Note on automatic rejection of low signal-to-noise data

Note on Automatic Rejection of Low Signal-to-noise Data

The AAVSO has a policy in place that states positive observations having errors equal to or greater than 1.0 magnitudes are not accepted as is. For a given measurement, an error of 1.0 magnitude is equivalent to a signal-to-noise ratio of 1, meaning that the purported signal is indistinguishable from the average noise fluctuation in the data and statistically is not a significant detection. In practice, your software might report a slight increase in flux from within your measurement aperture, and your mind is always tricked into thinking that this is real rather than a chance coincidence since you know that the star should be there.

While the AAVSO values all observations by our observers, sound science is paramount, and from a rigorously statistical standpoint an observation with a signal to noise of 1 is not valid. In such cases, we strongly encourage you to find the faintest comparison star within the field having a photometric error less than 0.3 magnitudes, and report the observation as "fainter than (comparison star)". A fainter-than measure is a perfectly valid observation, and in this case is actually much more useful, because it puts a firm upper limit on the brightness of the variable at that time.

If you have questions or concerns about this policy, please contact AAVSO headquarters via email at

Clear skies,

Arne Henden
Director, AAVSO