shows how the profile is signicantly broadened with a separation of only some 0.3“, and shows separate peaks when separated by about 0.8”.

These results indicate that the UV detectors will yield image resolutions of 1 arcsec or better, and the visible channel a little larger. The ultimate resolution of the UVIT instrument will combine these with the optical resolution of the telescope and the drift of the spacecraft during observations. The telescope should deliver images of 1 arcsec FWHM, and we expect to correct drift using the visible channel guide-star centroids, to a small fraction of that.

Figure 10 compares the integrate mode images (simply the sum of all the photon event splashes) with the centroided image resolution we report above. Integrate mode images can be centroided to 0.1 pixels, or 0.25 arcsec, to compensate for spacecraft drift.

Flux calibration
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The flux calibrations used the same hole-mask data. The wavelength was selected with the monochromator, and the flux monitored by photometer readings from a beam-splitter. A series of (usually four) flux settings was used for all images. Data were taken in photon-counting mode with 5S and 3S centroiding, and Integrate mode, and with full frame (29Hz) and a 