NEAR MSI Dark Current - Piecepart Calibration

Contents

Introduction

The dark current of the NEAR MSI has been characterized using data that was acquired during the piecepart environmental testing that took place at APL in Feb./March of 1995. Two important questions needed to be answered with respect to the dark current. First, how does the dark current vary as a function of the exposure time? Second, how does the dark current vary as a function of temperature?

Dark Current Versus Exposure Time

While the MSI was held at a nearly constant temperature, multiple images were taken at different exposure settings. The average DN value for the clean area of the detector was then plotted against the exposure time. Figure 1 shows this plot at the various temperature settings that were examined. Table 1 shows the same data in tabular form along with the names of the image files that were used in the analysis.

Reference lines have been drawn on Figure 1 to show the correlation between points that were acquired during three different room temperature conditions corresponding to 21.5, 28.4 and 31.6 degrees C.

Figure 1

Figure 2 shows an expanded view of Figure 1 to illustrate the variation at cold temperatures. The variability of the low and medium temperature points falls well within the 6 DN standard deviation of the data. It is unclear whether the increasing trend of the high temperature points is real since the standard deviation of these points is 5 DN (very close to the average offset between the 100 millisecond and 917 millisecond observations).

Figure 2

Dark Current Versus Temperature

Figure 3 shows a plot of the same numbers used in Figures 1 and 2, but plotting mean signal values against the temperature at which they were measured. Table 2 provides this data in tabular form, along with the names of the image files that were used in the analysis.

Figure 3

Conclusions

The graphs on this page argue that exposure time does not influence the observed dark current until the temperature is well above nominal operating temperatures. This analysis, however, has ignored the even/odd pixel variation inherent in the system. Accounting for this variation may provide insight into whether dark current is increasing with exposure time in the high temperature case. Refer to the full instrument dark current analysis for more information on this subject.