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Both phi and FWHM depend on the "airmass" a of the observation. Looking straight up corresponds to a=1. Looking at an angle of 60 degrees from the zenith has a=2. The minimum airmass for any given field occurs on the meridian, which is the great circle on the sky that include the zenith and the celestial pole at declination = -90 degrees.
The atmosphere attenuates the light by an amount that depends on the airmass as well as the passband. So the SNR as a function of airmass and exposure time is a sensible merit function that we seek to optimize, summed over all fields and over the 10 year duration of the survey. This is subject to some science-driven constraints, such as the desire to achieve a uniform survey-integrated SNR across the survey area and the desire to achieve some temporal sampling cadence across the different portions of the sky.
The FWHM varies depending on atmospheric conditions, and is usually expressed in units of arcseconds. LSST expects to achieve a median seeing of 0.6 arcsec, with a long term distribution as shown in the Figure below.
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