Spectroradiometric calibration of telescopes using laser illumination of flat field screens

摘要

It is standard practice at many telescopes to take a series of flat field images prior to an observation run. Typically the flat field consists of a screen mounted inside the telescope dome that is uniformly illuminated with a broadband light source. These flat field images are useful for characterizing the relative response of CCD pixels to light passing through the telescope optics and filters, but carry limited spectral information and are not calibrated for absolute flux. We present the results of performing in situ, spectroradiometric calibrations of a 1.2 m telescope at the Fred Lawrence Whipple Observatory, Mt. Hopkins, AZ. To perform a spectroradiometric calibration, a laser, tunable through the visible to near infrared, was coupled into an optical fiber and used to illuminate the flat field screen in situ at the telescope facility. A NIST traceable, calibrated photodiode was mounted on the telescope to measure the spectral flux reaching the aperture. For a particular filter, images of the screen were then captured for each laser wavelength as the wavelength was tuned over the filter bandpass. Knowledge of the incident flux then allows the relative responsivity of each CCD pixel at each wavelength to be calculated.