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To put extracted data onto a scale of flux density, it is necessary to observe spectrophotometric flux standards. There is a compilation of suitable stars in a folder in the control room. Ideally, the standard should be observed through each slit in a mask since it is possible that the instrument sensitivity might be a function of position within the field. Since this is a time consuming and difficult operation, it is more sensible to use a single observation either through one slit in the mask (although it will then be necessary to calculate an offset for the telescope to put the target on the desired slit) or through a longslit. Two longslit masks are provided (see Section 1.2): one has a normal slit width (1.5 arcsec) as used by default with INTERACT-4 masks; while the other has a much wider slit of 10-20 arcsec designed for spectrophotometric observations where it is necessary to get all the light in the slit.