A comprehensive approach to HST focal plane geometric calibration
Abstract
We present a new comprehensive approach to the focal plane geometric calibration of the Hubble Space Telescope (HST). This calibration consists in determining the relative locations and orientations of the Fine Guidance Sensors (FGS) and the HST science instruments. It is necessary for efficient operations, e.g. pointing and target acquisitions, and its quality directly affects the astrometric accuracy of archived products. Our approach is new both in how the necessary data are acquired and in their modeling and analysis. We demonstrate the viability and efficiency of the new approach with the help of four seasons of focal plane alignment data taken in Cycles 24 and 25 (April 2017 - October 2018). We uncover a significant scale error (∼ 7 · 10-4) in one axis of the operational FGS3 distortion. New alignments of FGS1, FGS2, ACS-WFC, and WFC3-UVIS are determined relative to the reference FGS3. We find that both cameras and FGS1 exhibit a stable offset of ∼150-170 mas relative to the latest previous measurement. The position of FGS2 is offset by ∼400 mas and continues to evolve, in agreement with previous independent findings. The methods and software developed in this work allow us to take full advantage of the unprecedented accuracy of the astrometric reference frame realized by Gaia and are directly applicable to the focal plane geometric calibration of the James Webb Space Telescope.
- Publication:
-
ISR TEL 2019-1
- Pub Date:
- August 2019
- Bibcode:
- 2019tel..rept....1S
- Keywords:
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- astrometry;
- focal plane;
- calibration;
- HST;
- JWST;
- FGS