"ivk" wrote in message
...
You may be interested in
thishttp://en.wikipedia.org/wiki/New_Worlds_Mission

Thanks a lot, that's exactly what I was asking about. From this
article, still a lot of things remain unclear:

1. James Webb Telescope is to be placed 1.5 mln km from Earth, on 1
year orbit (I think it is called Lagrange point). The article says
that the Starshade will be placed 238,600 miles (400,000km) from
Earth. How come it then will be 80,000 km from the telescope ?

2. Why does the article says that ~10 parsec is the limit ? By moving
the Starshade further from the telescope, they should be able to cover
more distant planet systems.

The problem would be that any planets of more distant stars would be much
closer to their primary stars, but the size of the diffraction pattern would
not change. You would instead need a bigger space telescope, which is
definitely not in the budget.

3. I am curious about the math - how this flower shape reduces
diffraction ? I thought one cannot reduce the diffraction much better
than w / D, where w is the wavelength, and D is the diameter of the
screen. If screen is ~10m (as the article says), it will be about the
order of 10 million, not 10 billion.

Do a Google or other search on "apodization". It's not so much a reduction
of the diameter of the pattern (it may actually increase the size of the
central peak) as a modification (reduction) of the diffraction rings.

See also http://planetquest.jpl.nasa.gov/tech...et_imaging.cfm

Indeed it is a serious but not fatal weakness of the proposed method that
the disk has to be so far from the telescope, making pointing at multiple
targets difficult.

Another perhaps more promising method is "nulling interferometry", in effect
building a multi-aperture telescope in which the light is combined to
"cancel out" or reduce the bright star sufficiently to see an image of a
planet.

--
Mike Dworetsky

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