The circular micrometer (also ring micrometer ) is a micrometer , which consists of a thin and precisely circular steel ring that is attached to a glass plate and placed in the focal plane of a telescope .
By observing and recording the times in which two astronomical objects close to each other enter and exit this ring, one can determine their right ascension and declination differences . The telescope required for this does not require an equatorial setup , but must remain completely unchanged during the observation time. This simple micrometer is particularly suitable for determining the location of faint objects such as comets and nebulae . In an emergency, the glare limiting the field of view of the telescope can also be used as a circular micrometer.