Can Telescopes Show Recognizable Reflections at Long Distances?

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Discussion Overview

The discussion centers around the capabilities of telescopes to show recognizable reflections from small objects at long distances, specifically addressing the range limits for reflection using high-powered scopes. It explores theoretical and practical aspects of telescope resolution and atmospheric effects.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether telescopes can show recognizable reflections off small objects at distances of about a mile, seeking to understand the range limits for such reflections.
  • Another participant introduces the Rayleigh Criterion, explaining that diffraction limits resolution based on the aperture size and the wavelength of light, stating that a good camera cannot resolve two objects close together at significant distances.
  • A later reply adds that atmospheric conditions, such as temperature distribution and turbulence, further complicate long-range visibility.
  • Another participant emphasizes that atmospheric factors like air temperature and haze can significantly affect the clarity of long-range observations.
  • One participant notes that telescopes are limited by the need to observe objects that reflect light, pointing out that most of the universe's mass is not visible.

Areas of Agreement / Disagreement

Participants express agreement on the limitations imposed by atmospheric conditions and the Rayleigh Criterion, but the overall question of whether recognizable reflections can be seen at long distances remains unresolved, with multiple factors contributing to the discussion.

Contextual Notes

Limitations include assumptions about atmospheric conditions being ideal, the dependence on the aperture size, and the unresolved nature of how these factors interact with the visibility of distant objects.

Newai
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Looking over CSI's "zoom and enhance" silliness, are there telescopes that could show a recognizable reflection off a small object, even a good mirror, at a mile or so? What range limits are there for reflection using high-powered scopes?
 
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There are a few limiting factors when it comes to the lenses involved, but there is an absolute limit on resolution for a single aperture.

It's called the Rayleigh Criterion: basically, due to the diffraction of light as it passes through the opening of a camera or telescope, you cannot resolve two objects (that is, you can't tell there are two and not one single object) if the ratio of their separation over the distance to the objects is less than the ratio of the wavelength of light over the size of the opening.

That ratio s/d (object separation over distance to the object) also equals the sine of their angular separation, so the Rayleigh criterion is usually written as
sin theta = 1.22 x lambda/ D

The 1.22 is a factor thrown in for circular openings.

So a good SLR type camera, with about a 2 cm aperture with f-stop open wide, cannot possibly resolve two dots half a mile away unless they are more than 2 cm apart, and that's with a perfect lens.
 
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Thank you.
 
...also assuming the atmosphere is perfectly still and of even temperature distribution, which it isn't.
 
Russ nailed it above. Air temp and haze (moisture/dust/pollution) often mess with any long range sight picture.
 
Last edited:
The biggest limitations of telescopes is that
1. You're looking through the atmosphere
and
2. You're only being up objects that reflect light, where the large majority of mass in the universe is not visible.
 

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