Six small secondary payloads will release from the fourth stage in the next few minutes.
The fuel load means the stage will burn for a shorter time than most PSLV flights.
The liquid-fueled Vikas second stage engine continues firing.
The boosters will ignite in a staggered sequence two-tenths of a second apart. Two more boosters will ignite at T+plus 25 seconds after the PSLV clears the launch pad.
The PSLV has received the final authorization to launch, and automated countdown sequence will oversee the final preparations to configure the 700,000-pound rocket for liftoff.
Today's launch will use the largest version of the Polar Satellite Launch Vehicle called the PSLV XL, with six strap-on motors with a length of 12 meters, or 39 feet.
The PSLV is a four-stage rocket with a solid-fueled first and third stages and liquid-fueled second and fourth stages.
The PSLV launch team reports the Sriharikota range and all weather conditions are "go" for an on-time launch.
The mobile service tower at the PSLV launch pad has been retracted to the launch position 150 meters away from the rocket, and no problems are reported in the countdown.
The first activity after the countdown began Saturday morning, Indian time, was the filling of the PSLV's liquid-fueled fourth stage with hydrazine and nitrogen tetroxide propellants.
The fourth stage will inject the Astrosat spacecraft into an circular orbit with about 404 miles (650 kilometers) above Earth at an inclination of 6 degrees to the equator. The twin-engine fourth stage, named the PS4 or L2.5, will fire for nearly 5 minutes in the last phase of the launch.
The mobile gantry at the launch complex, located on Sriharikota Island on India's east coast, was retracted more than 150 feet from the rocket early Monday, local time, to prepare for fueling of the PSLV's second stage with liquid propellant.
The PSLV's first and third stages, along with the six strap-on boosters, burn solid propellant already packed inside the motor casings.
Live video of the final countdown will begin about 30 minutes prior to launch, according to ISRO. The webcast will be seen on this page.
The Astrosat mission, estimated in Indian news reports to cost $27 million, will peer into the cosmos with a suite of X-ray and ultraviolet instruments, looking at collapsed stars, haze-enshrouded stellar factories and black holes.
Liftoff is set for 0430 GMT (12:30 a.m. EDT) Monday from a launch pad at the Satish Dhawan Space Center on India's east coast, or 10 a.m. local time in India.
The 3,335-pound (1,513-kilogram) Astrosat spacecraft hosts four X-ray science instruments and an ultraviolet telescope.
"Astrosat's five payloads are chosen to facilitate a deeper insight into the various astrophysical processes occurring in the various types of astronomical objects constituting our universe," officials wrote in a press kit released by the Indian Space Research Organization.
Astronomers in India plan to use Astrosat to survey the galaxy for black holes and measure the intense magnetic fields generated by neutron stars, the collapsed ultra-dense remnant left behind after supernovas.
Astrosat will also observe X-ray binaries -- systems where a black hole or neutron star can suck matter away from a nearby star, emitting powerful signals visible to X-ray telescopes.
The observatory's ultraviolet imaging telescope will scan the sky in visible, ultraviolet and far ultraviolet light, focusing on star-forming clouds throughout the nearby universe and infant solar systems closer to home.
Scientists will also coordinate observations between Astrosat's ultraviolet and X-ray sensors, which were developed by Indian scientific institutions and the University of Leicester in Britain.
The 145-foot-tall (44.4-meter) Polar Satellite Launch Vehicle will deploy Astrosat into a 404-mile-high (650-kilometer) orbit inclined 6 degrees to the equator about 22 minutes after liftoff.
Fitted with four enlarged strap-on solid rocket boosters, the PSLV will also deliver six commercial secondary payloads to orbit.
Four satellites owned by Spire Global, a San Francisco startup aiming to develop spacecraft for commercial weather applications, are fastened aboard the PSLV for Monday's launch. The Lemur 2 CubeSats, each about the size of a toaster oven, carry maritime tracking and GPS radio occultation payloads.
The exactView 9 ship-monitoring satellite built by the University of Toronto Institute for Aerospace Studies Space Flight Laboratory is also packaged as a secondary passenger on Monday's launch. About the size of a small briefcase, exactView 9 is owned by exactEarth Ltd. of Canada, which specializes in vessel tracking services.
Indonesia's Lapan A2 microsatellite carries video and digital cameras to image Earth from orbit, plus a ship-tracking payload and amateur radio equipment.
Monday's launch marks the 10th flight of the PSLV XL configuration, and the PSLV's 31st flight overall since 1993.