Almost three decades after the discovery of Pluto's large moon, Charon, a University of Hawai'i-Manoa astronomer has made the sharpest images to date of the celestial body.

And they were taken by a ground-based telescope atop Mauna Kea.

Last month, David Tholen used the adaptive optics system on one of Mauna Kea's twin Keck telescopes to take these images, which exceed the sharpness possible with the Hubble Space Telescope.

Tholen credits the several favorable factors that came together to yield these spectacular images of the Pluto system, including the use of more sensitive wavefront sensors installed on the telescope and the planet being at its maximum brightness.

The adaptive optics system on the telescope uses a deformable mirror and sensors that allow it to compensate for turbulence in the Earth's atmosphere that normally blurs the light from celestial bodies. The new images are about 20 times sharper than the images of Pluto taken 30 years ago.

When Tholen combined all 16 images to form a single image, Nix and Hydra — Pluto's small satellites discovered with the Hubble Space Telescope in 2005 — became clearly visible.

"It is our intent to obtain several more images of the Pluto system, hopefully with this same level of quality, so that we can track Nix and Hydra completely around Pluto several times," Tholen said in a news release. "By making extremely precise measurements of the satellites' positions, we will determine their masses by detecting the tiny displacements caused by their mutual gravitational attraction. Once the masses are in hand, we'll be able to say something more definitive about how big these new satellites are."

Astronomers have estimated that Nix and Hydra are less than 62 miles in diameter, compared with 753 miles for Charon and about 1,430 miles for Pluto.

Scientists planning the 2015 flyby of Pluto with the National Aeronautics and Space Administration's New Horizons spacecraft are interested in the results.

"Something as simple as selecting the proper exposure time to snap images of Nix and Hydra with New Horizons depends on knowing how big they are and how reflective their surfaces are," Tholen said. "One of our goals is to have those answers well in advance of the flyby."