magnification

[jaymz stealth]

first of all i want to star by asking what is it ment exactly by x30 does it mean an objext viewd in the telescope is 30 times bigger then with your eye or does it mean it decreses the distance 30 times for example 1000meters views though x30 would half that so it would be 500m and so forth till its halfed it 30 times

another thing my telescope has a max mag of 100 that is usefull what does it mean by this

also is it posible to increase the max usefull magnification from a telescope for example my focal lenth is 600 could i ad things to it to make the focal lenth 700 or even 800

[Peter Drew]

Yes, X30 means that the object will be 30X larger than seen without the telescope. All telescopes have a usefull maximum magnification above which the object either gets too dim or fuzzy. You can't increase the focal length of your telescope physically but you can use your Barlow lens as in your other post which will optically present the effect of a longer focal length. .

[umadog]

first of all i want to star by asking what is it meant exactly by x30 does it mean an object viewed in the telescope is 30 times bigger then with your eye or does it mean it

The right way to think about this is in terms of visual angle. The full moon, for instance, spans 0.5 degrees of visual angle when viewed naked eye. There are 90 degrees between zenith and horizon, so that's 90/0.5=180 full moon diameters. If you look at the moon through a telescope and magnify it 100x, then the moon will have an apparent angular diameter of 50 degrees.

another thing my telescope has a max mag of 100 that is useful what does it mean by this

The degree to which a telescope can magnify is limited by two things: the atmosphere and the optics. The atmosphere is turbulent and there's nothing you can do about that. On may nights, magnifications more than about 200x or 300x aren't possible due to this turbulence. However, the maximum useful power you are referring to is due to the optics. When light passes through a finite aperture (i.e. your telescope's objective lens or mirror) it diffracts, meaning that it spreads out slightly. The consequence of this can be seen if you look at a point source, such as a star. Although a star is effectively an infinitely small point, if you magnify it through a telescope it will eventually start to look like a fuzzy ball. This is a consequence of diffraction. The degree to which the light is spread by the aperture is known as the "point spread function." Roughly speaking, you can't discern details (e.g. double stars) which lie closer together than the width of this point spread function. When you over-magnify you don't see more detail, just more more blur. Hence, your telescope will have a magnification limit. Generally, about 40x to 50x per inch is cited as the limit, but in practice this depends on optical quality also.

also is it posible to increase the max usefull magnification from a telescope for example my focal lenth is 600 could i ad things to it to make the focal lenth 700 or even 800

No, I'm afraid not. Magnification is defined as objective focal length divided by eyepiece focal length. If you have a 600 mm focal length and stick a 10 mm eyepiece on, you will get 600/10=60x. You can double that magnification by using a 5 mm eyepiece, so that you get 120x. Alternatively, you could use a 2x barlow lens which has the effect of doubling your telescope's focal length: (2*600)/10=120x. Both of these will double the magnification but they won't alter the size of the point spread function, which depends entirely on the aperture. So you highest useful power doesn't change.

To go to higher powers you need a bigger lens or mirror. Smaller aperture telescopes have larger point spread functions (i.e. the fuzzy ball will look bigger) and larger aperture telescopes have smaller point spread functions. In other words, a larger telescope is capable of producing a sharper image because the light is spread out (blurred) less when it passes through the objective. The image will also look brighter at higher powers in a larger telescope and so this makes higher powered images more appealing.