In-Flight Determination of the Plate Scale of the Extreme-Ultraviolet Imaging Telescope

Using simultaneous observations of the Michelson Doppler Imager and Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory spacecraft, we determined in flight the plate scale of the EIT. We found a value of 2&farcs;629+/-0&farcs;001 pixel-1, in fair agreement with the 2&farcs;627+/-0&farcs;001 pixel-1 value deduced from recent laboratory measurements of the focal length and much higher by 7 sigma than the 2&farcs;622 pixel-1 value of the preflight calibrations. The plate scale is found to be constant across the field of view, confirming the negligible distortion level predicted by the theoretical models of the EIT. Furthermore, the 2 sigma difference between our results and the latest laboratory measurements, although statistically small, may confirm a recent work suggesting that the solar photospheric radius may be 0.5 Mm lower than the classically adopted value of 695.99 Mm.