The newtonian diagonal flat mirror reflects the diverging beam from the slit onto the collimator (figure 1 ). Two flats are available mounted on a common slide bar just below the slit. The "red" flat is silver coated with a sapphire overcoating and has excellent reflectivity (~98%) for wavelengths >4000 Å. The aluminized "blue" flat has good reflectivity from 3000 to 10,000 Å and should be used below 4000 Å with the blue collimator. Note that there may be a significant collimator focus position change when switching from one set of optics to the other.