# telescopes?

Hey there, so I was thinking about those mirrored telescopes (the kind with a big concave mirror in the back - Galilean I think?) and was wondering - how do they work? My confusion arises from the fact that concave mirrors reduce the size of an object when it is beyond twice the focal length, so how come it appears larger when you look into one? Random question, but I figured there are a ton of engineers on here

The eyepiece lens.

…R

What do you mean, 'appears larger' and look into what?

I mean that when you look into the telescope (through the eyepiece) the object you are looking at appears larger or magnified.

So is the eyepiece the only peice responsible for magnification? This is my understanding of the situation: When an object is outside twice the focal length of a concave mirror, a reduced image of the object is formed. This is directed towards an eyepiece by an angled mirror. The eyepiece acts like a microscope of this reduced image, "zooming in."

But the image you are actually looking at is tiny since it has to squeeze into the opening in your eyeball.

You're no longer looking at entire field of view that you're probably starting your baseline at. You're looking at a smaller portion, but in higher resolution, so this very loose term of 'appears larger' needs some clarification.

tinkerkid: This is my understanding of the situation: When an object is outside twice the focal length of a concave mirror, a reduced image of the object is formed. This is directed towards an eyepiece by an angled mirror. The eyepiece acts like a microscope of this reduced image, "zooming in."

The purpose of the mirror is to collect as much light as possible from the distant object and focus it so that it all goes into your eye (through the eyepiece lens) - or onto the CCD for the camera.

...R

They are Newtonian, not Galilean.

A galilean telescope is a refractor, which does not invert the image. It is usually limited to low power. The "telescoping" telescopes you see sea captains using are usually Galilean.

Reflector telescopes are much more common, and much less expensive to produce per unit of diameter.

The primary (parabolic) mirror focusses the light from the object into a tiny image inside the telescope body, where a secondary mirror directs it to the side. An ocular lens (the eyepiece) is used to magnify the image for viewing.

And yes, the image is many, many times smaller than the object. Distant galaxies, etc. are huge and would never fit inside your telescope.

ChrisTenone: Distant galaxies, etc. are huge and would never fit inside your telescope.

Nice :)

...R

Robin hit on the main point: the size of the telescope determines how much light it can gather. The bigger the better, and this is true for either reflector or refractor scopes. You might not notice that brightness effect since your eyes are so capable of adjusting to varying light levels, but in the end the brighter picture is just plain easier to see.

It is french wikipedia so it is in french but drawing have no language.

Notice that for us it is not Newton telescope but “Cassegrain” telescope
Edit :
This is not Newton principle but Cassegrain principle:
The two principles are very similar, mirrors are just not in the same place.

ChrisTenone:
Distant galaxies, etc. are huge and would never fit inside your telescope.

The Arquilian Galaxy would

Luv MiB.

tinkerkid: Hey there, so I was thinking about those mirrored telescopes (the kind with a big concave mirror in the back - Galilean I think?) and was wondering - how do they work? My confusion arises from the fact that concave mirrors reduce the size of an object when it is beyond twice the focal length, so how come it appears larger when you look into one? Random question, but I figured there are a ton of engineers on here

The real image may be small, but the virtual image is huge, but most importantly the angles subtended at the eye are much larger than those subtended by the actual stars.

The vitual image has to be far enough from your eye for your eye to have it in focus.

Concave mirrors produce a real image at their focus, typically the eyepiece converts that to a large virtual image, though it could be used to create a large real image (as when observing a solar eclipse onti a piece of card). In normal operation the eyepiece is simply a magnifying lens looking in detail at the real image produced by the concave reflector. A magnifying lens is basically allowing something close to the eye to be focused by creating a virtual image further back. Or you can think of it as increasing the power of your eye lens to be able to focus closer.