The resolving capacity indicates how small the angular distance between two objects may be whilst still being recognisable as two objects. The angular distance is measured in arc seconds("). The following rule applies: 1" = 1/3600°
Double stars are best suited as test objects, especially those where the two elements are almost equally bright. There are two criteria for the evaluation of the resolving capacity:
- The Rayleigh criterion is derived from diffraction theory: two objects can be observed as separate when the diffraction maximum of one object coincides with the diffraction minimum of the second. For a double star system, this corresponds to the image of an 8. From diffraction theory, for a wavelength of 550 nanometres (i.e. the limit of human visual sensitivity): resolving power in angular seconds = 138 / aperture in mm. A telescope with a 120mm aperture sees a double star system with a 1.15" separation between the two elements as an 8.
- The Dawes criterion is an empirical formula which is based on observations. According to this you can also identify a star as a double star if it looks oval, so more like a 0 instead of an 8. From experience the results of resolving capacity are as follows: resolving capacity in arc seconds = 117 / aperture in mm. A 120-millimetre telescope would depict a 0.9" double star as an oval object.
Technically, the resolving capacity is determined by the telescope’s aperture: the larger the aperture, the closer double stars or structures on planets may be, and still be recognised as such. In practice, air turbulence ("seeing") is the most important factor, which often limits the resolving capacity to one arc second. Added to this is the thermal behaviour of the telescope. It should be adjusted to the ambient temperature for observing.