Introduction

The measurement of double stars is central to the theme of this book and there are many ways of doing this, but this chapter is dedicated to the use of the filar micrometer which has been used seriously since the time of William Herschel. (For a thorough discussion of the history and development of the filar micrometer see the paper by Brooks.1) Much of our knowledge of longer period visual binaries depends on micrometric measures over the last 200 years. The filar micrometer is by far the most well-known device for measuring double stars. Its design remains largely the same as the original instrument which was first applied to an astronomical telescope by the Englishman William Gascoigne (c.1620-1644) in the late 1630s. The aim is to use fine threads located in the focal plane of the telescope lens or mirror to measure the relative position of the fainter component of a double star with respect to the brighter, regarding the latter as fixed for this purpose. This is done by the measurement of the angle which the line joining the two stars makes with the N reference in the eyepiece and the angular separation of the fainter star (B) from the brighter (A) in seconds of arc. These quantities are usually known as theta (6) and rho (p) respectively and are defined in Chapter 1.

The basic filar micrometer consists of two parallel wires, one fixed, one driven by a micrometer arrangement, with a third fixed wire at right angles to these two (Figure 15.1). The movable wire must be displaced

Fixed PA wire

n

n

Separation of wires

Fixed Moveable

Separation of wires

Figure 15.1. The arrangement of wires in a modern filar micrometer.

in the focal plane just far enough from the other two such that it can move freely and yet be in focus. It must also, of necessity, be very thin, preferably smaller than the apparent size of the star disks through the eyepiece. If the focal length of the telescope is too short then a Barlow lens is necessary. This has the advantage of boosting the focal length by two or three times and yet has no effect on the apparent size of the thread. (see Fig. 15.2)

The usual material for the wire is spider thread which was chosen for its fineness and relative ease of availability. (In fact it was a spider making its web in one of his telescopes that gave Gascoigne the idea for the filar.) Replacing spider thread in a micrometer is a relatively skilled job and these days commercially available micrometers use tungsten with a thickness of about 12 microns. The micrometer used by the author has been in regular use for 10 years and the wires have remained correctly set throughout, even though the micrometer has been fitted and removed from the telescope hundreds of times and many thousands of individual settings of the wires made.

In the modern Schmidt-Cassegrain the Barlow lens is a particularly useful accessory. For a 20-cm f/10, for instance, the focal length of 2000 mm is equivalent to a linear scale at the focal plane of 103'' per mm. This means that a 12 micron wire will subtend a diameter of about 1.25''. This is about twice the angular resolution of the telescope so it would limit the user to measuring pairs wider than about 3.0''. Even then the thickness of the threads would make accurate centring of star images difficult.

Figure 15.2. A RETEL micrometer fitted to the 8-inch refractor at Cambridge. The Barlow lens assembly is the brass tube immediately above and the power supply for the field illumination is attached to the tube within reach of the eyepiece. Comfortable observing positions such as this are rare. The chair collapsed entirely soon after this picture was taken!

The body of the micrometer must be able to rotate through 360° and its angular position is accurately measured by a circular gauge known as the position angle circle. This is usually graduated in degrees with a vernier available to read to 0.1°.

In the classical brass micrometer, another arrangement called the box screw is usually included. This allows both the fixed and movable parallel wires to be shifted in the focal plane by the same amount. This is useful when the double distance method of measuring separation is employed (described more fully later). For micrometers without this facility (and this tends to include the modern instruments that have become available over the last few years) it is necessary to move the whole telescope to bring the threads into position for double-distance measurement. Alternatively, the method described by Michael Greaney2 obviates the need to move the whole telescope

After setting the movable wire on the companion and noting the reading, the micrometer is rotated

around 180° so that the PA wire bisects the two stars again. The micrometer screw is then turned to move the movable wire across the primary back to the companion. The new reading is then noted and the difference between the two readings gives a measure of the double distance.

As the PA wire bisects the two stars a second PA reading can be taken. Add 180° to this second PA reading if it is less than 180°, or subtract 180° if it is more. The mean of the first and (corrected) second PA readings can be taken as the PA reading for that particular measurement.

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