Minimum entropy for the Space Solar Telescope automatic focus

Abstract

The novel remote sensor of the Space Solar Telescope (SST) is scheduled for launch in 2008. It will be uniquely designed to be the world’s first facility capable of observing with γ = 0.1 spatial resolution in vector magnetograms in the photosphere and the chromosphere, and 0.5 in soft X-rays. The high spatial resolution makes the on-orbit automatic focus (AF) the key technique to catch images. The paper brings forward a new method of the minimum entropy (ME) criterion for the astro-observation. Further more, we have applied such technology to the on-orbit AF of SST. The emulational program calculated the image entropies of different off-focus states. Data indicate that the minimum image entropy is corresponding to the optimal image plane; the ME criterion is more suitable for the heavenly bodies of low contrast and the focusing precision is 0.01 mm (δ' = 0.01mm).