Pupil amplitude control as a tool for high dynamic range imaging

Details Pupil amplitude control as a tool for high dynamic range imaging Pupil amplitude control as a tool for high dynamic range imaging

Sep 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002esoc...58..337l&link_type=abstract

Beyond conventional adaptive optics : a conference devoted to the development of adaptive optics for extremely large telescopes.

Physics

Optics

Scientific paper

In the pursuit of high dynamic range imaging, all factors have to be fought on an equal stance lest an effect that was previously thought negligible becomes dominant. It is known that the very core of an short exposure PSF is dominated by speckles. When these are corrected with the use of high dynamic range AO systems, the next dominating effect is the diffraction pattern of the telescope. A little further out (0.3'' < r < 1''), the dominating effect is uncorrected speckles and atmospheric residual (the seeing halo). Hopefully this can also be lowered with the use of high dynamic range AO, but it should be noted that the slope of the enveloppe of these residual is -11/3. At even larger radii (r > 2'') the dominating effect in the infrared is the scattering due to optical surface roughness, which decreases in r-2 and becomes the dominating effect. However, new polishing techniques will eventually bring this value down so it is will not contribute in the domain of interest to high dynamic range imaging. When all these effects have been taken care of, what is left? If one is not careful, diffraction from the telescope pupil can become an important effect to the achievable dynamic range. Studies have already shown the advantage of using an off-axis parabola as a primary to do away with the central obscuration. Also, it is well known that a circular pupil produces Bessel functions, the enveloppe of which decrease as r-3, while straight edges produce sinc2(r), the enveloppe of which decreases as r-2. However in two dimensions, a square pupil has an advantage because the r-2 decrease only affects two, small orthogonal axes in the focal plane, while the axes at 45 degrees experience a r-4 decay. very close to the PSF, though residual tipt-tilt inhibits this fast decay. Pupil shapes can therefore be driven by scientific goals, namely, the search for pointlike objects vs. the search for faint extended emission, and at what distance from the central star. This may lead to the idea of introducing pupil masks in front of the science detector. Another consideration is the segmentation of the primary mirror, which may be inevitable for extremely large telescopes. However if the segmentation is regular in the pupil plane, the diffraction effect add coherently from each segment (or more accurately segment boundary), thereby producing regular structures in the focal plane. New and innovative segmentation schemes are investigated and presented in this poster.

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