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In general arc frames should be obtained to bracket each object exposure in time, allowing us to check that no shift has occured during the exposure. An advantage of using a single frame (possibly generated by co-adding the before and after arc frames) is that this allows the resolution of the final spectrum to be easily investigated. This is because the arc spectrum is always extracted in an identical manner to that of the object; using the first (and in this case only) supplied arc frame. As with the flat field frame it may also be advantageous to take multiple arc frames, each exposed correctly to ensure high S/N in particular orders. The package is capable of using a different arc frame for each order if required.