tightly focus the reflected illumination at the spectrometer entrance aperture 22 the detector 34 may be at any orientation but is preferably oriented such that the incident illumination strikes the detector at near normal incidence note that fig3 represents a generalized optical metrology tool the precise form and function of the instrument are determined by the design and arrangement of the fore optics systems 16 and 18 with respect to the sample 14 the systems could comprise one or more instruments selected from the group consisting of spectroscopic reflectometers spectroscopic ellipsometers polarized beam spectroscopic reflectometers spectroscopic scatterometers or optical cd metrology tools in the following we illustrate the predicted wavefront errors for three different spectrometer designs the fastie ebert spectrometer 40 of fig1 and two different preferred embodiments 70 and 80 of the present invention illustrated in fig5 and fig7 respectively all of the simulations used a source wavelength of 670 nm a source na 0 1 unit magnification the object is u02dc 100 mm before mirror and the image is u02dc 100 mm after mirror and a concave mirror with a surface radius u2248 200 mm the grating and mirror positions were adjusted to produce the best focus fig4 shows the simulated wavefront error for the fastie ebert spectrometer 40 illustrated in fig1 plotting the predicted error in waves vs relative numerical aperture fig4 includes 4 plots derived for an axial point in the y z lower left and x z lower right planes and that for a 1 mm object in the y z upper left and x z upper right planes the design has.