ar X iv : a st ro - p h / 99 12 21 3 v 1 9 D ec 1 99 9 In Flight Determination of the Plate Scale of the EIT

Received ; accepted – 2 – ABSTRACT Using simultaneous observations of the MDI and EIT instruments on board the SoHO spacecraft, we determined in flight the plate scale of the EIT. We found a value of 2.629 ± 0.001 arc seconds per pixel, in fair agreement with the 2.627 ± 0.001 arc seconds per pixel value deduced from recent laboratory measurements of the focal length, and much higher by 7σ than the 2.622 arc seconds per pixel value of the pre-flight 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 2σ 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.