l**n
发帖数: 67 | 1 I guess most of the people have seen the picture of
the image of faraway stars which are gravititionally
lensed by an intermidiate star. But i notice that
in most of those pictures taken with Hubble telescope,
there are only two or four images of one star. I don't
know what caused this that we can only see discrete
number of images. Sometimes i do see continuous images
which appear as an arc. In my mind the image should appear
as a ring if not a circle. Who has any idea? THX | i*******n
发帖数: 166 | 2
Not stars. What picture you have seen is galaxies
are gravititionally lensed by
an intermidiate galaxy or cluster
of galaxies. For star-by-star, you cannot resolve
the images because the seperation of the images is
too small (about 0.001 arcsec) and what you can
observe is the brightness change (microlensing).
For a source lensed by a point source,generally speaking,
there will be two images. If the source, lens and the observer
are exactly on one line, there will be infinite images, i.e.
t
【在 l**n 的大作中提到】
: I guess most of the people have seen the picture of
: the image of faraway stars which are gravititionally
: lensed by an intermidiate star. But i notice that
: in most of those pictures taken with Hubble telescope,
: there are only two or four images of one star. I don't
: know what caused this that we can only see discrete
: number of images. Sometimes i do see continuous images
: which appear as an arc. In my mind the image should appear
: as a ring if not a circle. Who has any idea? THX
| l**n
发帖数: 67 | 3
I am sorry that i didn't look at it carefully. They are genuine
galaxies. But i still don't quite understand your explaination
of the two images. You said when everything lies on an exact
line and we can see Einstein ring but when any of them(lensed star
lensing star, and us) move away from the line, we suddenly see
the ring becomes to discrete images. Is that what you mean?
If this is true, i am very much surprised.
if it's not easy to explain, any reference could be fine.
Thx a lot.
【在 i*******n 的大作中提到】
:
: Not stars. What picture you have seen is galaxies
: are gravititionally lensed by
: an intermidiate galaxy or cluster
: of galaxies. For star-by-star, you cannot resolve
: the images because the seperation of the images is
: too small (about 0.001 arcsec) and what you can
: observe is the brightness change (microlensing).
: For a source lensed by a point source,generally speaking,
: there will be two images. If the source, lens and the observer
| i*******n
发帖数: 166 | 4
It is not surprising since this special case has a higher symmetry.
The conservation of angular momentum of the photons makes it move
in the plane with the source and lens in this plane (there are infinite
planes satisfy this condition). If, however, the source,lens and observer
are not on one line, the three define a plane. What the observer receives
are photons traveling in this plane, the observer can see two images in this
plane from the two sides of the lens. If the three are on a line,
【在 l**n 的大作中提到】
:
: I am sorry that i didn't look at it carefully. They are genuine
: galaxies. But i still don't quite understand your explaination
: of the two images. You said when everything lies on an exact
: line and we can see Einstein ring but when any of them(lensed star
: lensing star, and us) move away from the line, we suddenly see
: the ring becomes to discrete images. Is that what you mean?
: If this is true, i am very much surprised.
: if it's not easy to explain, any reference could be fine.
: Thx a lot.
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