Focal Length and Magnification Question

[Mick UK]

If you have 2, 10" reflector telescopes, one with say, a focal length of 1200 and on with say, a focal length of 2000.

using a 5mm eyepiece, you get a magnification of 240x and 400x respectively.

But why?

Why does it matter how long the distance between the primary and the secondary mirror is?..how does a longer focal length increase the magnification?

Im at a loss as to how light traveling the extra distance increases its magnification?..what causes it?

[ollypenrice]

It has nothing to do with the distance between the mirrors. It is the focal length that matters. It is easier if you look at a ray diagram but think of the angle between the rays. Wide angle for fast focal ratio, narrow angle for slow.

Olly

[Mick UK]

Thanks for that Olly, i checked a diagram out, and it was really quite easy to understand once i seen it!

So would i be right in think after looking at that diagram that because the light rays are spread futher apart (increasing the image size) that a longer focal length tube will have a larger secondary mirror to be able to capture the wider spread light rays?

Having said that..arnt you just capturing the same amount of light only spreading it out more, which surely would in effect be like watching the same image on a portable TV (Short focal length scope, smaller but higher definition image) as opposed to a larger TV(long focal length scope) but A less definition image?

Im confusing myself again now!

[ollypenrice]

Fast and slow scopes of the same aperture have the same light grasp but a wider field of view can collect more light. More light comes off the whole of the moon (fast scope) than from a small part of it (slow scope).

A fast scope of the same aperture surely needs a larger secondary? It has to intercept the steeply angled converging rays while they are quite wide in order to reflect them to a focal plane which clears the side of the tube for your EP. The slow scope can intercept the cone later, when it is smaller, and still reflect the light past the tube because the angle is shallower. I think!

Olly

[themos]

Primary mirrors create a real image inside the telescope.

Longer focal length mirrors create a larger real image. The shorter one's consolation prize is that the smaller image looks brighter.

The same eyepiece in the two telescopes magnifies those images by the same factor.

Result, the longer focal length mirror shows a bigger image.

[brianb]

Longer focal length mirrors create a larger real image. The shorter one's consolation prize is that the smaller image looks brighter.

The same eyepiece in the two telescopes magnifies those images by the same factor.

Result, the longer focal length mirror shows a bigger image.

Theoretically there is NO DIFFERENCE in brightness or size of an image provided that the same magnification is used when observing visually.

In practice, long focal length scopes are longer physically than short focal length scopes of the same aperture. Therefore heavier too. This makes them harder to mount

BUT

it's easier to make good quality optics in long focal lengths, long focus scopes are much easier to collimate, and short focal ratios (less than f/6) place heavy demands on eyepieces - you need to spend a lot of money to get an eyepiece that will allow a good f/5 scope to perform to its full potential.

[themos]

The two scopes capture the same number of photons every second.

One's real image is larger than the other's.

1000 photons look brighter if they come from a smaller area.

You'll need different eyepieces to see the real images at the same magnification (because you'll need to magnify the smaller image more) and then the images will appear as bright as each other.