Photometric-standard stars are stars that have had their light output in various passbands of photometric system measured very carefully. They provide a means to not only calibrate your photometric system, but give a common “yardstick” from which all measurements can be compared.
This video covers a high level view of the stellar life cycle as relating to variable stars, discussing pre-main sequence, the main sequence, after the main sequence, and after the AGB
The signal-to-noise ratio (SNR) characterizes the quality of a measurement and determines the ultimate performance of the system. The three primary sources of noise in a CCD imaging system are photon noise, dark noise, and read noise, all of which must be considered in the SNR calculation.