Diviner calibration

AbstractThis paper describes the methods to calbrate LRO's Diviner Lunar Radiometer Experiment. Like many radiometers, Diviner is sensitive to instrument temperature changes along the orbit of LRO. Regularly executed calibration blocks include instrument pointings to space and towards internal blackbodies at a known temperature. Data from these blocks serve to determine current offsets and current DN to radiance conversion value. A ground calibration campaign served to determine conversion tables over temperature.

General idea

a.k.a. the elevator pitch:

  1. 1.

    In regular intervals, Diviner looks at space, to define the counts for zero radiation, defining a \(counts_{space}\) with an error \(\sigma_{space}\).

  2. 2.

    Around the same time the instrument also points at an internal blackbody source at a measured temperature, defining a \(counts_{BB}\left(T_{BB}\right)\) with errors of \(\sigma_{sensor}\) and \(\sigma_{placement}\) of that temperature sensor.

  3. 3.

    The aforementioned measured temperature is used to look up the radiance \(R_{BB}\left(T_{BB}\right)\) for the given temperature in a previously determined calibration table.

  4. 4.

    By dividing this look-up radiance by the difference between the measured counts for space and black-body like so:

    \begin{equation} gain\left(T_{BB}\right)=\frac{-R_{BB}}{counts_{space}-counts_{BB}\left(T_{BB}\right)}\nonumber \\ \end{equation}

    we define a gain for this blackbody temperature.

  5. 5.
    \begin{equation} \mathrm{Radiance}=\left(\mathrm{counts}-\mathrm{offset}\right)\cdot\mathrm{gain}\nonumber \\ \end{equation}

This is the general idea, while the devil is in the detail:

  • Interpolation of HK blackbody temperature data. Interestingly, different time interval for telescope 1 and telescope 2.

  • above gain and offset are defined only defined for one calibration station, called ”block” in this paper. They need to be smoothly interpolated so that every data sample has one offset and sample available.

Ground calibration campaign

Radiometric ground calibration


Figure \ref{fig:gc_overview} shows the test configuration. The blackbodies were originally built for calibration of the PMIRR instrument, and are scaled copies of the blackbodies used to calibrate ISAMS (Nightingale 1991). The back plate has a corrugated surface for increased effective emissivity. The inside of the blackbodies are painted with Nextel black paint. The two blackbodies were pointed roughly ±17.5\({}^{\circ}\) above and below the horizontal. The DLRE instrument was positioned such that the blackbody apertures filled the two telescope apertures. The fixed temperature blackbody was flooded with liquid nitrogen throughout the test. The variable temperature blackbody varied in temperature from about 20 K to 415 K.

Temperature sensors

Two types of calibrated temperature sensors were used on the blackbodies. The Rosemount 118MF2000 PRTs were calibrated at JPL in March 2002 from 91 to 350 K. The silicon diode sensors are LakeShore DT-471-SD sensors, calibrated in February 2007 from 10 to 500 K. Calibration data is at the end of this report and in attached documents. The locations of the sensors are shown in Figures \ref{fig:bb_fixed_temp} and \ref{fig:bb_variable_temp}. The LakeShore sensors were read out with a LakeShore 330 temperature controller, which had been recently calibrated. The controller was set to use the standard LakeShore Curve 10 for all the sensors. Thus, the raw silicon diode sensor temperatures need to be translated from Curve 10 to the specific calibration curve for each sensor.

\label{tab:parameters_shared}Parameters shared by all channels.
Parameter Value Unit
Detector Nominal IFOV Cross Track 3.58 mrad
Detector Nominal IFOV In Track 6.15 mrad
Nominal Telescope Aperture Diameter 4 cm
Reduction in Aperture Area 6 %
Effective F number 1.87
Etendue 2.6E-04 cm\({}^{2}\)sr
Detector Dimension Cross Track 0.024 cm
Detector Dimension In Track 0.048 cm
Detector \(\mathrm{D^{*}}\) 8.0E+08 \(\frac{cmHz^{1/2}}{W}\)
Signal Integration Time 0.128 s
Detector Noise-Equivalent Power (NEP) 8.4E-11