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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?

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    $\begingroup$ You are right, there seem to be few precise definitions and many uses of the term. The most precise I can find are similar to "period" and "sampling interval/period" in experimental physics in general, but it seems to me that some astronomers also speak of "cadence" to describe complex instrument protocols (e.g. including calibration cycles, the linear or non-linear motion of the individual exposure area over the actual field of interest and such). It would be nice to have a practitioner chime in with the exact definition and a cursory overview of the "street talk". $\endgroup$
    – CuriousOne
    May 12, 2015 at 22:47
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    $\begingroup$ Well...could any professional astronomy please write down a precise definition on the arXiv which I could cite? :) $\endgroup$ May 15, 2015 at 0:56

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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.

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  • $\begingroup$ Thanks! That makes a lot of sense when I compare it to the ways I have seen it used. $\endgroup$
    – CuriousOne
    May 15, 2015 at 1:31
  • $\begingroup$ Hi @Rob Jeffries. May "cadence" means the same as "exposure time"? $\endgroup$
    – Stefano
    Sep 27, 2019 at 15:12
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    $\begingroup$ @Stefano it is never used as a synonym for exposure time - it is the interval between exposures. $\endgroup$
    – ProfRob
    Sep 27, 2019 at 15:39
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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!

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