mirror and eyepiece conundrum...

[ronnietucker]

This is probably going to sound daft, but this is my first telescope and I'm befuddled by this quandary:

If the primary mirror takes in a reflection of the outside world and bounces it to the eyepiece, how come the eyepiece doesn't get in the way and reflect onto the main mirror?

Not that I've actually seen anything yet. First night of having the 'scope and it was solid cloud. Now the weather predicts snow. Fan-tastic

[umadog]

You're right, the eyepiece does create a reflection in the primary mirror. You can verify this by looking into the empty focuser: you will see a reflection of your eye. You will also notice that you eye occupies a very small part of the view (it is reflected via the smaller secondary mirror) so most of the primary isn't affected in this way. Your eye is visible in the reflection of the secondary in the primary. Confusing...

You question also also hints at what the telescope is actually doing. A key thing to remember about optics is that they work both forwards and backwards. Let me explain the consequences of this. You're probably familiar with the idea that the primary mirror focuses light into what is known as the focal plane. The eyepiece magnifies the image formed at the focal plane. The primary mirror is a few inches across, but the light coming out of the back of the eyepiece is a beam only a few mm across. So in a sense, the optical system has minified the primary mirror: all of the light-gathering power of the primary mirror has been condensed so as to fit through your pupil. You can also think of this process as working the other way around: your pupil is being expanded and projected to fill the primary mirror. The telescope, then, is a device for expanding your pupil to a size several inches across. It is for this reason that your eye gathers more light when you use a telescope. You can even demonstrate this: Put your eyepiece into the telescope and shine a torch into the eyepiece and point the telescope at a wall: you will see a circle of light the same diameter as the primary mirror.

[ronnietucker]

Thanks umadog, that does make sense.