WFC3 Instrument Handbook for Cycle 25 |

help@stsci.edu |

Wide Field Camera 3 Instrument Handbook for Cycle 25 > Chapter 9: WFC3 Exposure-Time Calculation > 9.6 Estimating Exposure Times

•

•

•

• The detector background, or dark, (Bdet) in e- s−1 pixel−1 and the read noise (R) in e-. (Section 9.7 provides information for determining the sky-plus-detector background.)

• C = the signal from the astronomical source in e− s−1. (Note that the actual output image uses DN, which will be equal to Ct/G, where G is the gain.)

•

•

•

•

• R = the read noise in electrons; 3.1 e− for the UVIS channel and 12. e− for the IR channel (this is the effective read noise achieved by fitting the ramp of IR readouts, if close to the full sequence of 16 readouts is used).

• Nread = the number of detector readouts.

• The above equation assumes the optimistic (but often realistic) condition that the background zero-point level under the object that is subtracted could be well known (from integrating over many pixels) but is still noisy in Npix and does not significantly contribute to the error budget; in crowded fields this may not be true. In general, C in the numerator should be the signal in Npix from the component to be measured (e.g., the point source in an image or the line emission in a spectrum), while C in the denominator is the total astronomical signal in Npix (e.g., including light from the underlying galaxy in the image or from the continuum in the spectrum).In the regime where read noise is unimportant, and virtually all of the astronomical signal in Npix comes from the component being measured, the integration time to reach a given signal-to-noise ratio Σ is:

Wide Field Camera 3 Instrument Handbook for Cycle 25 > Chapter 9: WFC3 Exposure-Time Calculation > 9.6 Estimating Exposure Times