Telescope Limitations: What to Know

In summary, a telescope's resolution is limited by the length, width, and height of the telescope as well as the wavelength of light being observed.
  • #1
Ryan_m_b
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Hi there,

I was reading a novel recently where a character was using telescopes to watch vehicles moving on a planet at many light hours distant. The 'explanation' for his telescopes awesomeness is that it is designed to see distant stars and so seeing people moving on a planet in the same system is easy. However this does not seem right to me.

My knowledge of telescopes and astronomy is limited but I'm interested to know what the limitations are on a telescopes resolution, i.e. is it the length, width etc. I'm also fairly sure I've read somewhere before that whilst the Hubble telescope can see distant nebulae it would not be able to see the flag on the moon (which is what flagged up the passage in the aforementioned novel). Ideally I'd like to know how to answer questions such as "what dimensions would a satellite have to be in a 200km orbit to read size 10 font from my hand"

Thanks!
 
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  • #2
Try reading this: http://en.wikipedia.org/wiki/Optical_telescope#Angular_resolution . It basically tells you the resolving power of a telescope. For your example, 10 point font is about 2mm, so to read this at a distance of 200 km requires resolving an angle of (2E-3/2E5) = 1E-8 radians. So 1E-8 = lambda/D (ignoring the 1.22 since we're just making a rough estimate). Since the wavelength of visible light is about 5E-7 m, we need a telescope diameter of D = (5E-7/1E-8) = 50 m. Similarly, to see a flag (1 m) on the moon (400,000 km away), requires D = (5E-7 m * 4E8 m / 1m) = 200m, about 100 times bigger than the Hubble telescope. The reason that the Hubble telescope can see distant nebula that are much further away is that they are much, much bigger than a flag.
 
  • #3
Brilliant, thank you :)
 
  • #4
Oh, a potentially unintelligent question but does the brightness of the object make a difference?
 
  • #5
Not really. It is pretty easy to get around a brightness problem by increasing the length of the exposure.
 
  • #6
That makes sense. If I'd have thought about it for a little while I probably could have figured that out from all the microscopy I've done over the years. Cheers!
 
  • #7
russ_watters said:
Not really. It is pretty easy to get around a brightness problem by increasing the length of the exposure.

That is assuming that astrophotography is your choice of observing.
If visual observing is your flavor, then apparent brightness is critical.

Test,
 
  • #8
test4 said:
That is assuming that astrophotography is your choice of observing.
If visual observing is your flavor, then apparent brightness is critical.

Test,

Of course.
 
  • #9
ryan_m_b said:
Hi there,

I was reading a novel recently where a character was using telescopes to watch vehicles moving on a planet at many light hours distant. The 'explanation' for his telescopes awesomeness is that it is designed to see distant stars and so seeing people moving on a planet in the same system is easy. However this does not seem right to me.

My knowledge of telescopes and astronomy is limited but I'm interested to know what the limitations are on a telescopes resolution, i.e. is it the length, width etc. I'm also fairly sure I've read somewhere before that whilst the Hubble telescope can see distant nebulae it would not be able to see the flag on the moon (which is what flagged up the passage in the aforementioned novel). Ideally I'd like to know how to answer questions such as "what dimensions would a satellite have to be in a 200km orbit to read size 10 font from my hand"

Thanks!

In terms of your OP the observer is not using a "Hubble Like" scope.
Hubble uses photography for all observations. In many wavelengths and varying times.

You don't view in "real time" objects through Hubble.
Unless the object is close (such as the Jupiter comet collision)...
You take pictures of long past events.

Even if you could build a big enough scope to resolve detail on a "busy" planet, you would still be looking into the past of said planet regardless of weather or not you used a camera or your eyeball.

So, in reality, you could never watch in real time the events unfolding on a distant planet.

The farther away the planet, the longer in its past you would be viewing.

All telescopes (and all eyeballs...lol) no matter how sophisticated have to "wait" for the photons etc...to reach the objective lens/CCD (or retina) camera. The speed of light and the matter/energy in between rules what you see at any particular time.

Its so cool.

We will be forever looking into the past. Test,
 
Last edited:

What is the maximum distance a telescope can see?

The maximum distance a telescope can see is determined by several factors, including the size and quality of the telescope's optics, atmospheric conditions, and light pollution. The most powerful telescopes on Earth can see objects up to 13 billion light years away, while space-based telescopes have a much greater range.

Can a telescope see all the way to the edge of the universe?

No, a telescope cannot see all the way to the edge of the universe. The universe is constantly expanding, and the light from the most distant objects has not had enough time to reach us yet. Additionally, the observable universe is limited by the speed of light and the age of the universe.

What types of objects can a telescope not see?

A telescope is limited in its ability to see certain types of objects, such as black holes and dark matter. These objects do not emit or reflect light, so they cannot be observed with traditional telescopes. Instead, scientists use indirect methods, such as studying the effects of these objects on their surroundings, to gather information about them.

How does the size of a telescope affect its limitations?

The size of a telescope's aperture, or the diameter of its primary lens or mirror, plays a significant role in its limitations. A larger aperture allows for more light to be collected, resulting in a clearer and brighter image. This allows for the observation of fainter and more distant objects, as well as finer details in closer objects.

What are the main limitations of ground-based telescopes?

Ground-based telescopes are limited by the Earth's atmosphere, which can distort and blur images. This is known as atmospheric turbulence or "seeing." Additionally, light pollution from cities and other sources can hinder the quality of observations. Space-based telescopes, which are not affected by these limitations, are often used to overcome these challenges.

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