Photometry of umbral dots

Until now, the size of umbral dots has been considered to be below the resolution limit of large solar telescopes. We analyze observations of two sunspots and two pores, acquired in September 2003 with the new 1-m Swedish Solar Telescope, La Palma. White-light images with a resolution better than 0.'' 15 were taken simultaneously in blue ( 451 nm) and red ( 602 nm) wavelength bands. They were corrected for scattered light and restored for the instrumental profile of the telescope. Intensities, diameters and positions of umbral dots were measured in aligned pairs of images in the blue and red wavelength band. We find that observed intensities of umbral dots are correlated with local intensities of umbral background. On average, UDs are by about 1000 K hotter than the coolest area in the umbra and by 500 - 1000 K cooler than the undisturbed photosphere. Individual UDs may reach or exceed the average photospheric brightness and temperature. Histograms of observed diameters peak at 0.'' 23 ( 170 km). This indicates that the majority of umbral dots are spatially resolved with the 1-m telescope. The mean nearest-neighbour distance between umbral dots is 0.'' 4 and their average observed filling factor is 9%. The method of two-colour photometry is discussed and applied to obtain average true intensities and diameters. About 50% of umbral dots are brighter than the quiet photosphere and the average true diameter of umbral dots is 100 km. However, the latter results might be influenced by systematic errors of the method.