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S/N calculation

The "limiting magnitude" of an IR instrument, can be predicted when the observations are taken in the background-limited case (i.e. the readout noise is negligible compared to the background shot noise).

From the zeropoints and the background brightness in J, H, K and Ks, as measured during the INGRID's new foreoptics commissionig, we have calculated the expected limiting magnitude as a function of the required signal to noise ratio and of the exposure time for both point and extended sources, and as a function of the seeing for point sources only. Keep in mind that the background brightness are highly variable, so that this plots can vary from night to night. See the notes below.


Also you can use SIGNAL to calculate the number of object and sky photons which will be detected during an INGRID exposure of a point or extended source.

Point sources

limiting magnitude v. seeing

limiting magnitude v. exposure time

limiting magnitude v. signal to noise ratio

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Extended sources

limiting magnitude v. exposure time

limiting magnitude v. signal to noise ratio

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Last Updated: 4 May 2001
Almudena Zurita azurita@ing.iac.es