![]() ![]() This roll is not reset and continues to build over multiple orbits and reacquisitions, until the next full guide star acquisition. In single guide star guiding, typical gyro drift rates produce a roll about the guide star of 1.0–1.5 mas per 1 second exposure, which in turn introduces a translational drift of the target on the detector. To determine the quality of tracking during these observations, please refer to the Introduction to the HST Data Handbooksfor information about jitter files. These observations will suffer from small drift rates. O n some occasions, when two suitable guide stars are not available, single-star guiding is used with the telescope roll controlled by the gyros. The normal guiding mode uses two guide stars that are tracked by two of HST's Fine Guidance Sensors (FGSs). The Institute is monitoring the variations of the linear skew terms and will continue updating the corresponding astrometric reference files described in the above-mentioned ISR. (Please see ACS ISR 2015-02 and ACS ISR 2015-06).Īccurate astrometric measurements, especially for faint sources, should take into account the effects of CTE, as described in ACS ISR 2007-04. With the inclusion of a time-dependent skew in the ACS distortion model 1 used by AstroDrizzle, the accuracy of alignment between ACS/WFC images is ~0.05 pixels or better. In this case, the limitations are primarily the accuracy with which the geometric distortion of the camera has been characterized, see the DrizzlePac website for details. ![]() Second, the mapping from the guide star to the instrument aperture introduces a smaller, but significant error.Īlthough absolute astrometry cannot be done to high accuracy without additional knowledge, relative astrometry with ACS is possible to a much higher accuracy. First, the positions of guide stars are not known to better than about 0.3 arcseconds. As a result, the absolute astrometry attainable by using the image header world coordinate system directly is limited by two sources of error. The astrometric information in the header of an ACS image comes indirectly from the positions of the guide stars used during the observations. For more information about drizzling HST images, please refer to the DrizzlePac website. ![]()
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