In measuring each pair the position angle is always done first and although formally the wire should be reset at the end of this procedure to the mean value in practice this is not done since the individual values tend to agree closely enough for this purpose. Two to four settings are made and the individual angles remembered before writing them down. For wide pairs these will usually agree to within one degree and it is then only necessary to remember the decimal part. It is recommended that the quadrant in which the fainter star lies is noted. With equatorial telescopes the approximate directions of the cardinal points are usually fairly obvious so it is a simple matter to record whether the companion star is in the first quadrant (i.e. with a PA between 0 and 90°) or another quadrant. This is because the recorded PA from the micrometer is ambiguous by 180° depending on where the micrometer barrel is pointing. I happen to be right-handed so the micrometer barrel is usually in the first or second quadrant.

For separation, the technique used depends on the distance between the stars. For close pairs (< 15'') the double distance method is used and the two values of the screw are written down at the end of the procedure. For wider pairs it is too time-consuming to do this so four settings are made with the movable wire on one side of the fixed wire then another four settings made with the movable wire on the opposite of the fixed wire. This requires the use of the telescope slow motions and this is where a box screw would be useful. On the older brass micrometers this was an arrangement which allowed all the wires to be moved across the field of view whilst retaining their absolute position with respect to one another. With the RETEL micrometer the separation readings are in mm on the micrometer screw but each revolution of the screw is graduated in 50 divisions so care must be taken to note whether the reading is between x and x + 0.5 mm

Figure 21.4. An extract from the author's observing book. The two central columns record the settings of the movable wire in millimetres corresponding to the double distance method. The right hand column gives the observed position angle on the micrometer barrel. This is converted to the true PA and separation by using the reference pair 8 Boo. The final observed PA and separation are given along with the epoch of observation in decimals of a year. Note the correction to the mean PA of STF 1932. It is easy to misread the micrometer dials in the dome!

or x + 0.5 and x + 1.0 mm where x is the reading in whole millimetres on the barrel. In most cases, however, the error will stand out easily and be corrected when reducing the data.

As mentioned above for the Thorrowgood it is necessary to remove the micrometer and Barlow assembly at the end of each session and so one of the first pairs to be measured is a calibration pair. A list of bright pairs with separations from 14 to 100'' around the sky is used (and is given in Chapter 15). The relative position angles and separations are known to about 0° 1 and about 0.05'' - sufficiently small to be negligible compared to measurement or personal errors. The same pair, if possible is also measured at the end of the night. If it is possible to leave a micrometer in place on the telescope then this is the best option - even so, the zero of position angle should be checked at least once per night.

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