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Stochastic factors are
- seeing
- cloud cover
Point Source Photometric Merit Function
This suggests that we define a quantity point source photometric merit factor (PSPMF) that tracks how a given image contributes to SNR per band i, where
PSPMF_i=sqrt(t/15)*(T/1)*(1/FWHM)*(sqrt(1/sky)).
with T=atmospheric transmission, which depends on both cloud coverage and airmass and t=exposure time. Summing this quantity over all visits to a given location would provide a cumulative photometric merit factor.
Weak Lensing Merit Function
Another LSST objective is to measure the shapes of resolved galaxies. This is similar to the photometric merit function above, which determines the surface density of galaxies at a given SNR, but has an additional dependence on the seeing. Poor seeing degrades the ability to measure galaxy shapes, since it tends to circularize them.
This shape measurement only takes place in the r passband, and so this only applies in that one band.
We'll parameterize this with an exponent alpha and define the Shape Measurement Merit as
SMMF_r=sqrt(t/15)*(T/1)*(sqrt(1/sky))*(1/FWHM)^(1+alpha).
Temporal Sampling Merit Function
In order to facilitate the determination of the relative priority of fields at any given time, we need to track the amount of time since it was last observed compared to some desired cadence interval tau(field, passband, revisit history). The temporal merit is then given by
TMF_i=exp(t'/tau)
where t' in general contains information about a combination of the time since the field was last observed, and the conditions under which that took place. A first approximation might be
t'=(t-t_last)*(FWHM/0.8)*sqrt(median_sky/sky).
Field and Passband Prioritization
Not all locations on the sky are of equal interest to the survey, and this priority can differ across passbands. So we need to allocate a Field Priority Factor FPF(field, band) to tune the relative priority of observations.
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Oct 18 2013, C. Stubbs
Observations obtained at angles z from zenith suffer from two effects:
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