Dr. Bozidar Mitrovic: The second in importance is so called the resolving power of the
telescope.
What I mean to resolve is basically the following.
Suppose I have two stars that I am viewing through a telescope that
are actually close to each other in terms of their location on the
celestial sphere, so the question is, "Could I resolve them as
separate stars?"
I can actually see that these two directions are not the same, but
that there is an angle between them and it turns out that again the
larger the diameter, the bigger is the resolving power of the
telescope.
Big diameter is good for both, light gathering and
resolving power of the telescope.
Let me just point out what I mean by the fact that we can resolve
two stars as being separate stars.
An example is if you look at Big Dipper, this grouping of stars, it turns out
that here, at the break in the handle of the Big Dipper, there
are actually two stars, not one called Mizar and Alkaid, and
most people, you have to have really good vision to be able to see
that there are actually two stars there using your naked
eye.
You can resolve them as separate for sure by using the telescope
because that enables you to actually see these two stars at certain
angular distance, you can resolve that angular distance.
When I look there, I just don't see that there are
actually two stars.
That's what I mean by the resolving power, ability to actually
resolve objects in the sky as separate.
The problem is that the image near the focus is quite small, so
it's hard to see and then we need to do something in order to
enlarge the image.
Imagine that you have just a single lens and that you are viewing something, say a candle
as a source of light through this lens, the ray the lens refracts
is a ray that is traveling parallel to this so called optical
axis.
Well, as we saw, we'll pass through the focus, but the ray that is
passing through the middle of the lens will be un-deflected,
and the image where these two rays meet, the image
of the same is going to be inverted.
Now, you can imagine that as I move this candle farther and farther
away from the lens that this image is going to shrink.
If the source is very far away like planets are from
us and of course stars, the images are going to be tiny.
We need a second lens to enlarge this image, which is real, and I'll
explain in a moment what I mean by real.
What I mean by real image is that if I take a piece of canvas, a
white sheet, and put it here over the screen.
I would actually see, on that screen, the image of inverted candle.
I don't have to just put my eye here.
I can put any physical object projection screen and then I'll see the image,
but for distant objects, the image is small and a second lens
is needed to enlarge the image, so called eyepiece is needed to
enlarge the image.
I want to give you before going, like to [inaudible
06:09] but I'll just explain how it works.
Here is the real object, the objective lens, and then the first
image, just in the previous case of a candle is inverted, but it's
small, in particular, if the object is at a great distance from the
objective lens.
Then, I will use the second lens, eyepiece, to
actually enlarge the image.
Basically what happens is that this ray will be again refracted by this second lens,
so that it appears to us that it's coming from this spot here.
Basically, what has happened is that the light rays are redirected
again and appear to us as if they are coming from this object here
that looks bigger to us, but this is not a real image.
In other words, if I would put a screen here, I wouldn't
see anything because this is not the source of light rays.
It just appears to me by the way the light rays have been
redirected by the second lens as if they're coming from this object
here, and for that reason, the second image is called virtual
because if I would put a screen here, there would be no image
unlike if I put the screen where the first real image was.
The eyepiece, the second lens is used to increase
the magnification of the telescope.