HIRES has two cross dispersers blazed at different wavelengths. Switching from one to the other can not be done during the night, hence observers must request either HIRESb or HIRESr, optimized for short and long wavelengths respectively. Here is a plot of relative efficiency of between HIRESb and HIRESr:

It is possible to change cross dispersers from one night to the next. Observers are cautioned however, that doing so means a human presence inside HIRES during the day of the switch. By sunset during the day of the switch there is no guarantee that HIRES will have completely equilibrated in terms of temperature and humidity. The effects in terms of spectrograph stability are uncertain. Sometimes the vagaries of telescope scheduling force Keck to schedule different cross dispersers on subsequent nights. If observers notice that a different cross disperser was used on the night before their run, they are urged to discuss the implications with their support astronomer.

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