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An exposure time calculator (ETC) is a type of software application (often web-based) to calculate the necessary time to make an observation for photography or spectrography, generally oriented to a particular telescope. Generally they allow entry of a number of parameters (e.g., magnitude, passband) and yield information on setting up and using the telescope for the observation, naturally including the necessary exposure time. Tailored ETCs are often developed for particular research telescopes, or for a particular instrument; this assists in efficient and effective use of the instrument. Some ETCs are web-based, at the telescopes'/instruments' support websites.