Confused beginner - astrophotography

[Grumps]

Hi, I'm a complete beginner to astronomy, I know where the moon and sun are, and am looking to purchase my first telescope. I now know I need something with a motor drive and due to my complete ignorance a "goto" function. What I am confused with is the focal lenght of telescopes. In photography the greater the focal length, the greater the magnification. To photograph the moon you need at least a lens of at least 400mm to get a resonable sized dot in the frame. So, how is it telescopes with a focal length 650mm tell me that I will be able to see craters of the moon?

Please excuse my ignorance and potential use of incorrect terminology.

Any help would be much appreciated.

[JKB]

Perhaps try this Telescope Calculator

I'm no expert but let everyone here know what you want to do and I'm sure you'll have plenty of replies

Cheers

Jamie

[Ags]

A camera is lens + sensor. A telescope is lens + eyepiece plus eye. The eyepiece's role is magnify the image provided by the telescope's lens. Change the eyepiece nd you get a different magnification.

[cloudythoughts]

Magnification is focal length of the primary divided by focal length of the eyepiece.

so a 680mm primary with an 10mm eyepieces gives a magnification of x68

A camera 50mm focal length used with a 400mm lens will have magnification of 400/50 = x8

[smolloy]

It helps me to think about the light path.

In a camera, you have multiple beams of light (let's think about light coming from infinity to keep things easy) entering a lens, and being focused onto a sensor. If everything is working properly, all light coming from a certain point of the thing you want to image will be focused to the same point. Light coming from elsewhere in the object will be focused to a different point.

The field of view of such a system is calculated based on the maximum angle that light can come from and still hit the sensor -- i.e. the light that hits the edge of the sensor. For two cameras with the same sensor, but different focal lengths, the one with the longer focal length will have a smaller field of view. Thus the pictures it creates will contain a smaller chunk of the object spread over the same area -- thus we say that this camera system has a higher magnification.

All of this is the same when you have a telescope with a camera attached. The only lens is the one in the telescope (although it may be a mirror, or a lens, or both), and you do your best to put the camera sensor at the point at which the light comes to a focus.

Viewing through a telescope + eye-piece is quite different. Here the light is allowed to come to a focus (at the point at which you would place a camera sensor), and then diverge again. You can think of the eye-piece as a single lens that is placed a certain distance from the point of focus. This distance (all being well) is the same as the focal length of the eye-piece, and so the light emerging from the eye-piece is parallel again.

You then place your eye into this parallel beam of light, and your eye responds as if the light is coming from infinity (since it is parallel), and focuses accordingly, allowing you to see an image.

(Note: I'm not 100% sure, but I think most scopes will place the eye-piece *before* the light has focused, and will then convert the converging light to a parallel beam. Hopefully you can see that this system is mathematically almost identical to the situation where the diverging light is refocused.)

In this case, the magnification is calculated from the ratio of the focal length of the telescope and the focal length of the eye-piece. This should make a lot of sense if you think about what would happen if you used an eye-piece of the same focal length as the scope....

(Obviously a ridiculous thought experiment, but I hope you agree that the magnification would be X1, and you would simply be recreating the original beam of light.)

Does that make sense?

[Grumps]

Thanks to all for your help, I'm now a little less confused infact I'm now confident enough to start researching telescopes in my price range (£250 max) and taking the plunge. Hopefully in time I can return the compliment and help another newbie.