Re: Newtonian holiness

Quadibloc <jsavard@xxxxxxxxx> wrote:

On Apr 19, 8:37 pm, Dave Typinski <möb...@xxxxxxxxxxxxx> wrote:
Why don't you see a big hole in the image when you look through a
Newtonian telescope (or anything other than a refractor)?

Either I'm being exceedingly dense today or the explanation isn't all
that obvious.

Probably both.

Basically, the answer is that the secondary mirror is out of focus. If
you look at the eyepiece of a telescope from a distance, it's entirely
possible you'll see an image of the secondary in it.

But when you're looking through the eyepiece itself, the first lens
you're looking through is a really, really strong magnifying glass. It
focuses on a tiny image created by the telescope, which is usually at
the other end *of the eyepiece*, and magnifies it, in the process
making it look like it is at infinity.

Where the image actually is formed that the eyepiece magnifies,
there's usually another lens, called the field lens. (This is
oversimplified; many eyepieces do not have one element that obviously
performs the field lens function, they usually have elements in
between.)

What does the field lens do?

Think in terms of a refracting telescope. If you put your head behind
where the eyepiece would be, and just look at the objective lens, you
will see an upside-down image of what the telescope is pointed at.
Since that image is made up of light that went through the objective
lens, it doesn't extend beyond the lens. But if you move your head,
you will see other parts of what the telescope is looking at.

So there's an image in space in front of you that is bigger than what
you see, but only the part with the objective lens behind it is made
up of light going your way.

The field lens takes the light in the image, and bends it around so
that you can see the whole real image formed by the light from the
objective lens. So, with the field lens in place, you no longer see
the telescope tube and the lens itself, just a large upside-down image
showing more of what the telescope is pointed at. Since the field lens
is at the focal plane, it has almost no effect on the telescope's
magnification, it only redirects light.

The field lens and the magnifier in an eyepiece *both* conspire to
ensure that when you look through the eyepiece normally, with your eye
against it, you will see the image, and not the telescope. The
different parts of the image will have light from all, or at least a
large part of, the primary mirror or the objective lens, so that the
central obstruction only reduces the brightness of the image in a
nearly-uniform manner.

Thank you, John. That helps a lot.

So, let me see if I understand it by restating it.

If a star at infinity is right on the telescope's axis, some of its
light will be blocked by the secondary, but some of it also makes it
past the secondary to the "unshadowed" section of the primary. The
star's light rays are, after all, parallel--and the star is
undoubtedly larger than the primary mirror. It is this light that is
subsequently projected on to your retina at the position where one
would otherwise see the shadow of the secondary mirror if not for the
eyepiece.
--
Dave

.

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