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MPO Canopus - Time of Minimum Calculator

Variable star observers in particular use the time of minimum (usually the primary minimum) as a reference point. By comparing the measured time of minimum for a given set of data to the predicted time based on a known period and initial time of minimum, you can determine the accuracy of the stated period and if the period might be getting longer or shorter. Asteroid observations can use this concept to advantage to determine when a given feature in the lightcurve might reoccur or when to work the asteroid to catch a missing part of the curve.

Canopus uses the Hertzsprung method as detailed by Henden and Kaitchuck in their book to find the time of minimum and generate an ephemeris that shows the times of minimum past and future.

Use the period search routines in Canopus to find the period of the data before beginning unless you already have an assumed period. In which case, you still need to plot the observation data. The initial calculations to find the time of minimum are based on days, regardless of the configuration settings. The TOM Calculator can work in either hours or days once you’ve found the TOM.

TOMSearchParameters.gif (5495 bytes)

After generating a period, you start the TOM calculator by first telling Canopus the initial guess for time of minimum and the other search parameters. The initial guess is automatically entered as the JD of the data point with the faintest magnitude. Once the program finds the TOM, it displays the TOM Calculator.

TOMCalculator.gif (13926 bytes)

Here you can generate an ephemeris for past and future times of minimum for planning future observations or comparing the derived values against actual observations.

When working with the TOM calculator, you must use data from one and only one session. If you include data from more than one session, the results are unpredictable. If you have multiple sessions covering the same time of minimum, e.g., data from different observers, you can then compute an improved average value and error using a statistical calculator or spreadsheet.


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This page was last updated on 01/19/11 05:14 -0700.
All contents copyright (c) 2005-2011, Brian D. Warner
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