Near-Field Calibration Of An Objective Spectrophotometer To Nist Radiometric Standards For The Creation And Maintenance Of Standard Stars For Ground- And Space-Based Applications

NIST-calibrated detectors will be used by the ground-based 100mm diameter Astronomical Extinction Spectrophotometer (AESoP) to calibrate the spectral energy distributions of bright stars to sub-1% per mm spectral resolution element accuracy. AESoP will produce about a hundred spectroradiometrically calibrated stars for use by ground- and space-based sensors. This will require accurate and near-continuous NIST calibration of AESoP, an equatorially mounted objective spectrophotometer operating over the wavelength range 350nm - 1050nm using a CCD detector.To provide continuous NIST calibration of AESoP in the field a near-identical, removable 100mm diameter transfer standard telescope (CAL) is mounted physically parallel to AESoP. The CAL transfer standard is calibrated by NIST end-to-end, wavelength-by-wavelength at mm spectral resolution. In the field, CAL is used in a near-field configuration to calibrate AESoP. Between AESoP science observations, AESoP and CAL simultaneously observe clear sub-apertures of a 400mm diameter calibration collimator. Monochromatic light measured simultaneously by AESoP and CAL is dispersed by the objective grating onto the AESoP pixels measuring the same wavelength of starlight, thus calibrating both wavelength and instrumental throughput, and simultaneously onto a unique low-noise CAL detector providing the required throughput measurement. System sensitivity variations are measured by vertically translating the AESoP/CAL pair so that CAL can observe the AESoP sub-aperture.Details of this system fundamental to the calibration of the spectral energy distributions of stars are discussed and its operation is described. System performance will be demonstrated, and a plan of action to extend these techniques firstly into the near infrared, then to fainter stars will be described.