TopHat was a scientific experiment launched from McMurdo Station in January 2001 to measure the cosmic microwave background radiation produced 300,000 years after the Big Bang.

The working payload was shut down on January 10, 2001 after the liquid cryogens cooling the detectors were exhausted, and the balloon simply circled the continent until it was safe to land.

The telescope was called part of the "Submillimeter Astrophysics Experiment" for Dr. Edward Cheng of NASA’s Goddard Space Flight Center.

TopHat experiment was the first of its kind in that it placed the telescope on top of the actual balloon, where it rotated at a constant rate around a vertical axis and covered a 48-degree-diameter window of the sky.

TopHat was built in part to follow up the observations of the BOOMERanG experiment which also studied the cosmic background radiation.