It's difficult to definitively say which Australian astronomer is the most famous, as fame is subjective and depends on the audience. However, some of the most notable Australian astronomers include:

* Sir John Frederick William Herschel (1792-1871): While born in England, he spent a significant portion of his career observing the Southern Hemisphere from South Africa and is considered an important figure in Australian astronomy.

* Barnaby Normanby, 1st Earl of Strafford (1797-1862): An amateur astronomer, he established an observatory at his property in New South Wales and made significant contributions to the study of the Southern Hemisphere.

* Harold Leslie Chillingworth (1877-1962): He was instrumental in founding the Australian Astronomical Observatory and played a key role in establishing Australia as a leader in astronomy.

* Sir Frank Dyson (1868-1939): He served as Astronomer Royal of England but spent significant time observing from Australia and making discoveries about the Milky Way.

* Sir Fred Hoyle (1915-2001): Although not strictly Australian, Hoyle was a prominent British astrophysicist who spent a significant portion of his career working in Australia and made significant contributions to our understanding of the universe.

* Professor Brian Schmidt (born 1967): He is a Nobel Laureate for his work on the discovery of the accelerating expansion of the universe and is currently the Vice-Chancellor of the Australian National University.

These are just a few examples. Ultimately, it's up to individual perception to determine which Australian astronomer is considered the most famous.