largest single-dish radio
telescope in the world until its collapse in 2020. Arecibo was used for
many different types of astronomical research, including the study of
hydrogen gas in the Milky Way and other galaxies. {[}{\color{blue}16}{]} Another
important observatory for hydrogen line research is the Green Bank
Telescope (GBT) in West Virginia. The GBT is one of the largest fully
steerable radio telescopes in the world and is optimized for observing
the 21 centimeter line emission from hydrogen gas. It has been used to
study the distribution and motion of hydrogen gas in the Milky Way
galaxy and other galaxies. The Parkes Observatory in Australia is
another radio observatory that is well-known for its hydrogen line
research. The observatory is home to the Parkes radio telescope, which
was used to discover the first pulsar in 1967 {[}{\color{blue}18}{]}. The telescope is
also used for studying the properties of hydrogen gas in the Milky Way
and other galaxies. The Effelsberg Telescope in Germany is another radio
observatory that is used for hydrogen line research. The telescope has a
100-meter diameter dish and is one of the largest fully steerable radio
telescopes in the world. It is used for many different types of
astronomical research, including the study of hydrogen gas in the Milky
Way and other galaxies. The Australia Telescope Compact Array (ATCA) is
a radio interferometer located in New South Wales, Australia. It
consists of six 22-meter dishes that can be used together to create
high-resolution images of astronomical objects. The ATCA has been used
for many different types of research, including the study of hydrogen
gas in nearby galaxies.The Square Kilometer Array (SKA) is a
next-generation radio telescope that is currently under construction in
South Africa and Australia. When complete, the SKA will be the largest
and most sensitive radio telescope ever built. It will be used for many
different types of astronomical research, including the study of
hydrogen gas in the early universe and the epoch of reionization.{[}{\color{blue}8}{]}