Space Telescope Science Institute
DrizzlePac 2012 Handbook
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The DrizzlePac Handbook > Appendix A: Plate Scales and Polynomial Distortions > A.3 Effects of Velocity Aberration

A.3 Effects of Velocity Aberration
Plate scales can vary as a result of the telescope’s orbit through velocity aberration. The Hubble Space Telescope has an orbital speed about the Earth of about 7 km/sec., and the Earth’s orbital velocity about the Sun is approximately 30 km/sec.
Figure A.2: Velocity Aberration Angular Change
The net velocity causes stellar image displacements of some tens of arcseconds. In Figure A.1, represents the angle between the telescope direction of motion relative to the Sun and the direction of a star in barycentric coordinates. The term is the angle measured towards the instantaneous apparent direction and is given by
Differentiating this expression gives us the change in scale due to velocity aberration:
This scale change turns out to be isotropic over the field of view, allowing it to be corrected for through updates to the CD matrix. The plate scale increases by the reciprocal of . The magnitude of this effect is of order 1 part in 104 and can vary by 5 parts in 105 within an orbit. More details on this calibration for ACS can be found in the 2002 HST Calibration Workshop proceedings; specifically in the paper “The Effect of Velocity Aberration on ACS Image Processing”.

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