What is the precise definition of “cadence” in astronomy? I'm finding it difficult to find a precise definition of "cadence" in astronomy. This term is commonly used to describe the data of astronomical surveys. For instance, one of the data products for the Kepler mission is "30-min or 1-min cadence images around targets".
Does anyone have a precise definition and a source which I could quote?
 A: I don't think there is an exact definition. The term is used as a means of referring to an array of observation times of some astronomical phenomenon.
In common usage, the terms "long cadence", or less often "low cadence", means that there is generally a longer time interval between observations. On the other hand, a "short cadence" or "high cadence" means a shorter time interval between observations (note the possible confusion here, but I can't be held responsible for common usage!).
Cadence is distinct from sampling rate or duty cycle, since the cadence can be irregular or non-periodic, either by design or circumstance. To me, if the observations are periodic, then it makes more sense to talk about a sampling interval or sampling frequency. Indeed, in the example you quote, the Kepler cadence is regular, so that you get exposures read out every 30 minutes or 1 minute respectively. The exposure times would be these intervals minus any deadtime or readout time for the instrument.
A: I understand it as the time between each image taken by the instrument. On one of the instrumentation papers on NASA's STEREO spacecraft carrying multiple imagers have written it as number of images to accumulate every 1 hour cadence times. And I can also see in their movies that is a collection of stacked images, the images have been taken every 1 hour!
