multiplexing arrangement is preferred of the types described in u s pat nos 5 442 439 or 5 559 597 the content of both being incorporated herein by reference however with the configuration shown in fig2 there is not enough area on the ccd to multiplex without shifting binning between data and calibration acquisitions accordingly shift binning is used for both low frequency and high frequency grating types fig5 illustrates a 4 channel data binning arrangement in conjunction with a multiplexed grating ccd image plane and fig6 illustrates 5 channel calibration binning note that no light impinges on the data channels when calibrating and vice versa the redefinition of ccd binning between sequential calibration and data acquisitions enables higher density multi grating or multi channel multi grating operation with tilted images the dedicated calibration channels also u201c surround u201d the data channels on 2d ccd dispersed slit image facilitating an u201c over under u201d calibration interpolation approach the approach to the calibration data acquisitions may either be quasi simultaneous or sequential although as depicted in fig1 optimal calibration is achieved by feeding calibration light through the data fiber s to the data collecting detectors this embodiment of the invention compromises by interpolating calibrations on immediately surrounding channels although many interpolation algorithms are possible experimental results have shown that a simple linear interpolation agrees with an optimal approach to less than u02dc 0 05 pixel rms u02dc 0 1 cm 1 for worst case multiplexed grating implementation the design corrects for camera tilt yaw slit image curvature lens distortion an so forth and is therefore more accurate than a single neon channel approach which assigns one calibration curve to the entire array fig8 illustrates actual ccd camera images.