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The Minor Planet Observer
Palmer Divide Observatory

2007 Shoemaker Grant Recipient

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MPO Canopus - The Period Spectrum

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Whenever you do a period search in MPO Canopus, the Period Spectrum form appears. The Plot tab of the form shows the range of periods in the search along the X-axis while the RMS error values of the Fourier analysis are displayed along the Y-axis. The lowest data point represents the period with the lowest RMS error and, in many cases, corresponds very closely to the actual period. Using this criteria and the screen shot above, the period for the given curve would seem to be a little less than 5.9h. However, note that there are many dips, though not as deep. These dips represent other possible solutions due to "aliasing."

In simple terms, an alias period is one that differs from the true period by an integral fraction, e.g., 1/2, 5/4, 22/19, and so on. In other terms, the data set is not complete enough to eliminate the possibility that you've found a period that represents, for example, 1.5 revolutions instead of 1.0. This often happens if a single run does not cover a complete cycle and the next run, also incomplete, is many cycles removed - a few days.

There is also the possibility that the selected range of periods in the initial search did not cover the true period. In this case, even if there is a minimum, it may not represent the true period. It's no wonder then that the cry of "more data!" is heard so often in lightcurve work. However, with some understanding of what the Period Spectrum tells you and practice, you can soon become proficient in period analysis and not be fooled too often.


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This page was last updated on 01/19/11 05:14 -0700.
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