Posted by
timothy
on from the no-doubt-retouched-by-area-51 dept.
Phrogman writes: "The most detailed images of the surface of Saturn's mysterious moon Titan have now been made public. There is more information in an article on Spaceref located here." Interestingly, the photos show three distinct bright areas around Titan's equator.
Poor resolution caused by laws of physics
by
goingware
·
· Score: 5
The resolution you get is determined by the laws of physics. You can work it out either classically, as was originally done using diffraction of electromagnetic waves, or you can do it quantum mechanically, using the uncertainty principle.
Classically, the blurring is caused by scattering of light by the edges of the telecope. The narrower a "slit" you pass light through, the greater the scattering. If you mount a pair of razor blades a hair's breadth apart and shine a laser through, you'll see bands of light on a wall on the other side from the diffraction. As you move the blades apart (make the opening bigger), the bands get smaller.
A telescope is like a really big diffraction slit. The problem here also is that the amount of diffraction is determined by the wavelength of light, so that an image taken in the infrared will always have poorer resolution than the same object taken in visible light. There is also atmospheric distortion to contend with.
Radio waves have very long wavelengths, so a single dish telescope resolves very poorly (like a degree or so). But because the frequency is manageably low you can record the phase on tape and later digitize the signals and combine them from more than one dish you can do interferometry with radio telescopes in a practical way - even interferometry using the whole width of the earth as a baseline. In principle you could have dishes orbiting in different places around the sun and get enormous resolution.
There is a limited amount of optical interferometry going on too, combining the beam of two or more telescopes, as with the 10 meter on mauna kea - they're building a second to use as an interferometer.
Quantum mechanically, you cannot know the product of the location of a particle and its momentum better than about Planck's constant h/2 PI. Putting a particle through a slit determines its position with some certainty and so its momentum across the slit becomes undetermined by a corresponding amount.
With a very narrow slit and a laser beam the uncertainty is large so the photons are scattered as much as a foot or two to either side passing across a room. But passing through the aperture of a 3.6 meter mirror the uncertainty introduced is actually fairly low, so the resolution of a big telescope is high.
One thing you gotta realize is the apparant diameter as viewed from earth is really miniscule. Look up the actual diameter of Titan in an ephemeris and the distance of both Saturn and the Eart from the Sun (earth is 93 million miles) and you can figure out the solid angle it takes up in the sky at closest approach. It's really tiny!
For comparison, the usual resolution of most earth-based telescopes is limited to about a quarter of an arc-second on a really good night.
Up until recently the biggest telescope in the world was the 200" at Palomar Mountain, and it couldn't get nearly the quality of photos of Jupiter and Saturn that the space probes sent by NASA did.
Titan's atmosphere is about four times as dense as Earth's and is composed primarily Nitrogen laced with methane and ethane I had lunch a Las Margaritas today. There is an identical atmosphere around my cubicle.
--Shoeboy
It's not "the most detailed".
by
efuseekay
·
· Score: 5
The images are of ground-based adaptive optics enhanced of Titan at the 1.3-2 microns ranges. That is because optical wavelengths cannot penetrate the thick methane atmosphere.
Now, HST (as several posters have pointed out) has "higher resolution" pictures. But that's at 0.9 microns, which is a factor of 2 smaller than 2 microns. HST does not have a 2 microns filter (methinks), so they can't see Titan from there.
So the phrase "most detailed" has a lot of qualification to it.
The interesting result is that they found bright spots, but the statement in their report of mountaineous regions corresponding to albedo (i.e. reflected light) peaks is flaky and almost careless given their scientific pedigree. Bright spots does not correspond to high areas.
The point is that a liquid sea of methane usually is low in albedo, since methane absorbs light even at low wavelength. SO bright spots means that there might actually be "dry" land on the surface.
-- Mode (3) smart-aleck mode. Press * to return to main menu.
Slashdot Mirror
Classically, the blurring is caused by scattering of light by the edges of the telecope. The narrower a "slit" you pass light through, the greater the scattering. If you mount a pair of razor blades a hair's breadth apart and shine a laser through, you'll see bands of light on a wall on the other side from the diffraction. As you move the blades apart (make the opening bigger), the bands get smaller.
A telescope is like a really big diffraction slit. The problem here also is that the amount of diffraction is determined by the wavelength of light, so that an image taken in the infrared will always have poorer resolution than the same object taken in visible light. There is also atmospheric distortion to contend with.
Radio waves have very long wavelengths, so a single dish telescope resolves very poorly (like a degree or so). But because the frequency is manageably low you can record the phase on tape and later digitize the signals and combine them from more than one dish you can do interferometry with radio telescopes in a practical way - even interferometry using the whole width of the earth as a baseline. In principle you could have dishes orbiting in different places around the sun and get enormous resolution.
There is a limited amount of optical interferometry going on too, combining the beam of two or more telescopes, as with the 10 meter on mauna kea - they're building a second to use as an interferometer.
Quantum mechanically, you cannot know the product of the location of a particle and its momentum better than about Planck's constant h/2 PI. Putting a particle through a slit determines its position with some certainty and so its momentum across the slit becomes undetermined by a corresponding amount.
With a very narrow slit and a laser beam the uncertainty is large so the photons are scattered as much as a foot or two to either side passing across a room. But passing through the aperture of a 3.6 meter mirror the uncertainty introduced is actually fairly low, so the resolution of a big telescope is high.
One thing you gotta realize is the apparant diameter as viewed from earth is really miniscule. Look up the actual diameter of Titan in an ephemeris and the distance of both Saturn and the Eart from the Sun (earth is 93 million miles) and you can figure out the solid angle it takes up in the sky at closest approach. It's really tiny!
For comparison, the usual resolution of most earth-based telescopes is limited to about a quarter of an arc-second on a really good night.
Up until recently the biggest telescope in the world was the 200" at Palomar Mountain, and it couldn't get nearly the quality of photos of Jupiter and Saturn that the space probes sent by NASA did.
-- Could you use my software consulting serv
Titan's atmosphere is about four times as dense as Earth's and is composed primarily Nitrogen laced with methane and ethane
I had lunch a Las Margaritas today. There is an identical atmosphere around my cubicle.
--Shoeboy
The images are of ground-based adaptive optics enhanced of Titan at the 1.3-2 microns ranges. That is because optical wavelengths cannot penetrate the thick methane atmosphere.
Now, HST (as several posters have pointed out) has "higher resolution" pictures. But that's at 0.9 microns, which is a factor of 2 smaller than 2 microns. HST does not have a 2 microns filter (methinks), so they can't see Titan from there.
So the phrase "most detailed" has a lot of qualification to it.
The interesting result is that they found bright spots, but the statement in their report of mountaineous regions corresponding to albedo (i.e. reflected light) peaks is flaky and almost careless given their scientific pedigree. Bright spots does not correspond to high areas.
The point is that a liquid sea of methane usually is low in albedo, since methane absorbs light even at low wavelength. SO bright spots means that there might actually be "dry" land on the surface.
Mode (3) smart-aleck mode. Press * to return to main menu.