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Author Topic: How much magnification in gravitational lensing?  (Read 3911 times)

Offline sheepman

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How much magnification in gravitational lensing?
« on: 21/02/2008 23:14:56 »
For the last few years I have read about gravitational lensing and recently seen some impressive photos of Galaxy Cluster SDSS J1004+4112.  In one photo, three galaxies and five quasars are lensed.  What orders of magnitude does gravitational lensing magnify?
« Last Edit: 21/02/2008 23:17:15 by sheepman »


 

another_someone

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How much magnification in gravitational lensing?
« Reply #1 on: 22/02/2008 00:47:32 »
I am not sure it is appropriate to talk about the magnification of a lens, since any lens that magnifies is capable of any arbitrary magnification.  The more appropriate question would be what the focal length of the lens is, and so we can calculate what distance we need to be to gain a particular magnification.

I suppose one could also ask what the spherical and chromatic aberrations are like for a gravitational lens, and so what is the limit of resolution we can achieve.
 

Offline Soul Surfer

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How much magnification in gravitational lensing?
« Reply #2 on: 22/02/2008 09:54:48 »
It is difficult to say and depends on the configuration in theory there is no limit  but gravitational lenses can make distant objects many times brighter than they would be if you looked at them directly and unlensed but you must remember that the images are always very distorted although I believe it is possible to analyse the lens and reduce the distortion in the image with image processing
 

lyner

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How much magnification in gravitational lensing?
« Reply #3 on: 22/02/2008 22:01:45 »
A 'point source' at great distance will still be a point source. It won't look any bigger. It might be expected to be BRIGHTER, though. working on the principle that gravitational lensing can produce two images of one source we could expect the brightness could increase by a factor of at least 2.
 

another_someone

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How much magnification in gravitational lensing?
« Reply #4 on: 23/02/2008 04:01:24 »
A 'point source' at great distance will still be a point source. It won't look any bigger. It might be expected to be BRIGHTER, though. working on the principle that gravitational lensing can produce two images of one source we could expect the brightness could increase by a factor of at least 2.

But they only approximate to 'point sources' due to their distance.  Taking that argument to its logical conclusion, there is very little point in pointing any telescope at a heavenly body (which, with the exception of the Moon and Sun, all look pretty much like point objects to the naked eye).
 

Offline syhprum

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How much magnification in gravitational lensing?
« Reply #5 on: 23/02/2008 09:02:41 »
The gravitational well produced by a massive black hole or a galaxy can be compared to a converging lens, no doubt a skilled mathematician could work out the equivalent "F" number although there would be abberation.
Knowing this and the distances involved it is simple to work out the magnification

Apologies for repetition!
« Last Edit: 23/02/2008 09:05:16 by syhprum »
 

lyner

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How much magnification in gravitational lensing?
« Reply #6 on: 23/02/2008 19:07:07 »
There are very few stars (if any) whose diameter you can measure, due to limits imposed by diffraction. What you get from a lens is an increase in brightness (light gathering power, if you like) but no measurable increase in actual size. You can magnify the image you see in a telescope as much as you like but all you will see, ultimately, is a picture of the diffraction pattern caused by the telescope itself.
I never implied that a telescope doesn't achieve anything; it makes dim objects visible and helps you resolve detail but it doesn't magnify stars' images; they will always be 'blurred points'.
« Last Edit: 24/02/2008 01:18:04 by sophiecentaur »
 

lyner

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How much magnification in gravitational lensing?
« Reply #7 on: 24/02/2008 22:36:34 »
The concept of 'f number' really only applies when you are dealing with a 'real' image formation. The effect of gravitational lensing is not the same as in a camera or a real, inverted image would be formed on 'our side' of the lens. This doesn't happen, does it?
I think the lensing effect produces a virtual image, adding in effect another element to the telescope lens / mirror. In any case, the ratio of focal length to aperture would be absolutely HUGE (f thousands). the f number which counts is that of the telescope itself. The effective focal length of the telescope plus the gravitational lens would be slightly shorter and there could be a net change of f number - but not much, I think.
 

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How much magnification in gravitational lensing?
« Reply #7 on: 24/02/2008 22:36:34 »

 

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