Resolution Of A Telescope

A question that gets asked frequently is "what is meant by the resolution of a telescope".

Basically when anyone refers to how clearly a telescope can see they are referring to its resolution or resolving power.

In other words how much fine detail can be seen or how closely two celestial objects can be and still be observed as two separate objects.

The resolving power of a telescope is measured in degrees which are further sub-divided into arcminutes and arcseconds.

1 degree = 60 arcminutes = 3600 arcseconds

The most effective telescopes used by amateur astronomers can hardly ever see detail that is less than 1 arcsecond wide or make a distinction between 2 stars that are less than 1 arcsecond apart. The reason being that turbulence in the Earth's atmosphere causes images to ripple and shimmer.

Whenever we observe the moon or planets through a telescope, we are looking at an extended object. As we increase magnification under good conditions, we see far more detail. In contrast, whenever we focus on a star, we're looking at a point source and regardless of how much we actually increase the magnification, it is so distant that will only get a point of light.

The fact is that due to diffraction, we don't even see a point of light, all we see is a circle of light referred to as an Airy disk. The arc-second diameter of this disk decreases as the aperture of the telescope increases.

The Airy disk is encompassed by progressively faint concentric rings of light and the whole grouping is known as a diffraction image.

One method that is frequently employed to calculate the resolution, is to split two stars that are close together. The ability to separate two stars is in fact the capacity to separate their Airy disks.

There is a formula that you can use to determine the distance apart that two equal brightness stars must be in order to separate them.

Known as Dawes' limit after its discoverer W.R Dawes it expresses the resolving power of telescope or microscope.

Resolution Of A Telescope Formula

  • R = 4.56/D D in inches, R in arcseconds
  • R = 11.6/D D in centimeters, R in arcseconds

where D is the diameter of the aperture and R is the resolving power of the telescope.

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