FOC Instrument Science Report # 83 21 Feb 1995 "The Photometric Accuracy of the FOC" by Gerhardt R. Meurer Abstract excerpted from the report: A comparison is made between the expected photometric accuracy of the FOC, and that observed. The presently known systematic effects are listed and quantified. Typically when both internal and external factors are included one only expects to obtain photometry accurate to ~0.2 mag in the ultraviolet. This expectation is tested by comparing magnitudes extracted from F220W images of nine starburst galaxies with those extracted from IUE spectra. After removing one discrepant observation, it is found that relative to the IUE, the FOC external errors are 0.05 mag, three times lower than expected. By carefully removing some of the internal zeropoint variations, one should be able to reduce the total systematic errors to ~0.10 mag.