In order to facilitate reading this manual, various fonts are used. Responses you are asked to type in at the command level appear in these fonts: hds_open,filename. In this example, a lower-case italic font indicates numeric or character parameters you have to supply. Procedure names in the text appear like this: average, whereas widget buttons are typed like this: OPEN or DISPCORR, depending on whether or not they bring up a new widget or take immediate action, respectively. Note, that IDL is not case sensitive Messages from the OYSTER software or IDL are denoted in this style: Averaging complete.