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A: You can look at this chart and equation here for A0 type stars, which was derived from the Exposure Time CalculatorETC, and generally applies well to all stars. You can also calculate your own signal-to-noise estimates using with the Exposure Time Calculator (ETC) which can be downloaded here. Note that XXX Chan et al. (2014) found that the S/N estimates from the ETC should be multiplied from the ETC are about 1.2 times greater than the actual S/Nby 0.8.