For large telescopes located on Earth, the primary factor limiting their resolving power is air turbulence in the atmosphere.
The Impact of Earth's Atmosphere on Telescope Resolution
While a telescope's aperture (the size of its main mirror or lens) is fundamentally related to its theoretical resolving power (its ability to distinguish fine details), this theoretical limit is often not achieved in practice when observing from the ground.
Atmospheric Seeing
The Earth's atmosphere is constantly in motion. Pockets of air with slightly different temperatures and densities act like tiny lenses, refracting and distorting the incoming light from celestial objects. This phenomenon is known as "seeing," and it causes stars to twinkle and blurs images observed through telescopes.
- Blurring Effect: Atmospheric turbulence causes the light from a single point source (like a star) to spread out over a small area on the detector, rather than remaining a sharp point.
- Resolution Limit: As stated in the reference, this turbulence typically limits the resolving power of ground-based telescopes to around 1 arc second.
Why Large Apertures Don't Fully Help (on Earth)
The reference highlights a crucial point: because atmospheric turbulence imposes this 1 arc second limit, ground-based telescopes with apertures larger than approximately 120 mm do not bring any real gain in resolving power solely based on their size. While a larger aperture collects more light (making fainter objects visible), it doesn't overcome the blurring effect of the atmosphere. All the light collected is still being smeared by the same turbulent air above the telescope.
This means that building larger and larger telescopes on Earth primarily improves light-gathering ability for observing fainter objects, rather than dramatically increasing their ability to resolve finer details beyond the atmospheric limit.
Overcoming the Limit
Scientists employ various methods to mitigate the effects of atmospheric turbulence:
- Observing from Space: Telescopes in orbit, like the Hubble Space Telescope, operate above the Earth's atmosphere, allowing them to achieve their theoretical, diffraction-limited resolution based purely on their aperture.
- Adaptive Optics: Modern large ground-based telescopes use sophisticated systems called adaptive optics. These systems use deformable mirrors to rapidly correct for the atmospheric distortions in real-time, significantly improving resolution.
- Choosing Ideal Locations: Observatories are often built on high mountains in dry climates where the atmosphere is thinner and more stable, reducing the severity of turbulence.
In summary, while a telescope's design dictates its potential resolution, the practical limit for large ground-based telescopes is primarily set by the turbulent nature of the Earth's atmosphere, typically around 1 arc second.
