questions about optics

[ArneB]

Does anyone know?

1) When stars are viewed through a telescope, they appear as discs of different radii. Is the radius the star's real radius, or is it just a function of the stars brightness?

2) Why is viewing extrasolar planets directly so difficult? Is it because the planet "drowns" in the light of the star, or is it because the planet emits too little light? Is it possible to block the light from the star, or filter it to see the planet?

3) What is the reason a small telescope cannot produce a high resolution?
I understand it gathers less light, but why can't that be compensated by a long exposure? Is some of the information destroyed by diffraction?

4) Is it likely that the sun will be used as a gravitational lens in the near future? What would be the resolution of that "telescope"?

[ElPelado]

1) When stars are viewed through a telescope, they appear as discs of different radii. Is the radius the star's real radius, or is it just a function of the stars brightness?

First of all, no they dont appear as disk, and if they appear as disk, they shouldnt. As far as I know, the only telescope I that can see a disk when seeing a star is the Hubble. Normal telescopes see stars as simple dots because they are tooooo far. But telescopes are not perfect, and some limitations of the optics of thetelescopes make the star look like a small disk. When light/water passes trough a small hole(that hole can be the aperture of teh telescope), if the hole is very small(I think that its diammeter should be smaller than the light's wave lenght) the light/water is distorted. That is what happens when light from a star enters your telescope, and that makes the start look as a disk.

2) Why is viewing extrasolar planets directly so difficult? Is it because the planet "drowns" in the light of the star, or is it because the planet emits too little light? Is it possible to block the light from the star, or filter it to see the planet?

Have you tryed to see neptune or uranus with a small telescope? What do you see? Almost nothing... just a dot, or sometimes a small disk. Imagine seeing a planet of that size(even with a very big telescope). Remember what i told that you cant see the disk of a star... the planets are smaller than stars so its impossible with today's technology to see planets. But we "see" them using different technics. And the planets DONT emit light! They only reflect the light from the star they orbit... I dont think that blocking the light from the start would help...

3) What is the reason a small telescope cannot produce a high resolution?
I understand it gathers less light, but why can't that be compensated by a long exposure? Is some of the information destroyed by diffraction?

Well, I am not sure about this one... but I think you are right... if you take long exposures it should work... but it will take longer to make a hi resolution pictures with a small telescope than to make it with a bigger one... but I am not sure at all... I would reall ylike to know this one...

4) Is it likely that the sun will be used as a gravitational lens in the near future? What would be the resolution of that "telescope"?

I have no idea about this one... But I am not sure if it can be used as a telescope... it can deflect the light, but I dont think it can be used as a telescope. With deflect I mean that you will see something not in its real position, you will see the light coming from a different palce in space...

[maxim]

2) Why is viewing extrasolar planets directly so difficult? Is it because the planet "drowns" in the light of the star, or is it because the planet emits too little light? Is it possible to block the light from the star, or filter it to see the planet?

Have you tryed to see neptune or uranus with a small telescope? What do you see? Almost nothing... just a dot, or sometimes a small disk. Imagine seeing a planet of that size(even with a very big telescope). Remember what i told that you cant see the disk of a star... the planets are smaller than stars so its impossible with today's technology to see planets. But we "see" them using different technics. And the planets DONT emit light! They only reflect the light from the star they orbit... I dont think that blocking the light from the start would help...

There are first tests in blocking the light of the main star. The Problem is that this can't be done by just applying a black filter matte, but by inducing identical light with inverse waveforms - so both lighwaves eliminate each other and just the light from the planet remains.

3) What is the reason a small telescope cannot produce a high resolution?
I understand it gathers less light, but why can't that be compensated by a long exposure? Is some of the information destroyed by diffraction?

Well, I am not sure about this one... but I think you are right... if you take long exposures it should work... but it will take longer to make a hi resolution pictures with a small telescope than to make it with a bigger one... but I am not sure at all... I would reall ylike to know this one...

Because you will have to move your telescope to compensate for movement and rotation of earth. You can't do this by hand, only with some hardware - so it might be cheaper to just buy a larger telescope.

maxim

[granthutchison]

3) What is the reason a small telescope cannot produce a high resolution?
I understand it gathers less light, but why can't that be compensated by a long exposure? Is some of the information destroyed by diffraction?

