Dome-diffuser flat-fielding for schmidt telescopes

The Beijing-Arizona-Taiwan-Connecticut Color Survey of the Sky employs the 0.6/0.9 m Schmidt telescope of the National Astronomical Observatories, located at its Xinglong station (150 km northeast of Beijing), combined with a 20482 CCD data-taking system. Key to the success of this program is the development of a simple and efficient way to achieve supersky-quality, large-scale flat fields for our telescope-CCD combination. This is done by placing an isotropic diffuser right in front of the Schmidt corrector plate (the entrance pupil of the telescope) and illuminating the diffuser with scattered light off the dome screen. When we test this methodology by obtaining supersky flats at the zenith, we find that our "super" dome flats define the same large-scale inhomogeneities of the CCD that the supersky flat does, but with far greater signal-to-noise ratio. Hence, we show that defining our large-scale flat fields with our diffuser-dome technique is at least as good as using supersky flats for a ∼1°-wide field of view. This methodology can be implemented with any Schmidt telescope with similar results. Various other data acquisition effects that can affect CCD results at the 1 % level are also discussed.