Measuring the Atmospheric Influence on Differential Astrometry: A Simple Method Applied to Wide Field CCD Frames

Abstract

Sets of short exposure, guided CCD frames are used to measure the noise added by the atmosphere to differential astrometric observations. Large nightly variations that are correlated with the seeing have been found in the data obtained over 2 years at the KPNO and CTIO 0.9-meter telescopes. The rms noise added by the atmosphere, after a linear transformation of the raw x, y data, is found to be 3 to 7 mas, normalized to 100 seconds exposure time and a field of view of 20 arcminutes near the zenith. An additional nearly constant noise (base-level) of 8.5 mas = 0.012 pixel is found for the KPNO and 6.0 mas = 0.015 pixel for the CTIO telescope. This implies that a ground-based, all sky, astrometric survey from guided CCD frames is more likely limited by the base-level noise than by the atmosphere and could reach an accuracy better than 10 mas under good seeing conditions.

Ground-based CCD astrometry with wide field imagers

“…6 (lower panel). Zacharias (1996) found the global effects of the atmosphere on differential astrometry to be smaller than the predictions of Lindegren (1980), so our global residuals are likely indicative of small distortion errors (as discussed in Sect. 5), in addition to the atmospheric effects.…”

Astrometric Quality of the USNO CCD Astrograph (UCA)

“…The σ f f values already include the error contribution from the atmospheric turbulence, σ atm in addition to the real fit precision of the image profiles. For our exposure time and field size we estimate σ atm ≈ 5 to 10 mas (Zacharias 1996b). This implies that for centrally overlapping frames the UCA performs close to the limit as set by the atmosphere for these 3-minute exposures.…”

Astrometry of satellites I to V of Uranus 1990-1991

“…The star position errors roughly agree with the photographic measuring errors in Table 2. The CCD errors are 0.04 pixel which is three times the value found by Zacharias (1996). However, Zacharias made contiguous exposures with the telescope autoguided where our exposures are spread over five hours and without autoguiding.…”