|
Hi, A few weeks ago I heard an item on the radio which referred to a paper which had appeared in Nature. The reference was to a flower-petal-shaped 'occulter' which was proposed to be put in space to block the light from a target star, permitting a space telescope to directly see circling planets (if any!) as faint stars. I eventually found the paper by Webster Cash entitled "Detection of Earth-like planets around nearby stars using a petal-shaped occulter" (Nature 6 July, 2006)( http://www.nature.com/nature/journal/v442/n7098/pdf/nature04930.pdf). Cash describes having developed a shape for the petals of his proposed occulter which "suppress(es) diffraction over a broad spectral band". In such a device "the sum of the phases of the light from all the paths through and around the shade must be extremely close to zero. When such an occulter is flown in formation with a telescope of at least one metre aperture, terrestrial planets could be seen and studied around stars to a distance of ten parsecs." His proposed occulter would be 30-50 metres in diameter and would have to be positioned 50,000 km from the telescope (!) to permit detection of nearby planets around a distant star, something he suggests is quite feasible… Since suppression of diffraction around occulters is pretty much what we would like to do to sharpen the shadows of noduses (nodi?) and noon marks, I thought I'd make a very small, 'quick and dirty', model (compared to Cash's heroic flowers) only 4.5cm inner/7 cm outer diameter made by enlarging and cutting out a copy of the picture in Nature (see below) made in a photocopier. It has been far too overcast and I far too busy (the beginning of the new semester) for anything other than a very quick comparison with a circular disk 4.5 cm in diameter but the result was promising. At about 3 metres distance, the shadow of the ordinary circular disk was barely perceptible: very faint and fuzzy. The shadow of the occulter beside it was also blurry but far sharper, so I feel this is a promising line of investigation. Perhaps the petals can be placed to face inwards in a ring to sharpen an annulus? Happy dialing, Peter Mayer
-- Peter Mayer Politics Department The University of Adelaide, AUSTRALIA 5005 Ph : +61 8 8303 5606 Fax : +61 8 8303 3443 e-mail: [EMAIL PROTECTED] CRICOS Provider Number 00123M ----------------------------------------------------------- This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. No representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. |
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
