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SNR=(273*sqrt(itime*expnum)*10^(-0.2*(Kmag-1))) /(1.66*ln(seeing in ")+1.9)
For example, four 400s 600s exposures on a K=10 target with 0.6" seeing will give you:
SNR=(273*sqrt(6600*4004)*10^(-0.2*(10-1))) /(1.66*ln(0.6)+1.9) ~ 200
The same target with the same exposure time but in 1.0" seeing will give:
SNR=(273*sqrt(6600*4004)*10^(-0.2*(10-1))) /(1.66*ln(1.0)+1.9) ~ 110
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