This can be a single quartz exposure or a sum or an average of several quartz exposures. This non-saturated quartz exposure should have sufficient counts such that its division into the object image does not significantly change the signal to noise.
HIRES NOTE: It is important that the cross disperser angle (xdangl) match the object xdangl to within about 0.002 units. This is especially important when using a narrow slit where the orders overlap in the blue. The effective slit width can become rather small in this case; any mismatch in the flat to object xdangl can decrease the effective slit width further.
ESI NOTE: It is a good idea to take flats with the same slit width as your object for better fringe correction (although not required). It is difficult to get enough counts in the bluest ESI order, so several exposures should be used. You could take a some flat exposures which saturate in the red to raise the blue count total, however, as of now (November 2000) there is not yet any MAKEE program to "blank" or mask out the saturated orders.