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The following improvements of the instrument are foreseen:
- Improved adative stabilisation of the occulter centering.
- Lyot stop: as mentioned in section 2.3, the use of a
pinhole-shaped diaphragm introduces residual
optical defects, such as static bright speckles at the crossing of a
spider arm and the second Airy ring. For
an optimization of the apodizing device, we will use in the
future a diaphragm matching the telescope pupil shape.
- The Risley prisms adjustement needs to be automated.
- Optimized detectors: regarding the photon-counting detector, AsGa
photocathodes now available extend the
photon-counting range to 900nm. They will be used on future photon-counting
cameras. With their clean detection of photon-events, electron bombarded
CCDs appear of interest, in spite of the rather slow readout
currently available. Non intensified photon-counting CCDs are also announced.
- Speckle noise: In the cleaned image generated by the dark speckle algorithm,
and also in a conventional long-exposure a fixed pattern of speckles
tends to remain visible in the smoothed halo.
Long integrations, whether in the dark-speckle imaging mode or
with conventional long exposures, tend to smooth the atmosphere-
induced speckles to the point where fixed speckles become
apparent. These originate in the fixed bumpiness of the main
mirror and the smaller optics, including the adaptive mirror. The
fixed amplitude pattern in the pupil, caused by secondary
obscuration, spiders, mirror dirt, etc... also contributes, but to a
lesser degree since the Lyot stop is configured to remove much of
the corresponding straylight. Attaining a more accurate
measurement of static residual bumpiness in the Shack-Hartmann
wave sensor should improve very much the situation. The
accuracy of such measurements being rather immune to the photon
noise affecting fast wave corrugations, there is hope of
significant improvements.
An easier but less efficient method, which can be used simultaneously,
involves adding bias signals, randomly variable in space and time, to the
actuators.
The issue will be analyzed in more detail in a forthcoming article.
Next: Conclusion
Up: Present performance of the coronagraph
Previous: Psc
6/15/1998