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Star Focus

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#1 KennyJ

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Posted 28 February 2004 - 01:56 PM

Given the vast differences between the respective distances of stars from earth ( e.g Deneb 1800 light years away compared with Sirius only 8.6 light years ) why is it that the focusser on a binocular or scope remains in the same place to attain crisp pin -point images of both objects ?

And a completely different question on a related topic.

When Vega is low in the north as it is presently , ought it be possible to resolve it as a "blue" colour as I do when it is overhead in summer ?

It always looks like a Kaleidoscope to me at this time of year -- is this due to reflections from the Sun ?

Yours curiously , Kenny.


#2 Guest_**DONOTDELETE**_*

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Posted 28 February 2004 - 02:15 PM

Kenny, I've also been interested in this. I posted an explanation of it in the off topic forum’s FAQ thread and asked that it be corrected, but no one did so, I assume my explanation is correct. Here is a part of my post:


Why does the eyepiece need to be closer to the front objective lens when focusing on far away objects, and farther away when focusing on closer objects?


The answer has to do with the angle of the light coming from the object in focus:


O< - - - - - - - - - - - - - L

As can be seen in the above diagram, by the time the light from the object O reaches the lens L, only the light rays traveling close to perpendicular to the objective lens will enter it.

O<L

In this second diagram you can see that some of the light entering the objective lens is at an angle.


Basically there is no difference between focusing on a star 4 light years away, or a galaxy much further, because the angle at which the light enters the objective is the same in both. As usual I don't know this as a fact, and I'm anxiously awaiting a definite answer from someone more knowledgeable.



For Vega, the only thing I can think of is the atmosphere interfering since you're looking through so much more of it then you do in the summer.

#3 sftonkin

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Posted 28 February 2004 - 03:22 PM

The focus remains the same because the distances are indistinguishable (by the optics) from infinity. Or, put another way, the rays of light that come from it to the big end of your binocs are indistinguishable from parallel.

The colours of bright stars near the horizon are due to the atmosphere -- specifically to turbulence (bad seeing). Because different parts of tubulent air are at different temperatures, they have different densities and refract (and hence disperse) light differently. Even when the air is still, you do get spectral fringes on objects near the horizon due to dispersion (try Sirius or Venus through a reflector when they are low down over the sea or something), when the air is moving, it gets a tad more interesting!

#4 KennyJ

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Posted 28 February 2004 - 04:11 PM

Thanks Daniel and Stephen.

Regards --Kenny.



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Posted 28 February 2004 - 04:27 PM

The difference in color is better explained by scattering of light. Objects from the horizon travel a greater distance in the Earth's atmosphere to reach you thus a given photon has more opportunities to experience was is called Rayleigh scattering. When the light's wavelength is much much less than the length of the atmospheric molecules (which is mostly diatomic nitrogen here on Earth) the light will be scattered strongly. This is explains why we see the sky as blue (very short wavelength). Nearly all of the longer wavelength photons (red colored) pass right through the atmosphere without scattering. This preferred scattering causes the object (whether it's the Sun or a distance star) to look more red when it's viewed through more atmosphere. Along the line of sight more blue has been scattering out, thus a reddish color remains. The Sun at sunset or sunrise is very red and not so red at noon as an example.

#6 KennyJ

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Posted 28 February 2004 - 04:43 PM

As a side - issue , I must confess that I find the Kaleidoscopic effect with Vega quite fascinating in itself.

Another effect which intrigues me is when deliberately putting a planet WAY out of focus -- by focussing the bino or scope as if you were looking at an object 100 yards away.

The patterns thus formed , filling up almost the entire field of view I find quite wonderful to behold.

Regards --Kenny.

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Posted 28 February 2004 - 04:47 PM

I like looking at bright stars sometimes too when its turbulent out....the twinkling can be quite amazing.

#8 sftonkin

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Posted 28 February 2004 - 05:57 PM

Are you sure about that? Whilst scattering certainly explains the reddening of objects near the horizon (as per your description), but it cannot explain the flashing of different colours of stars, or the colour differentiation on the upper and lower limbs of planets when they are at low altitude. The fringing on this image is due to refractive dispersion:
Posted Image

The flashing of bright stars in different colours is, I am sure, due to turbulent variation of refractive dispersion.

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Posted 28 February 2004 - 11:55 PM

Oh no, you are right about the possible turbulence and disperion combination resulting in chromatic flashes and limb discoloring. I was merely explaining why a blue star, like Vega, is not blue when its low in the sky.

#10 sftonkin

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Posted 29 February 2004 - 01:55 AM

Ah. Apologies for the misunderstanding.


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