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eclipsing binary section

Determining the Time of Minimum

This is one step that is unique to observing eclipsing binaries.  Once you're taken images of the star field through an eclipse, and produced a time series of V-C magnitudes (and C-K to ensure quality), you must now determine the mid-time of the eclipse.  If your data is of good quality, and the star is not badly spotted, the time series will be symmetric around the midpoint of the eclipse.  Most algorithms for determining the ToM assume this is so.  They numerically "fold" the time series around a hypothesized midpoint, and compute how well the ingress and egress legs match.  The algorithm


Photometry for eclipsing binaries is not different from other stellar photometry.  In fact, as mentioned, most reported ToMs are from untransformed differential data, which simplifies the process.  However, because you might have hundreds of images from one night, all of the same star field, you will appreciate having an automated procedure that does not require picking stars with the computer's mouse on more than a few "starter" images.

How to Observe EBs

This page will assume you are an amateur astronomer with some experience taking and processing CCD images.  The procedures for doing this are described in the AAVSO CCD Observing Manual, except that it is not essential that you be able to perform transformations.  Also, see our Bibliography, and Software Resources pages.


In a Nutshell

EB observing generally involves the following:

In a Nutshell

People, Contacts

The AAVSO has also set up forums, accessible via the main website, to discuss everything from governance to photometry to different types of variable stars.  There is a forum for Eclipsing Binary Stars -- please consider using that as a source for information and place for discussion.

Other EB Sites

  • GCVS query page, for information about a particular variable star.  The home page for the site has links to where you can download the entire GCVS catalogue.  This is useful if you want to filter the database so identify observing targets.

Times-of-Minimum Databases

The AAVSO does not maintain a current database of times of minima, because there are several other groups already doing so.  Here are links to the most extensive of those resources:


Analysis (O-C)


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