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Sporally
2007-Jan-25, 12:22 AM
Do we know how far Hubble can see? 13.6 billion light years, well, not a surprice, i can peer that far aswell:lol: I don't know if this is something that can be calculated by looking at the mirror size and and lenght of the telescope, but if two stars are placed for example 1 light year from each other and they both has the same distance to Earth (and therefore the same angle between the line between the stars and the line to Earth), how far away can they be so that Hubble will still be able to see that there are two stars? I hope you get me:) I don't know if there are better ways of asking this question, maybe astronomers have a better method of saying how far a telescope can peer...

01101001
2007-Jan-25, 12:39 AM
SpaceTelescope.org FAQ (http://www.spacetelescope.org/about/faq.html)


3. Q: I was wondering a few things. First how far can the Hubble Space Telescope actually see?

A: Actually the telescope itself has no limits - but the Universe itself does. Hubble is a medium-sized (2.4 meter) telescope with very sharp optics and very good instruments. This enables the telescope to see very faint objects despite its relatively modest size. According to the theory of Big Bang, the absolute observational limit to telescopes (as we know them today) is a 'sphere' of opaqueness surrounding us positioned approximately 13-14 billion light years away. It is called the 'surface of last scattering', and is also known as the source of the 'microwave background radiation'. Up to 300.000 years after Big Bang, the Universe was totally opaque to light. This means that we know that we (when we look out in the Universe and thus back in time) will never see past, or through, this barrier.

Today galaxies that have been seen with Hubble are at a distance of approximately 12-13 billion light years. In the coming years more distant galaxies will undoubtedly be detected, but the limit for our observations will not progress a lot for two reasons. Firstly, the galaxies have to have time to form stars after Big Bang (this takes roughly one billion years). Secondly, the young galaxies will be enshrouded in large amounts of dust that will - at least to some degree - obscure our view of the early Universe.

But, forget the idea that a telescope lets you see farther. If a photon comes from a maximum distance, according to our understanding of the age of the Universe, and enters your eye (or a camera or other device) then have you not "seen that far" whether there was a telescope in the path or not?

Telescopes do let you see better because they are big light buckets; their larger openings let them collect more photons.

I'll let someone else define angular resolution, how sharply one can see.

Doodler
2007-Jan-25, 01:22 AM
SpaceTelescope.org FAQ (http://www.spacetelescope.org/about/faq.html)



But, forget the idea that a telescope lets you see farther. If a photon comes from a maximum distance, according to our understanding of the age of the Universe, and enters your eye (or a camera or other device) then have you not "seen that far" whether there was a telescope in the path or not?

Telescopes do let you see better because they are big light buckets; their larger openings let them collect more photons.

I'll let someone else define angular resolution, how sharply one can see.

Don't forget exposure time. Ever tried holding your eyes perfectly still for a million seconds?

01101001
2007-Jan-25, 02:45 AM
I'll let someone else define angular resolution, how sharply one can see.

No one? Well, how about Wikipedia: Optical telescope :: Angular resolution (http://en.wikipedia.org/wiki/Optical_telescope#Angular_resolution)?


Essentially; the larger the aperture, the better the angular resolution

It should be noted that the resolution is NOT given by the maximum magnification (or "power") of a telescope. Telescopes marketed by giving high values of the maximum power often deliver poor images.

[...]

Light-gathering power
The light-gathering power of an optical telescope is directly related to the diameter (or aperture) of the objective lens or mirror. Note that the area of a circle is proportional to the square of the radius. A telescope with a lens which has a diameter three times that of another will have nine times the light-gathering power. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light.

Sporally
2007-Jan-29, 06:40 PM
Thx, but is it possible to calculate how far away you could place two stars with, let's say, one light year in between and still see them as two seperate stars?

Nicolas
2007-Jan-29, 07:44 PM
Yes, through the angular resolution.

tan(angular resolution) = 1 lightyear/maximum distance

angular resolution in radians. Not that it really matters for these small angles, but anyway I assume here that one of both stars is ligned up with the scope.

This should do for normal definitions of angular resolution.

Amber Robot
2007-Jan-29, 08:12 PM
Thx, but is it possible to calculate how far away you could place two stars with, let's say, one light year in between and still see them as two seperate stars?

It will depend on wavelength to a degree. First assume that HST is diffraction limited, so the angular resolution will be about lambda/D, where lambda is the wavelength you are observing at and D is the diameter of the primary mirror.

For the sake of argument let us assume that we observe the stars at 500 nm. The primary mirror of HST is 2.4 meters in diameters. So that means that lambda/D is about 2.1x10^-7 radians, or about 0.043 arcseconds.

If you want a pair of stars 1 light year apart to be resolved then they can be at a distance of up to 1/tan(.043 arcseconds) in distance, which is about 4.7 million light years or 1.5 Megaparsecs.

If I did all my calculations right, that should be the answer you are looking for, right?

Sporally
2007-Jan-29, 11:56 PM
Very nice, i will write this nice formula down. So actually HST should be able to see most of the stars not near the Andromeda galaxy's center.