Measuring Celestial Dimensions with Micrometers

摘要

To find the linear size of celestial objects with a telescope one must use a measuring device such as a micrometer. Micrometers are usually made with adjustable webs, needlepoints, or an eyepiece reticle with graduated lines ruled in the glass. The webs, needlepoints, or reticle lines are positioned at the focal plane, in focus with the image, and magnified. An image can be aligned between the micrometer webs, points, or the ruled lines of the reticle and the separation noted on a dial or scribed on the reticle in either fractions of an inch or millimeters (See Figures 1 - 3). To translate this separation to some usable angle or linear dimension the image scale of the telescope must be calculated. Image scale is usually expressed in degrees, minutes, or seconds of arc per inch or millimeter. To find the image scale in seconds of arc (arcsec) per millimeter, divide 206,265 (seconds of arc in 360 degrees) by the focal length (F.L.).