NIS Calibration Report #9

Jim Bell and Andy Switala; 7/13/95

Issue:

Leakage of unblocked second-order signal onto Ge detector

Methods:

(1) Use the May 13-15 OCF data for NIS placed on retro by Scott. In particular, we examined the file NIEMH03.135, blocks 26 to 43, which are pairs of 1650 to 1850 nm monochrometer scans taken with and without the order-sorting filter. The tests were done at a temperature of -18.1 C

(2) 10 measurements were made in each block of data. We calculated averages and standard deviations for each block 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) Use the available dark measurements to subtract the dark signal from each monochrometer measurement.

(5) Determine the peak signal values for each detector at each input monochrometer wavelength. The following variables are defined:

G_f: Peak value in Ge measurement with filter in

G_uf: Peak value in Ge measurement with filter out

I_f: Peak value in InGaAs measurement with filter in

I_uf: Peak value in InGaAs measurement with filter out

Figures 1 and 2 show examples of G_f, G_uf, I_f, and I_uf values for an input monochrometer signal at 1700 nm.

Figure 1: NIS Ge detector spectra of 1700 nm light input from a monochrometer. The top spectrum shows the second-order signal (at 1700/2 = 850 nm) detected even with the order-sorting filter in place. The bottom spectrum shows the unfiltered second-order signal at 850 nm.

Figure 2: NIS InGaAs detector spectra of 1700 nm light input from a monochrometer. The top spectrum shows the first-order signal detected with the order-sorting filter in place. The bottom spectrum shows the unfiltered first-order signal. The order-sorting filter is obviously very transparent in this wavelength region.

(6) Define the order-sorting filter Leakage Coefficient, h, as the ratio of the Ge to InGaAs filtered peak values divided by the ratio of the Ge to InGaAs unfiltered peak values. This double-ratio technique effectively removes the effects of the variable transmission of the order-sorting filter as a function of wavelength:

		h = (Gf / If) / (Guf / Iuf) 

Figure 3 shows a plot of h vs wavlength:

Figure 3: Order-sorting filter leakage coefficient, versus wavelength. This graph shows the percentage of second-order light that is leaking past the order-sorting filter and which must be removed from all of our returned Eros spectra. The center wavelength positions of Ge detector channels 1 through 6 are shown by the vertical dashed lines.

Summary of Results:

Second-order light leakage is going to be a serious concern in our data. The effect will be noticed in Ge detector channels 1 through 5, (centered on 818 to 905 nm) with the amount of second-order leakage following the trend in Fig. 3 above. For example, fully 30% of the DN's that we detect from Eros at 1700 nm will be transmitted through the order-sorting filter onto the Ge detector as 850 nm light, and this component will be added on top of whatever DN's we see from Eros at 850 nm proper. The effect reachs a maximum of about 60% in Ge channel 1.

This leakage region is a critical spectral region, as it is the short-wavelength end of the 1-micron pyroxene feature, and also includes our overlap region with the MSI camera filters. However, there is no reason that we should not be able to easily correct for this second-order leakage in our data, provided we have adequate calibration data. We must also consider that we will have to model expected signal-to-noise ratios at the shortest Ge wavelengths assuming that the additional leakage DN will be added to the "expected" signature.

I would recommend that when these data are re-taken during the ongoing OCF run, that:

(1) the monochrometer be stepped through at finer sampling to better characterize the leakage curve, and

(2) the limits of the monochrometer scan be set at 1550 to 1900 nm, so that the leakage curve can be characterized over and slightly beyond the entire responsive spectral range of Ge channels 1 through 6.