Separation

The most common technique for the measurement of distance is called the "double-distance" method (see Figure 15.3). Basically the fixed wire of the micrometer is placed on the primary star and the movable wire on the companion. The reading of the movable wire is noted. The telescope and micrometer screw are then moved until the fixed wire is placed on the companion and the movable wire placed on the primary star. The difference between the two positions of the screw is twice the separation of the pair in millimetres (or whatever unit the screw is calibrated in). This is repeated several times, depending on the difficulty of the pair. The separation of the pair in arcseconds is then calculated by k(r2 - r ! )/2 where k is the screw constant and rj and r2 are the mean values of each separation setting. I make four double distance measures for wide pairs and up to six measures for close pairs. This procedure, like that of the determination of position angle, is repeated for several nights before a mean value is determined for each. It is better to make the measures of separation close to the position

Figure 15.3. Double-distance method of determining separation.

angle wire, since if the separation wires are not strictly parallel then the measure of separation will be in error and in any case the images will be better near the centre of the field.

An alternative method by Michael Greaney2 is illustrated in Figure 15.4. The CD-ROM contains Delphi 5 programs for calibrating and using filar micrometers.

Figure 15.3. Double-distance method of determining separation.

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