Eclipsing binaries and comments on Imaging Exoplanets


If you are thinking about jumping in and trying you hand at exoplanets the first thing you need to try are eclipsing binaries. Just like exoplanets, EBs vary in magnitude because a fainter companion is orbiting the primary star in such a way that the transit is in line with Earth, causing magnitude to vary.

The primary difference is, you guessed it, the signal-to-noise (S/N) ratio needed to successfully obtain a light curve.  For EBs, S/N can be quite modest, in range of 0.01 magnitude error. This is because the magnitude variation of many EBs can be as much as 1 magnitude or more. And, you are facing only the usual S/N variation.

With exoplanets the variation in magnitude is quite small, on the order of less than a 0.03 magnitude drop or less. This puts a premium on S/N and the average successful exoplanet observer is look at S/N rations on the order of 0.002-0.005 magnitude (2 to 5 mmag). Further, many exoplanets are bright, which means that you have to worry more about phenomena like  scintillation noise in addition to the usual sources of noise (including noise caused by measuring aperture size).  However, just like EBs, exoplanet work has one saving grace; it demands precision, not accuracy.

In other words, it is not the actual magnitude of the primary that is of prime importance, it is generating an accurate timing of the minimum and the shape of the light curve that are of primary importance.

So, you say: “Ed, how do you image and measure exoplanets.” I answer: “I don’t!” It is just not something I have done.  But if you want to give it a try, I can suggest a book about exoplanet observing  that has been invaluable to me in understanding sources of error that have proven valuable for regular photometry.

“Exoplanet Observing for Amateurs” is Bruce L. Gary’s outstanding book for the amateur contemplating exoplanet research. Gary takes a very common sense approach to guiding us through the demands of this most demanding field of amateur photometry. Even if you, like me, are not particularly interested in exoplanets, you will benefit greatly from Gary’s discussions of calibration, setting differential photometric  apertures, sources of noise, calibration and a host of other topics.

Understanding the demands of this most rigorous program has helped me in my less demanding  pursuits of eclipsing binaries and other variables. Better yet, the book is available as a PDF for free and as a hard copy (which I recommend) for a very modest price and also visit Bruce’s web site.

So, you are still interested in exoplanets and are not put off by the rigor of the program. Begin with some easy eclipsing binaries using the AAVSO EB program as a guide. Start with some fairly easy ones that vary in their light curves and work up to the really “hard” ones that vary only slightly. Finding the target and times are easy, a day-by-day ephemeris is available for the program EBs through the AAVSO program and instructions for using the ephemeris are available at the RAS Wiki on the EB page along with several links I hope you  will find useful.  And, make sure to see if any of our GRAS observers are imaging exoplanets and ask them about their observing program.

Dr Ed Wiley