We present an in-flight calibration of the ROSAT PSPC using the incident spectra of the hot white dwarf HZ43 and the polar AM Her. We derive an absolute flux calibration of the PSPC using the accurately known soft X-ray spectrum of HZ43. Corrections to the PSPC response matrix are derived from a comparison of predicted and observed PSPC spectra of HZ43, supplemented by results for AM Her. The calibration of the PSPC for photon energies E < 0.28 keV is found to be accurate to better than 5% refuting earlier reports of a major miscalibration. Our corrections to the detector response matrices remove systematic residuals in the pulse height spectra of soft sources.
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