Covering direct light from a star

One way to directly observe planets of remote stars is to use a space
telescope with a resolution better than the orbit of the planet, and
cover the direct light from the star with a screen. The screen has, of
course, to be very far from the telescope (order of millions km, to
avoid covering the planet), and its diameter has to exceed the
aperture of the telescope. My question is - how many times it has to
exceed the telescope's aperture to prevent substantial portion of the
direct star light from still getting into the telescope by way of
diffraction on the edge of the screen ?

"ivk" wrote in message
...
| One way to directly observe planets of remote stars is to use a space
| telescope with a resolution better than the orbit of the planet, and
| cover the direct light from the star with a screen.

http://antwrp.gsfc.nasa.gov/apod/ap061114.html

I'm sorry to have to tell you this, but you are not realistic.

you are not realistic.

Not realistic about what ? My post was a question rather than a
statment.

"ivk" wrote in message
...
| you are not realistic.
|
| Not realistic about what ? My post was a question rather than a
| statment.

"| One way to directly observe planets of remote stars is to use a space
| telescope with a resolution better than the orbit of the planet, and
| cover the direct light from the star with a screen."


Looks like ****in' stupid statement to me, you lying idiot.

This is a photograph of a star with planet of it, you stupid moron:

http://antwrp.gsfc.nasa.gov/apod/ap061114.html

I'm sorry to have to tell you this, but you are not realistic.

Got it now, cretin?

On Feb 18, 8:21*am, ivk wrote:
One way to directly observe planets of remote stars is to use a space
telescope with a resolution better than the orbit of the planet, and
cover the direct light from the star with a screen. The screen has, of
course, to be very far from the telescope (order of millions km, to
avoid covering the planet), and its diameter has to exceed the
aperture of the telescope. My question is - how many times it has to
exceed the telescope's aperture to prevent substantial portion of the
direct star light from still getting into the telescope by way of
diffraction on the edge of the screen ?


Ignore Androcles, he's been a bit incontinent recently.

You may be interested in this
http://en.wikipedia.org/wiki/New_Worlds_Mission

It's a large occulting sunflower shape which sits appx 80,000 km in
front of a space telescope (such as the James Webb Telescope). Being
such a distance from the scope makes it difficult to switch from one
target star to another.

I believe the 'petals' modifies the diffraction pattern, so as to
enhance the visibility of any orbiting planets.

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.

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.

"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

(Remove pants sp*mbl*ck to reply)

Mike, thanks a lot for your reply. Now I see that the petal shape
spreads diffraction pattern, making its peaks lower.

BTW, this whole project, as proposed by prof. Cash, probably won't
work, because the Sun light spread in all directions from the edges of
the occulter will totally dim out the light from the remote planet. So
to make the whole thing work, one will need a huge screen (a few 100s
m in diamater) independently floating in several km from the occulter.
While it is feasible, it will enourmously increase the price tag.

And if they're going to place it at a Lagrange point, in 1.5 Mkm from
the Earth, they will need the 2nd screen to protect from Earth
reflected light. And then the 3d one, for the Moon light...