Proceedings Article | 25 November 1996
KEYWORDS: Coronagraphy, Chlorine, Mirrors, Image resolution, Diffraction, Charge-coupled devices, Spatial resolution, Point spread functions, Distortion, Space telescopes
The LASCO C1 mirror coronagraph onboard the SOHO satellite (launched on 2 December 1995) was designed to observe the fine structure of the solar corona from 1.1 to 3.0 R. Even though the optical resolution is approximately 3 arc sec, the nominal achieved resolution is set by the CCD pixel size of 5.6 arc sec. A pixel size of 1.5 arc sec or less is needed to obtain diffraction limited observations according to the Nyquist criteria, and therefore the actual coronagraph images are under sampled by a factor of 4. We have explored improving the spatial resolution of the LASCO C1 images using the technique of dynamic imaging. Successive images are obtained with sub-pixel displacements of the steerable primary mirror. Typically a set of 4 images is obtained with 1/2 pixel displacements in the x and y axes. Using simulated data we have studied the improvement resulting both from simple co-addition of the multiple observations, and from a deconvolution algorithm we call Fractional Pixel Restorations (FPR). We studied the effects in numerical simulations of noise, contrast variations, modest differences in the scene observed in the multiple images, etc. WE have also applied co-addition and the FPR algorithm to laboratory pre-flight images of a wire mesh target, which significantly improved the resolution. Using dynamic imaging with 16 images and 1/4 pixel steps, it would in principle be possible to reach the diffraction limit of the telescope in some circumstances (low noise, sufficient image contrast, no temporal changes in the observed scene, a well characterized instrumental point response function). By the time of this meeting we hope to have high resolution solar images from the LASCO C1 telescope to show. LASCO is a cooperative project of an international group of scientists at the Naval Research Laboratory, Washington, DC, the Max- Planck Institute fur Aeronomie, Germany, the Laboratoire d'Astronomie Spatiale, France, and the Space Research Group at the University of Birmingham, Great Britain.
