Abstract
Context. This study is made in the context of the future solar coronagraph ASPIICS of the ESA formation-flying mission Proba-3. Aims. In the context of solar coronagraphy, we provide a comparative study of the theoretical performance of serrated (or toothed) external occulters by varying the number and size of the teeth, which we compare to the sharp-edged and apodized disks. The tooth height is small (a few centimeters), to avoid hindering the observation of the solar corona near the limb. We first analyze the diffraction pattern produced by such occulters. In a second step, we compute the umbra profile by integration over the Sun. Methods. We explored a few methods to compute the diffraction pattern. Two of them were implemented. The first is based on 2D fast Fourier transformation (FFT) routines and a multiplication by the Fresnel filter of the form exp(−iπλzu2). Simple rules were derived and discussed to set the sampling conditions. The Maggi–Rubinowicz representation is then proposed as an alternative method, and is proven to be very efficient for this study. Results. Serrated occulters tend to create a two-level intensity pattern, the inner being the darker, which perfectly matches a previously reported geometrical prediction. The diffraction in this central region is lower by two to four orders of magnitude when compared to the sharp-edged disk. The achieved umbra level at the center ranges from 10−4 to below 10−7, depending on the geometry of the teeth. Conclusions. Our study shows that serrated occulters can achieve a high rejection and can almost reach the performance of the apodized disk when very many teeth are used. We prove that shaped occulters must be preferred to simple disks in solar and stellar coronagraphy.
Highlights
The first experiment using an external occultation was made by Evans (1948) to measure the brightness of the sky near the Sun
In the context of solar coronagraphy, we provide a comparative study of the theoretical performance of serrated external occulters by varying the number and size of the teeth, which we compare to the sharp-edged and apodized disks
We prove that shaped occulters must be preferred to simple disks in solar and stellar coronagraphy
Summary
The first experiment using an external occultation was made by Evans (1948) to measure the brightness of the sky near the Sun. On a highly reduced scale, this experiment simulates the phenomenon of a lunar eclipse and has been the precursor of a series of coronagraphs using an external occulter in space, as was well described by Koutchmy (1988) in his review of space-borne coronagraphy The principle of these experiments appears to be much simpler than the classical Lyot coronagraph (Lyot 1939), but characterizing the straylight involves Fresnel diffraction, the effects of which are very complex to model accurately. A simple disk would reject the light toward the center, resulting in an inferior good darkness Based on this reasoning, Boivin (1978) deduced by geometrical considerations that such a serrated occulter would produce a dark inner region whose radius, which we name the Boivin radius, depends on the number and the size of the teeth
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