The Effect of Magnification

The term "resolving power" is rather misleading as it implies that the amount of resolution depends on the magnification, which it does not. A more accurate term might be the limit of resolution or the angle of resolution. If the two images appear separate in the eyepiece then an increase of magnification should separate the images still further, assuming that the atmosphere will allow higher magnification.

The resolution of the human eye depends on the diameter of the pupil, which can vary from 1.5 mm to 8 mm depending on the individual and conditions of illumination. For double stars it is generally accepted7, 8 that the limiting resolution is about 2-2.5', lower than might be expected from the pupil diameter, but when the eye is fully dark-adapted, the image definition is impaired by inherent aberrations in the eye.

In terms of measuring close pairs, Couteau9 defines a resolving magnification which makes the radius of the first dark ring equal to the visual limit for the average eye. This magnification is numerically equal to the diameter of the objective in mm, i.e. mr = 200 for a 20-cm telescope. Couteau considers that the minimum useful magnification for double stars is 2mr, or x400 for a 20-cm telescope.

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