
MPO Canopus  The Period Spectrum
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 Xaxis while the
RMS error values of the Fourier analysis are displayed along the Yaxis. 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. 