calibration results by item flat fields internal lamps AUTHORS : William Jon McCann, Gerhardt R. Meurer
DETECTOR : SBC
PURPOSE :
Characterize the global count rates achieved through each filter for the internal deuterium calibration lamp. Compare these data to the rates predicted by the throughput model to reassess the degree of attenuation required to optimize signal while remaining below the global count rate limit. Also assess the deuterium lamp output degradation over time.
DATA :
SBC internal deuterium flats were obtained during the February - March 1999 Thermal Vacuum tests at GSFC. For comparison with these data is the ACS Throughput Model provided by Eric Johnson.
METHOD :
Straightforward. Two dark frames were taken at the beginning and end of the series. Each was summed and divided by its exposure time and mama tube temperature (SBCTEMP). These two values when averaged gave the dark rate per degree C (cts/sec/°C) for this series. In this temperature stable region the dark rate was assumed to be a linear function of MAMA tube temperature. Each image was summed and divided by its exposure time to give the global count rate for the image. The dark rate per degree C multiplied by the tube temperature gave the dark rate for the given image. This rate subtracted from the global count rate gave the actual count rate. The sbcd2_rates.pro routine does the work.
A line was fit to the global count rate versus time. Since the deuterium lamp remained on throughout this series, the time is essentially lamp-on time. See Figure 1. The d2_lamp_deg.pro routine does the work.
RESULTS :
The data in Table 1 indicate that decreasing the ND attenuation by a factor of four will result in exposure levels matched well to the maximum SBC global count rate of 3 × 105 cts/sec. This suggests replacing the existing ND 2.0 filter with a ND 1.3 filter, based on relative attenuation of the ND filters in the model.
A linear fit to the data indicates that the deuterium lamp degrades at the rate of 0.1585 global counts per second, and is down 0.92% in one hour of operation.
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