Space Telescope Science Institute   9.3 Computing Exposure Times  9.3.2 Exposure Time Estimates for Red Targets in F850LP

9.3.1 Calculating Exposure Times for a Given Signal-to-Noise


The signal-to-noise ratio, is given by

:

Where:

This equation assumes the optimistic (and often realistic) condition that the background zero point level under the object is sufficiently well known (and subtracted) to not significantly contribute; in crowded fields this may not be true.

Observers using the CCD normally take sufficiently long integrations that the CCD read noise is not important. This condition is met when:

For the CCD in the regime where read noise is not important and for all SBC observations, the integration time to reach a signal-to-noise ratio , is given by:

If your source count rate is much brighter than the sky plus detector backgrounds, then this expression reduces further to:

i.e. the usual result for Poisson statistics of .

More generally, the required integration time to reach a signal to noise ratio is given by:


 9.3 Computing Exposure Times  9.3.2 Exposure Time Estimates for Red Targets in F850LP
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