NIS Calibration Report #8

Jim Bell and Andy Switala; 7/13/95

Issue:

Detector noise induced by holding the NIS slit in closed position

Methods:

(1) Use the May 13-15 OCF data for NIS placed on retro by Scott. In particular, we examined the file NIEMH06.135, which are multiple blocks of dark data obtained at four different conditions:

(a) Source (light) off, shutter open, gain 10

(b) Source off, shutter closed, gain 10

(c) Source on, shutter closed, gain 10

(d) Source off, room lights off, instrument pointed at cold wall, shutter open, gain 10

The tests were done at a temperature of -18.2C

(2) 100 measurements were made in each test condition. We calculated averages and standard deviations of all 100 measurements and plotted the results to verify data quality.

(3) Use the nominal wavelength calibration defined in Bell and Switala's NIS Calibration Report #7 to convert channels to wavelength.

(4) Treat condition (d) above as the "reference dark", as it is probably as close to true deep-space dark as could be obtained in the lab at that time. Then ratio the three other dark averages to that one and plot the results to search for evidence of additional noise.

Figures 1 and 2 show the results without error bars for Ge and InGaAs. Figures 3 and 4 show the results with error bars for Ge and InGaAs.

Figure 1: NIS Ge detector spectra of averaged dark data obtained in the four conditions described above. The mean values are plotted without error bars in order to see the scatter. The RMS noise is about 0.5%. The two most important cases of concern (open circles and filled triangles) show possible evidence of increased noise at the longest Ge wavelengths (> 1300 nm). The systematic increase in the case of source on, shutter closed (triangles) may be indicative of a very weak light leak beyond the shutter. However, see Figure 3.

Figure 2: NIS InGaAs detector spectra of averaged dark data obtained in the four conditions described above. The mean values are plotted without error bars in order to see the scatter. The RMS noise is about 1.0%, and it is highly wavelength dependent. The two most important cases of concern (open circles and filled triangles) show possible evidence of increased noise in the 2100 to 2600 nm region. However, see Figure 4.

Figure 3: NIS Ge detector spectra of averaged dark data obtained in the four conditions described above. The mean values are plotted with error bars in this case and the scale is expanded relative to Fig. 1. This figure shows that the apparent increase in dark noise at the longest wavelengths is probably not statistically significant relative to other wavelengths.

Figure 4: NIS InGaAs detector spectra of averaged dark data obtained in the four conditions described above. The mean values are plotted with error bars in this case and the scale is expanded relative to Fig. 2. This figure shows that the apparent increase in dark noise at the long wavelengths is also not statistically significant relative to other wavelengths.

Summary of Results:

It does not appear that the motor that holds the shutter in the closed position adds any statistically significant noise to either the Ge or InGaAs detectors. The increased standard deviation of the dark signals at the longer wavelengths in each detector are most likely a result of lower SNR in those elements. We would recommend that a few additional OCF tests be performed, if time allows, to examine the possibility that a small amount of light is leaking beyond the closed shutter onto the Ge detector.