Using an Fe-55 Source for Gain Fluctuation Characterization in Sounding Rockets

Abstract:

CCD cameras used in sounding rocket instruments and satellites have a commonly known response to changes in temperature, pressure, and other factors. Characterizing the signal noise for our Full-sun Ultraviolet Rocket SpecTrograph (FURST) instrument has shown that of these responses, the fluctuation in gain is the main driver for variance in the readout signal when compared with calibration consistency (see Vigil et al. 2020 in SPIE). For this instrument, we must obtain spectral knowledge of Doppler-shifts smaller than 3 km/s (R > 10,000). Acquiring such a high level of precision in the calibration of the optics and cameras requires small error margins in the fluctuation of the gain. Previous work with the ESIS instrument has provided numerous camera calibration test data, the same camera which will be used in the FURST instrument. Using these data, we were unable to show that external factors such as temperature changes can accurately predict gain fluctuations for our instrument setup. However, we have found that the use of an Fe-55 radioactive source as a control spectrum can directly quantify the gain variation contributing to the readout of the solar spectrum. Using flux rates of our current Fe-55 source, we perform a statistical analysis of the integration time required for an acceptable characterization of gain fluctuation. This analysis allows us to calculate the necessary size and position of the radioactive source within the instrument payload, making it possible to characterize gain during testing and flight of the sounding rocket.