  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.
