Observing

To make an observation the micrometer is fitted to an equatorially mounted, correctly aligned, and carefully collimated telescope. The 50 mm or the 25 mm grating frame is mounted, depending on the expected separation of the pair to be measured and as high a magnification as possible should be used, preferably 400x or more. The first step is to align the two star components and their satellite images exactly by rotating the whole micrometer (Figure 14.5c). This is called the alignment method and it gives position angles with great precision. Only when the stars and satellites appear properly aligned in a straight line is the position angle read on the 360° dial. At this point it should be noted in which quadrant the fainter star lies in case a correction of 180° needs to be made to the measured position angle. Then the micrometer is rotated exactly 90° further and a configuration as shown in Figure 14.5d will be seen. Now it is time to start tilting the grating frame. This is an easy procedure because when observing with a short 20-cm Schmidt-Cassegrain telescope the grating frame can still be directly reached and operated from the eyepiece end. Great care and judgement is necessary to determine the frame's inclination which produces the correct star configuration. There are two alternative patterns: perfect squares or perfectly right-angled crosses as shown in Figure 14.5e. The idea behind this is, of course, to set the angular distance of the satellite images exactly equal to the double star separation. The mode of operation quickly becomes second nature with the observer and, of course, the larger the series of settings and readings the more reliable the result. In order to compensate for instrument inaccuracies and to increase the precision further, the frame should be swung to both sides and readings on either side on the cosine scale should be made. Furthermore as the satellite images appear on either sides of the stars, two squares or crosses are shown, hence both of them should be judged. As a final verification, the angles A'BA' as well as B'AB' can be checked for perfect orthogonality. Incidentally if a diagonal prism is used the "cross" pattern can be arranged vertically or horizontally for better judgement simply by turning the diagonal. Experience shows that judgement seems to tire quickly so decisions have to be made quickly and alternate glances with either

Figure 14.5. Star and satellite patterns as seen in the eyepiece.

eye yield a clearer result instead of staring for too long at the patterns. Only when perfect accord is obtained is the tilt angle cosine read directly from the scale. To get the final value for p the grating's nominal spacing, i.e. 50 or 25 mm, is multiplied by this cosine. Now the diffraction formula can be used to calculate the double star separation, p.

It is not necessary to use the "cross" configuration in Figure 14.5E. By swinging the frame, the aligned stars and satellites as shown in Figure 14.5C could for instance be brought directly to exactly equal distances A'-B'-A-B-A'-B' which makes the next step, the instrument's 90° position angle turn, superfluous. Depending on the chosen, lined up star and satellite arrangement, the cosine reading will then need a correction before using the value in the formula. In the described example it has obviously to be multiplied by two. Other alignments - with corresponding correction factors - are possible and thereby the range of the micrometer could be extended considerably. Occasionally, when crowded stars and satellites are lined up in this way, it is perhaps not easy to distinguish stars and satellites. Hence the "cross" configuration as described earlier and shown in Figure 14.5e is preferred, as it works without this added difficulty.

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