In this final section, we will retrieve the rotational period of 10 Hygiea and see if it is in agreement with the publised value of 13.8224 +/- 0.0005 h.

  1. We already know from Tutorial 7 that the FALC method is particularly effective in determining rotational periods of minor planets. Let's use that method to retrieve the rotational period of 10 Hygiea. We also know from Tutorial 7 that the proper alignment of Observation Sets (ObsSets) is critical to finding the right period, since the FALC method can find a different period for different ObsSet alignments. It implies that we have to align ObsSets so that they mesh well together, before we start the period analysis. 

    We will first use a crude approximation for the alignment by simply subtracting for each Observation set the average brightness from all observations in that Observation set. This is explained in Tutorial 3. Execute the Subtract Avg Brightness command on the ObsWin.

    Further on in this section, we will work with an Observation window in which the Observation sets have been well meshed.

  2. Press the Period Determination button in the ObsWin toolbar and select the FALC method. Enter the values shown in the below dialog box.

    Click Apply to start the FALC calculation.  

  3. This creates the below Period Window showing a dominant period at 13.8200 +/- 0.0067 hours. This value is - despite the crude meshing we did of the Observation sets - in very good agreement with the published value of 13.8224 +/- 0.0005 h. 

  4. If you click the PhaseWin at Frequency Cursor value button in the toolbar of the above Period window, you will see the Phase window corresponding to the dominant period. That Phase window immediately illustrates that our Observation sets have not been well aligned. 

    You can now use that Phase window to adjust the brightness of each individual Observation set in relation to an Observation set that you use as reference. It's a time consuming step, but if well done, you will arrive at a clean Phase window, identical to the one published in Figure 2 of the Ferrais paper, mentioned in the introduction of this Tutorial.

  5. You will actually find an Observation window with well aligned Observation sets in the Tutorial folder, which we mentioned at the beginning of previous section. In that folder is a Peranso file called 10 Hygiea meshed. Opening that file will produce the below Phase window.