Reducing Observations

The observed micrometer settings are taken home where they are copied with a little more neatness into an observing book (Figure 21.4). The original

recordings are kept in case of a query or transcription error. It is at this point that the mean settings are calculated and the position angles and separations worked out.

The two observations of the calibrations are done first. This gives a mean value for the observed position angle at the beginning and end of the session. This usually agrees to better than 1 degree. The difference between the instrumental value and the value from the calibration list is the correction to be applied to all the other mean position angles. Similarly a mean screw value is obtained from the calibrations and applied to the remaining observations. The final touch is to convert the calendar date to a decimal of a year. This can be done via a lookup table which can be found in the Explanatory Supplement to the Astronomical Ephemeris or the program JD&Epoch in the "soft" folder on the accompanying CD-ROM can be used. High-resolution work such as speckle interferometry on rapid visual binaries demands using the date to four decimal places but for visual work with small telescopes, three places of decimals is more than adequate.

Chapter 22

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