Poisson Variance

Once the readnoise variance has been added and the bias has been optionally subtracted, the variance due to Poisson noise is added. For all instruments the Poisson variance is calculated as:

V_P = S * gain * N_e

where S is the signal in ADU per exposure, gain is the detector gain in electrons per ADU, and N_e is the number of exposures per integration.

At this stage the number of pixels that are background limited is displayed. This number is simply the percentage of pixels where the Poisson noise is greater than the readnoise.
[_ADD_POISSON_VARIANCE_]