Drift Method of Measurement

Once the CCD image has been taken and the normal post processing completed, such as, removing the dark

Figure 16.1. The author and his 8-inch Schmidt-Cassegrain

Figure 16.1. The author and his 8-inch Schmidt-Cassegrain

frame and dividing by the flat field, then the position angle and the separation of the pair can be determined. The simplest method to determine the position angle and separation is the drift method. This method consists of taking an image of a pair and during the exposure turning the telescope's clock drive off. This produces an image like the one in Figure 16.3. The lines produced by the drifting star images define the east-west line in the image. A protractor can be used to measure directly from the image the position angle.

Figure 16.2. The

SBIG ST-8E CCD.

Figure 16.2. The

SBIG ST-8E CCD.

Figure 16.3. A drift field image of WDS 02473 + 1717.

The measurement of the separation requires that the distance between two points on the image be calibrated in arcseconds per unit length. The calibration can be accomplished using the standard pairs from Table 15.1 in Chapter 15.

To illustrate this method, consider the quadruple system WDS 02473+1717 (Figure 16.3). The AB and CD pairs are too close for the CCD image to resolve, therefore, they are treated as single stars in this example. The WDS lists the AB pair at V magnitude 8.57 and the CD pair at magnitude 9.71. The AB-CD separation is 104'.'59 and the position angle is 135°0. The image in Figure 16.3 is a 30 second exposure taken with the author's 8-inch Schmidt-Cassegrain telescope with a ST-8 CCD camera.

The simplest approach to measuring the pair consists of first enlarging the image as much as practical and printing it. With a straight edge, draw a line joining the AB and CD points. Use a protractor and measure the angle between the east-west line (formed by the drift of the star's images) and the AB-CD line. This turns out to be 45°. Since the east-west line is perpendicular to the north-south line, 90° can be added and the result is 135° - amazingly good agreement with the WDS! For widely separated pairs this method works quite well. Closely spaced pairs and pairs where the stars nearly align with the east-west line cause problems for this method.

Once the position angle has been measured, the next step is the separation. Measurement of the separation requires that the field be calibrated in arcseconds per

Figure 16.3. A drift field image of WDS 02473 + 1717.

pixel. This can be accomplished using the standard pairs from Table 15.1 or by using the method covered in the next section of this chapter. The separation is determined by counting the number of pixels between the two stars. This requires the x andy coordinates of the two stars be known. In this case, the position of the AB pair is at (521, 547) and the CD pair is at (582, 518). The distance between the pair is found using the following formula:

The resulting separation is 67.5 pixels. The telescope/CCD combination has 1.52'' per pixel. This gives a separation of 102.7''. This is within 2% of the WDS value of 104.59''.

The drift method of measurement is the probably the simplest way to measure double stars. However, this simplicity has a price, that is, the accuracy of the measurements is not as good as the CCD camera is capable of delivering. To access the full potential of the CCD camera image the method in the next section is required.

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