Progress in Wide Field CCD Astrometry

Extract

Instrumental parameters and astrometric results from five telescopes are summarized in Table 1. The KPNO 0.9m has field-corrector optics. The CTIO 0.9m is a classical Cassegrain. The 4-meter telescopes both have doublet field-correctors. The USNO 0.2m (8-inch) astrograph has a 5-element lens which is designed for a 9° flat field of view for photographic plates. A bandpass of 570-650 nm is used at the USNO 0.2m while most frames with the other telescopes have been taken through a Gunn r (600-710 nm) filter. Stellar images on the CCD frames have been fitted with a 2-D circular symmetric Gaussian profile, giving the centroiding error σ_fit . Plots of σ_fit vs. instrumental magnitude look similar in shape for all telesopes. The asymptotic fit precision, σ_afp , is the limit in σ_fit achieved for bright stars, given in milli pixels (mpx) in Table 1. This centering error is overestimated by an amount depending on the deviation of the real image profile from the model function (Winter 1997).