Jim Bell, Andy Switala; 11/07/95

NIS sample measurements online

Description:

Spectra of many different materials were measured in the APL Optical Calibration Facility (OCF) using the NEAR NIS instrument on July 20, 1995. The goal of these measurements was to provide the team with experience prior to the mission using NIS spectra of real geologic samples. The spectra can also be used to assess the differences in spectral detectability and SNR between the wide and narrow NIS slits. These spectra have been handed out to NEAR Science Team members in hardcopy format; they are presented here digitally.

We modified the OCF setup by using a pickoff mirror to allow observations of samples lying flat in small (2-inch circular) cups; the setup was CRUDE in that no attempts were made to rigorously control the viewing geometry or environmental conditions.

We measured spectra of 15 powders, 16 rocks, meteorites, and rock fragments, 3 miscellaneous samples, and Halon using both wide and narrow slits at Ge gains 1 and 10. We also measured spectra of the NIS onboard calibration target.

We obtained data for most of the types of materials that we had hoped to measure when we first identified a possible sample suite.

Methods:

Samples were measured in the following sequence: First, a group of 25 dark measurements were obtained at gain=1. Then, groups of 25 spectra (1 sec each) were obtained using the narrow slit gain=1, wide slit gain=1, narrow slit gain=10 and wide slit gain=10. Then, a group of 25 dark measurements were obtained at gain=10. This process was repeated for each sample.

Each raw spectrum has had the appropriate dark signal subtracted. Then, a nominal correction for Spectral Crosstalk has been applied (background can be found in NIS Calibration Report #9; more details will be forthcoming in a new report in progress). This crosstalk-corrected spectrum has then been ratioed to a spectrum of halon (also crosstalk-corrected and with its dark substracted) obtained at the same gain and slit setting. The sample/halon spectrum has then been multiplied by the reflectivity of halon to derive an estimate of the absolute reflectance of the sample.

Error bars for the NIS represent the average over the 25 spectra measured for each sample.

Results:

Graphs of each sample measured under the four different slit and gain states have been compiled. Click on the first choice to see the sample spectra with the sample names identified. Some people may not wish to see the names of the samples identified in case they want to try to identify the spectra on thier own. In this case, click to second choice.

(1) Spectra with sample names identified

(2) Spectra without sample names identified

Click here to see spectra of many of these same samples as measured by the MSI instrument and compiled by Mark Robinson.

Click here to see a comparison of the NIS and MSI spectra of many of these same samples.

Click here to go back to Jim's NEAR/NIS home page.