The SNR of a Transit. (arXiv:2305.06790v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kipping_D/0/1/0/all/0/1">David Kipping</a>
Accurate quantification of the signal-to-noise ratio (SNR) of a given
observational phenomenon is central to associated calculations of sensitivity,
yield, completeness and occurrence rate. Within the field of exoplanets, the
SNR of a transit has been widely assumed to be the formula that one would
obtain by assuming a boxcar light curve, yielding an SNR of the form
$(delta/sigma_0) sqrt{D}$. In this work, a general framework is outlined for
calculating the SNR of any analytic function and it is applied to the specific
case of a trapezoidal transit as a demonstration. By refining the approximation
from boxcar to trapezoid, an improved SNR equation is obtained that takes the
form $(delta/sigma_0) sqrt{(T_{14}+2T_{23})/3}$. A solution is also derived
for the case of a trapezoid convolved with a top-hat, corresponding to
observations with finite integration time, where it is proved that SNR is a
monotonically decreasing function of integration time. As a rule of thumb,
integration times exceeding $T_{14}/3$ lead to a 10% loss in SNR. This work
establishes that the boxcar transit is approximate and it is argued that
efforts to calculate accurate completeness maps or occurrence rate statistics
should either use the refined expression, or even better numerically solve for
the SNR of a more physically complete transit model.
Accurate quantification of the signal-to-noise ratio (SNR) of a given
observational phenomenon is central to associated calculations of sensitivity,
yield, completeness and occurrence rate. Within the field of exoplanets, the
SNR of a transit has been widely assumed to be the formula that one would
obtain by assuming a boxcar light curve, yielding an SNR of the form
$(delta/sigma_0) sqrt{D}$. In this work, a general framework is outlined for
calculating the SNR of any analytic function and it is applied to the specific
case of a trapezoidal transit as a demonstration. By refining the approximation
from boxcar to trapezoid, an improved SNR equation is obtained that takes the
form $(delta/sigma_0) sqrt{(T_{14}+2T_{23})/3}$. A solution is also derived
for the case of a trapezoid convolved with a top-hat, corresponding to
observations with finite integration time, where it is proved that SNR is a
monotonically decreasing function of integration time. As a rule of thumb,
integration times exceeding $T_{14}/3$ lead to a 10% loss in SNR. This work
establishes that the boxcar transit is approximate and it is argued that
efforts to calculate accurate completeness maps or occurrence rate statistics
should either use the refined expression, or even better numerically solve for
the SNR of a more physically complete transit model.
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