Yes, there's a diffraction effect called the Dawes' limit which limits resolution and is inversely proportional to the aperture of the telescope. Divide 116 by your telescope's aperture in millimetres, and you have its best resolution in arcseconds.

Grant

[ArneB]

ElPelado,
The sun is not so different from a lens in the sense that it actually has a focal point. Space is distorted near massive objects, but in this respect the sun is not very massive. I think I've read that the focal point lies beyond Plutos orbit.

[granthutchison]

I think I've read that the focal point lies beyond Plutos orbit.

About 550AU out from the Sun ... we won't be doing anything with it soon.

Grant

[ArneB]

hehe, thanks. If the resolution is one arc second, that means that you can visually separate two objects that are one arcsecond apart?
So a 10" telescope has a theoretical maximum resolution of 0,46 arcseconds? Wouldn't that almost be enough to make out surface "details" on neptune? I guess turbulence would destroy the view

http://cfao.ucolick.org/pgallery/neptune.php

This is really impressive.

[granthutchison]

If the resolution is one arc second, that means that you can visually separate two objects that are one arcsecond apart?
So a 10" telescope has a theoretical maximum resolution of 0,46 arcseconds?

Yep. My recollection is that even good atmospheric seeing prevents you getting a better resolution than one arcsecond with a conventional optical telescope.

Grant

[Evil Dr Ganymede]

I think I've read that the focal point lies beyond Plutos orbit.

About 550AU out from the Sun ... we won't be doing anything with it soon.

Grant

So would that be a really good spot for telescopes and so on, that use the sun's gravity as a lens? Would things seen from there be magnified?

[granthutchison]

So would that be a really good spot for telescopes and so on, that use the sun's gravity as a lens? Would things seen from there be magnified?

Well, one thing would be seen magnified ... and it would be smeared into a ring shape. To "point the scope" you'd need to fly around the Sun at 550AU out. And to see, say, an extrasolar planet, you'd need to position your spacecraft very precisely at the correct focus for that object, and then move with the focus as the planet moved around its parent star.
But apart from that it's really a perfect system.

Grant

[Evil Dr Ganymede]

So much for that idea

But is there anything special about being at the gravitational focus (which from the sound of it is a radius, not a point) of a star, other than the fact that light bent by the star focuses there?

[granthutchison]

But is there anything special about being at the gravitational focus (which from the sound of it is a radius, not a point) of a star, other than the fact that light bent by the star focuses there?

I can't think of anything. As you say, there's a huge volume of space out there in which the Sun's gravity is focussing something, so it's difficult to see how it could be that special.

Grant

[t00fri]

If the resolution is one arc second, that means that you can visually separate two objects that are one arcsecond apart?
So a 10" telescope has a theoretical maximum resolution of 0,46 arcseconds?

Yep. My recollection is that even good atmospheric seeing prevents you getting a better resolution than one arcsecond with a conventional optical telescope.

Grant

There are two crucial aspects in telescope observation (assuming perfect optics):

-- diffractive effects that e.g. turn the image of a 'pointlike' source of light (<-> a far away star) in the focal plane into a so-called Airy disc, that appears surrounded by a number of faint diffractive rings. The radius of this diffractive disc decreases inversely proportional to the telescope's aperture D. That's where the Dawes limit for the separability of two (equally bright) stars arises as

Code: Select all

min distance [arcsecs] ~ 4.5/D[inches]


For an 8 inch telescope this gives about 0.56 arcsecs.
The effect is a direct consequence of the wave nature of light. Diffraction always becomes apparent in regimes where the familiar geometrical ray approximation is bound to break down...

This limit is affected by the presence of the atmosphere, but not so much in optimal situations and notably for smaller telescopes. With my 8 inch Celestron, I can easily split equally bright doubles about 0.6 arcsecs apart in good nights! This is quite close to the Dawes limit. With bigger telescopes the atmospheric effects become much more severe, though.

--contrast Contrast is a really crucial aspect in almost any telescope observation. Contrast strongly depends on the optical perfection of the telescope, but also much on the atmospheric conditions. A tiny amount of turbulence will make most delicate surface markings on planets invisible, for example...

The light from the extended surface of a planet may be viewed as a superposition of many Airy discs, and thus the resulting contrast is much more sensitive to atmospheric effects than the distinction of just two neighboring point sources of light (double stars).

Bye Fridger

[ArneB]

That's interesting reading! Makes me want to take the wave physics and opitcs courses.