across its diameter. The actual spot image sizes are shown in Fig 5. Given the very linear relationship, it is evident that the diffraction and spot size effectively add, and we have used the extrapolation to zero hole size as a measure of the detector resolution, as shown in the diagram. The measured spot sizes for the 25 micron hole mask data were similar to that in Fig 5. The same slope and extrapolation was therefore applied to the image size measured with the 25 micron hole-mask used with the flight detectors. There was insufficient calibration time to get images with the multi-sized hole-mask with the flight detectors, but the geometry and window thickness was the same for all detectors, so Fig 5 was applied to all cases. In the next section, we note other ways to make this measure, and their good agreement.

Spot sizes are defined as the FWHM values from the images. These are expressed in readout pixels, and can be converted to values at the photocathode or arcseconds on the sky as seen with the UVIT telescope. Signal levels were integrated over the final spot images.

Spatial resolution
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The spatial resolution was also estimated by 