On the Long-Term Evolution of the Sensitivity of the STEREO HI-1 Cameras |
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Authors: | S. J. Tappin C. J. Eyles J. A. Davies |
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Affiliation: | 1.RAL Space,STFC Rutherford Appleton Laboratory,Didcot,UK;2.Laboratorio de Procesado de Imagenes,Universidad de Valencia,Valencia,Spain |
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Abstract: | The Heliospheric Imagers (HI) on the Solar TErrestrial RElations Observatory (STEREO) observe the solar wind and disturbances therein as it propagates from close to the Sun to 1 AU and beyond. In this article we use stellar photometry over much of the mission to date to make a determination of the long-term evolution of the photometric response of the inner (HI-1) cameras. We find very slow degradation rates of the order of 0.1 % per year, similar to those found for HI-2 by Tappin, Eyles and Davies (Solar Phys. 290, 2143, 2015) and significantly slower than rates found for other comparable instruments. We also find that it is necessary to make a small (({approx},1~%)) revision to the photometric calibration parameters used to convert instrument units into physical units. Finally, we briefly discuss the effects of pointing instabilities on the measurement of stellar count rates. |
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