Posted 28 February 2004 - 01:56 PM
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.
Posted 28 February 2004 - 02:15 PM
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.
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.
Posted 28 February 2004 - 03:22 PM
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!
Posted 28 February 2004 - 04:27 PM
Posted 28 February 2004 - 04:43 PM
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.
Posted 28 February 2004 - 04:47 PM
Posted 28 February 2004 - 05:57 PM
The flashing of bright stars in different colours is, I am sure, due to turbulent variation of refractive dispersion.
Posted 28 February 2004 - 11:55 PM