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The Ultraviolet Imaging Telescope (UIT) obtained ultraviolet images of 66 astronomical targets during the ASTRO-1 mission in 1990 December. UIT has a $40'$ diameter field of view and the $\sim 3''$ resolution images are recorded on image-intensified film. The UIT absolute calibration is based on comparison of stars observed in common with IUE and the Hopkins Ultraviolet Telescope (HUT), which was co-pointed with UIT. A white-dwarf based calibration was applied to the IUE data. A linearity check is an important aspect of the absolute calibration, since the faintest stars observed by UIT (at m(2490) $\sim 19.5$) are about 5 magnitudes fainter than any of the calibration stars.
We discuss the expected accuracy of the UIT absolute calibration. We then discuss the discrepancy between the absolute fluxes of blue horizontal branch stars measured with UIT in 3 globular clusters, and the predicted flux levels from theoretical zero-age horizontal branch models.
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