--- Joe Bauman <bau@desnews.com> wrote:

I don't want to flog a dead horse, but the blocking object doesn't have to block only the star. It could

block a huge swath of space to one side of the star,

as long as its edge just cuts out the star too. . . .

Joe, You might find these pages interesting. NASA Terresterial Planet Finder (TPF) Technology Milestone Page http://planetquest.jpl.nasa.gov/TPF/detectionMilestone.cfm NASA TPF Technology Plan (6.2 Mb) http://planetquest.jpl.nasa.gov/TPF/TPF-CTechPlan.pdf TPF Homepage http://planetquest.jpl.nasa.gov/TPF/tpf_index.cfm The problem isn't that much different from our amateur experience of predicting whether a binary star can be split. Whether a binary star can be split depends on the distance between the stars, _the contrast ratio between the stars_ and the Airy disk size of your telescope (which depends on aperature). By masking the central star, you increase the contrast ratio between the fainter secondary binary (or extrasolar planet). In the TPF, everything is working on much higher tolerances. As the NASA press release states, JPL has been able to make it work on simulated stars and planets in the laboratory. http://planetquest.jpl.nasa.gov/TPF/detectionMilestone.cfm TPF will consist of a visible light corongraph satellite and a compound, formation-flying infrared interferometer satellite. As you can see from the simulated pictures on this page - http://planetquest.jpl.nasa.gov/TPF/tpf_what_is.cfm - NASA does not seem to be trying to take what we would call a usual astrophotograph of a planet - rather they are trying to see the planet's signal in the diffraction rings and interference pattern of the light of the star. Wikipedia on a Coronagraph http://en.wikipedia.org/wiki/Coronagraph Another simulated TPF picture - http://en.wikipedia.org/wiki/Image:Coronagraph_starburst.jpg - Canopus56 __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

--- Joe Bauman <bau@desnews.com> wrote:

Thanks, Kurt and Michael. The "airy disk" doesn't involve light scattering, but the telescope's optics -- is that right? I am starting to understand it a little better. Best wishes, Joe

I'm reaching a little beyond my optics knowledge, but I'll take a shot. The Airy disk that you see when you star test your telescope at extreme defocused magnification - http://www.skywatchertelescope.com/EducationST.html - is an inherent property of a lens. The lens, through diffraction, changes the probability of a particular photon reaching a specific point. Therefore, the light spreads out into a circular diffraction pattern seen above, whenever it passes through a lens. You would see an Airy disk even if your personal SCT was in space. (Getting to the eyepiece is another matter.) Atmospheric turbulence just makes things worse. Since Sirius is still up, the easiest way to see the Airy disk diffraction rings in your SCT is to point it at Sirius, put in the highest magnification lens you've got, focus it, and then slightly defocus the scope either inside (towards the scope) focus or outside (away from the scope body) focus. Then compare the diffraction ring image using Regulus, another bright star but that should be high enough that it is outside horizon atmospheric turbulence. The coronagraph complicates things a little. The edge of the coronagraph - or blocking disk - also acts as diffraction edge. Light gets bent or scattered by the edge. So in this TPF simulated image - http://en.wikipedia.org/wiki/Image:Coronagraph_starburst.jpg - what you proably are seeing is a fringe, or coma, caused by light scattering from around the coronagraph blocking disk edge and then two Airy disks - one for the star at the center - and one for the planet off to the side. In the above image, the circular Airy disk rings like those seen in the first image - http://www.skywatchertelescope.com/EducationST.html - and are around the planet in the second image - http://en.wikipedia.org/wiki/Image:Coronagraph_starburst.jpg - But the diffraction ring around the planet is pixelated into an odd "Star of David" pattern. That is just an artificat of the individual detectors on the CCD chip. The same effect is seen in the red coronagraph simulated image in the third picture - http://planetquest.jpl.nasa.gov/images/coron_inter.jpg In the third image the circular Airy disk diffraction rings of the central star are scattered by the blocking disk so the light is no longer concentrated in neat concentric circles around the star. The Airy disk circles of the central star are dispersed into the small concentric points and probably outside the frame. As a result, you can now see the small Airy disks of the planets that would normally be whited-out by the circular diffraction rings of the central star. The blue right-hand image in the third picture - http://planetquest.jpl.nasa.gov/images/coron_inter.jpg - is an interferometer image. This works on a completely different principle from your usually optical telescope. See the Wikipedia entry for "Interferometer" - http://en.wikipedia.org/wiki/Interferometer - under the topic "Astronomical optical interferometry." Here's a schematic of an interferometer built with a mask and a signal lens - http://www.geocities.com/CapeCanaveral/2309/image33.gif and a second using two telescopes - http://www.geocities.com/CapeCanaveral/2309/image34.gif The interferometer works by taking two beams of light from the same object, time delaying one beam, and then projecting both beams on to the same surface. One method of time-offsetting light from a star is to use a second telescope offset in space from the first telescope, e.g. - the TPF array of interferometer telescopes - http://planetquest.jpl.nasa.gov/images/tpf_formationFlying-162.jpg If the time-offset of the two light waves is equal to a whole number of the light's wavelength, diffraction rings cancel or add to each other out such that summing occurs - light from objects will intensify, the dark areas get darker. That's what you see in the blue right-hand interferometer image in the third picture - http://planetquest.jpl.nasa.gov/images/coron_inter.jpg - Canopus56 P.S. - You may be asking yourself how there can be an image of the central star on the picture where there is a blocking disk in the line-of-sight - put there specifically to block out the light of the star. The answer is that whenever light passes an edge, the probability is that some of it will go right and that some of it will go left. While most of the photons will turn outside the picture frame, as a matter of statistics, a small fraction of the light will turn the other direction around the diffraction edge and still make an image on the "blocked" line-of-sight side of the disk. It's the just the "spooky" dual nature of light and quantum physics. Light does not act only like a ballistic particle; light also acts like a wave. Waves can turn corners; ballistic particles cannot. __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

--- Joe Bauman <bau@desnews.com> wrote:

Thanks so much, Kurt. Does the spooky action of light mean they won't be able to block enough light from a star to directly image a planet?

Per the images, they seem to have made it work on a testbed. What's different about the TPF context is that the "mask" is probably only going to be few microns across. Thus, diffracted light can "leak" by diffraction around the sides. That's different from the traditional solar chronograph, where the mask is much larger, both physically and in terms of the 1/2 deg size of the Sun. http://sohowww.nascom.nasa.gov/data/realtime/c3/512/ - Canopus56 __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com