CANBERRA, Jan 14 (Reuters) - Raging wildfires destroyed at least another 28 homes and licked at Australia's leading optical space observatory on Monday, officials said, but spared giant telescopes that have mapped far-away galaxies and discovered new planets.

Less fortunate were a father and son who police arrested after a fire was lit deliberately to destroy illegal drug laboratories they were alleged to be running in dense bushland. Police were closing in on the drug labs when the fire was lit.

More than 140 fires are burning across vast areas in the north and west of New South Wales state (NSW), Australia's most populated state, and in the island state of Tasmania despite cooler weather giving firefighters some respite.

A searing heat-wave had fuelled the fires over the past week. Only one person, an elderly firefighter working alone in Tasmania, has died so far in the fires.

The biggest blaze, with a perimeter of 100 km (60 miles), destroyed around 40,000 hectares (100,000 acres) of bushland and 28 homes around the Warrambungle National Park in NSW.

That fire also forced the evacuation of the Siding Springs Observatory, which houses 15 major telescopes.

Cameras inside the mountain-top observatory showed large flames and thick smoke sweeping over it. There appeared to be little damage to telescopes and dishes but scientists have been unable to visit the site yet to assess any damage.

"We do not yet know what impact the extreme heat of the ash might have on the telescopes themselves," said Erik Lithander, acting vice chancellor of the Australian National University, which operates the observatory.

The fire damaged five buildings at the observatory, including accommodation for visiting astronomers, but Lithander said scientists were confident the telescopes would still work.

Siding Springs is home to the 4-metre (13 ft) Anglo-Australian Telescope, which has surveyed 200,000 galaxies and was instrumental in confirming the existence of dark energy.

That discovery led to Australian Brian Schmidt sharing the 2011 Nobel Prize for physics.

The observatory has also helped find more than 30 new planets over the past decade and is being used to map the southern sky.

In Sydney, police arrested two men late on Sunday over a fire that broke out in the Blue Mountains National Park west of the city last week. The fire destroyed more than 50 hectares of bushland in the Blue Mountains, a popular tourist destination.

Police said they had been aware of the illegal, outdoor drug labs but were forced to postpone a raid due to the extreme fire danger in the area last week.

"The two sites ... were only accessible by foot and required police to trek through tick, leech and snake-infested scrubland to reach them," NSW police said in a statement on Monday.

Police said a father and son had been charged with "the large commercial manufacture of a prohibited drug" and contaminating a water catchment area. The younger man was also charged with lighting the fire. (Editing by Paul Tait)

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  • In this photo provided by the New South Wales Rural Fire Service a wildfire near Deans Gap, Australia, crosses the Princes Highway Tuesday, Jan. 8, 2013. (AP Photo/NSW Rural Fire Service, James Morris)

  • In this Jan. 4, 2013, photo provided by the Holmes family, Tammy Holmes, second from left, and her grandchildren, two-year-old Charlotte Walker, left, four-year-old Esther Walker, third from left, nine-year-old Liam Walker, eleven-year-old Matilda, second from right, and six-year-old Caleb Walker, right, take refuge under a jetty as a wildfire rages near-by in the Tasmanian town of Dunalley, east of the state capital of Hobart, Australia. (AP Photo/Holmes Family, Tim Holmes)

  • In this Jan. 4, 2013, photo provided by the Holmes family the Walker siblings six-year-old Caleb, left, four-year-old Esther, second from left, eleven-year-old Matilda, holding two-year-old Charlotte, second from right, and nine-year-old Liam, right, prepare to enter the water to take refuge with their grandparents under a jetty as a wildfire rages nearby in the Tasmanian town of Dunalley, east of the state capital of Hobart, Australia. (AP Photo/Holmes Family, Tim Holmes)

  • In this Jan. 4, 2013, photo provided by the Holmes family, a building burns near a jetty where Tim and Tammy Holmes attempt to shelter their five grandchildren as a wildfire rages nearby in the Tasmanian town of Dunalley, east of the state capital of Hobart, Australia. The family credits God with their survival from the fire that destroyed around 90 homes in Dunalley. (AP Photo/Holmes Family, Tim Holmes)

  • In this Jan. 4, 2013, photo provided by the Holmes family, the Walker siblings six-year-old Caleb, left, four-year-old Esther, second from left, nine-year-old Liam, and eleven-year-old Matilda, right, holding two-year-old Charlotte, prepare to enter the water to take refuge with their grandparents under a jetty as a wildfire rages nearby in the Tasmanian town of Dunalley, east of the state capital of Hobart, Australia. (AP Photo/The Holms Family, Tim Holmes)

  • Burnt trees from the Deans Gap fire are shown near Nowra on the south coast of Australia's New South Wales state on January 9, 2013. Cooler conditions helped firefighters battling blazes across Australia on January 9 but up to 30 were still out of control. (GREG WOOD/AFP/Getty Images)

  • A firefighter douses smouldering trees from the Deans Gap fire near Nowra on the south coast of Australia's New South Wales state on January 9, 2013. Cooler conditions helped firefighters battling blazes across Australia on January 9 but up to 30 fires were still out of control. (GREG WOOD/AFP/Getty Images)

  • Firefighters douse burning logs from the Deans Gap fire near Nowra on the south coast of Australia's New South Wales state on January 9, 2013. Cooler conditions helped firefighters battling blazes across Australia on January 9 but up to 30 were still out of control. (GREG WOOD/AFP/Getty Images)

  • John O'Reilly takes refuge in the cool water behind a waterfall of a fountain in central Sydney, Australia, Tuesday, Jan. 8, 2013, as the temperature reaches 42 degrees Celsius (107.6. degrees Fahrenheit). Temperatures across much of New South Wales state are expected to reach 45 degrees Celsius (113 degrees Fahrenheit) creating severe wildfire conditions. (AP Photo/Rick Rycroft)

  • This aerial photograph taken on January 5, 2013 shows the devastated town of Dunalley after bush fires swept through the area. (Chris Kidd/AFP/Getty Images)

  • This aerial photograph taken on January 5, 2013 shows the devastated town of Dunalley after bush fires swept through the area. (Chris Kidd/AFP/Getty Images)

  • This aerial photograph taken on January 5, 2013 shows the devastated town of Dunalley after bush fires swept through the area. (Chris Kidd/AFP/Getty Images)

  • This aerial photograph taken on January 5, 2013 shows the devastated area Boomer Bay after bush fires swept through the area. (Chris Kidd/AFP/Getty Images)

  • This aerial photograph taken on January 5, 2013 shows Westpac Police Rescue Helicopter crewman Sgt. Matthew Drumm surveying the devastated town of Dunalley after bush fires swept through the area. (Chris Kidd/AFP/Getty Images)