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Q & A: Telescopes

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Most recent answer: 10/22/2007
Q:
whats the difference between ASTRONOMICAL and TERRESTRIAL telescope
- aabhushan
kathmandu
A:
As you may have guessed, astronomical telescopes are used to look at things not on the Earth and terrestrial telescopes look at things on the Earth. But there is a big difference in how they are made. To understand this, I'll need to talk a little about how refractive telescopes work.

A refractive telescope (like astronomical and terrestrial) uses lenses to bend light. Since stars or mountains that we want to look at a long ways away, we want to gather as much light as we can from the object and focus it all into our eye. To do this, there is a really big lens in the front called an objective lens. The objective focuses the light down to a very small area. Now, we can think of what we're looking at as if it was in that little area that the objective focused the light to.

Here's where the different telescopes come into play. An astronomical telescope uses another, smaller lens to focus the light from the small image into our eye. The problem with this is that the image is upside down! A terrestrial telescope has another lens to turn the upside down image right side up.

Now, you might think that it would be better to just use terrestrial telescopes all the time. But that third lens has to be placed away from the second lens. So terrestrial telescopes are a lot longer than astronomical telescopes and therefore more expensive.

They each have their advantages. When we look at stars, we don't really care that the image is upside down. And since the telescopes are so big, it would be a lot more expensive to make the telescope bigger just to see the image right sid up. On the other hand, when you are looking at something on Earth, you want to see it right side up. But since it doesn't need to be as powerful as the ones that look at the sky, it is also a lot smaller and less expensive.


Adam

(published on 10/22/2007)

Follow-Up #1: Astronomical/Terrestrial magnifications

Q:
what is the magnification of an astronomical and terestrial telescope?
- prasanna9mitra@gmail.com
India
A:
There are two important criteria for astronomical telescopes: light gathering power and angular resolution.   The magnification is not that important since the image is captured by film or ccd camera and you can always enlarge the image as much as you want, keeping in mind the resolution of the recording medium.  For both of the two criteria the diameter of the primary lens is the biggest factor.  If you increase the diameter of the lens by a factor of two the light gathering power is increased by a factor of four and the angular resolution is improved by a factor of two.   Big lenses, or big mirrors in the case of most modern telescopes, are better.
The best telescopes these days have diameters of the order of eight meters.

For a terrestrial telescope the magnification is important.  If too small you canít resolve details of the object.  If the magnification is too big you donít get a large enough field of view.  Many good quality binoculars have magnifications of around seven.  A spotting telescope may have much more but, again, the field of view is much smaller.

LeeH


(published on 10/22/2007)

Follow-up on this answer.