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.