How Does Telescope Size Affect Resolving Power?

The larger a telescope, the better its resolving power.
••• Comstock/Stockbyte/Getty Images

Telescopes enhance our ability to see distant objects in a number of ways. First, they can gather more light than our eyes. Second, with the help of an eyepiece, they can magnify an image. Lastly, they can help distinguish objects that are close together. This last enhancement is called a telescope's resolving power. In general, the resolving power of a telescope increases as the diameter of the telescope increases.

The Light-Gathering Apparatus

The resolving power of a telescope depends on the diameter of the telescope's light-gathering apparatus, or objective. In a refracting telescope, the objective lens is the first lens the light passes through. In a reflecting telescope, the objective is the telescope's primary mirror. In a Schmidt-Cassegrain telescope, the objective is also the primary mirror. As the diameter of the telescope's objective increases, the resolving power increases.

The Diffraction Limit

The degree to which objects can be resolved by a telescope is called the diffraction limit. The diffraction limit describes the smallest angular separation between two visible objects. The typical unit of this measurement is the arcsecond. The diffraction limit is inversely related to the diameter of the telescope's objective. Therefore, as the diameter increases, the diffraction limit decreases; you can resolve increasingly smaller objects with larger telescopes.

Wavelength and Resolving Power

The diffraction limit depends on the wavelength of light being collected. At higher wavelengths, the diffraction limit increases. In other words, these images will not be as clear as lower wavelength light sources for a given telescope diameter. For example, near infrared observations through a one meter telescope would have a diffraction limit of 2.5 arcseconds. Blue light observations through the same telescope, on the other hand, would have a diffraction limit of 0.1 arcseconds.

Other Limitations

The Earth's atmosphere presents an optical obstacle to even the largest terrestrial telescope. As light from the stars and planets passes through the atmosphere, it gets refracted. This causes a blurring of the objects image known as "seeing." In order to avoid the complications of seeing, large telescopes tend to be located on mountaintops or, as is the case with the Hubble Space Telescope, in space.

Related Articles

How to Calculate Angular Resolution
What Astronomical Instrument Measures the Brightness...
How to Calculate Focal Length of a Lens
How to Use a Bushnell Reflector Telescope
What Is Magnification on a Microscope?
How to Use a Refracting Telescope
How to Convert Magnification to Dioptors
How to Use the Bushnell Telescope 78-9512
How to Calculate Total Magnification of a Microscope...
Things to Do With Magnifying Glasses
What Do Stars Look Like?
How Do Reflecting Telescopes Work?
Concave Lens Uses
What Is Magnification Power?
What Is the Resolution of a Microscope?
How to Use Bushnell Telescopes
How to Convert Lux to Candela
How to Assemble a Bushnell 565 Telescope
How to Measure a Magnification Mirror