HTTM: Responsivity & Compression

[noqsi]

Implement formula and inverse to convert electron counts to digital units and make transformations for modifying slices back and forth given these conversions

• When converting a calibrated FITs image to RAW, this is done by multiplying a constant ψ, which is approximately .2 digital units per electron and comes from calibration data ; consider talking to Joel Villasenor to get an estimate

  To handle compression, apply a near identity, slightly non-linear transformation function (derived empirically). Note that a theoretical model for this exists already due to John Doty.

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Make sure to represent whether data is in ADU or Electrons for bookkeeping purposes

[noqsi]

More details:

Constant: FPE_MAX_ADU (65535).

Parameters from calibration database: video_scale (electrons/adu, ~5.5) and compression_parameter (dimensionless, ~0.01).

Parameter from input image: exposure_count (900 for FFI).

I/O: data_electrons, an array of pixel values and data_ADU, an array of pixel values.

Derived parameters: (note that responsivity is named ψ earlier in this thread).

    responsivity = 1/video_scale
    exposure_max_ADU = FPE_MAX_ADU * exposure_count
    compression_per_ADU = compression_parameter/exposure_max_ADU
    compression_per_electron = responsivity * compression_per_ADU

Function: electrons->ADU

    compression = 1.0 + compression_per_electron * data_electrons
    data_ADU = responsivity * data_electrons/compression

Function: ADU->electrons

    decompression = 1.0 - compression_per_ADU * data_ADU
    data_electrons = video_scale * data_ADU/decompression