The basic formula that governs the sensitivity calculations is the definition of the signal-to-noise ratio, S/N:
where P is the power illuminated on the detector and depends on the
measurement mode and the flux of the source.
is the noise equivalent power determined as the total of
all noise contributions along the signal chain. Noise sources are:
t is the integration time. The factor of 2 results from sampling theory;
detector signals are derived from the slope of the integration ramps using
several non-destructive read-outs (see section 2.6 of the IOM3.1).
The values of the power P and depend on the
observing mode such as photometry with PHT-P and PHT-C, spectrophotometry,
maps or polarimetry.
From eqn. 1 the integration time becomes:
Note that eqns. 1 and 2 do not account for detector drifts. Therefore these equations should be considered as an approximate estimate of the detection limits attainable rather than an exact measure of the photometric accuracy.
