===============================
fluxcalibaverage-ARM-NIGHT.fits
===============================

:Summary: Flux calibration file contains an average calibration model for a
    given camera including airmass and seeing dependencies.
:Naming Convention: ``fluxcalibaverage-{ARM}-{NIGHT}.fits``, where where ``{ARM}`` is
    one of the spectrograph arms (*e.g.* ``b``) and ``{NIGHT}`` is a date in YYYYMMDD format.
:Regex: ``fluxcalibaverage-[brz]-[0-9]{8}\.fits``
:File Type: FITS, 84 KB

The model is parameterized as

calibration_model(lambda) = average_cal(lambda) * 10^( -0.4*( (seeing-pivot_seeing)*seeing_term(lambda) + (airmass-pivot_airmass)*atm_ext(lambda) ))

calib_flux [1e-17 erg/s/cm2/Angstrom] = uncalib_flux [counts/Angstrom] / (calibration_model * exptime [s])

Contents
========

====== ================================== ===== ===================
Number EXTNAME                            Type  Contents
====== ================================== ===== ===================
HDU0_  FLUXCALIB                          IMAGE Average flux calibration
HDU1_  ATERM                              IMAGE Airmass dependent term
HDU2_  STERM                              IMAGE Seeing dependent term
HDU3_  WAVELENGTH                         IMAGE Wavelength grid
HDU4_  ATERM_ERR                          IMAGE Uncertainty on airmass dependent term
HDU5_  STERM_ERR                          IMAGE Uncertainty on seeing dependent term
HDU6_  FFRACFLUX_WAVE                     IMAGE TODO: *Description needed.*
====== ================================== ===== ===================


FITS Header Units
=================

HDU0
----

EXTNAME = FLUXCALIB

Average flux calibration model such that calibrated flux = uncalibrated photons / model.


Required Header Keywords
~~~~~~~~~~~~~~~~~~~~~~~~

.. collapse:: Required Header Keywords Table

    .. rst-class:: keywords

    ======== ========================= ==== ==============================================
    KEY      Example Value             Type Comment
    ======== ========================= ==== ==============================================
    NAXIS1   2380                      int
    BUNIT    10**+17 cm2 count s / erg str  i.e. (elec/A) / (1e-17 erg/s/cm2/A)
    CHECKSUM 2pfC2ofA2ofA2ofA          str  HDU checksum updated 2018-11-27T11:46:41
    DATASUM  3135063907                str  data unit checksum updated 2018-11-27T11:46:41
    ======== ========================= ==== ==============================================

Data: FITS image [float32, 2380]

HDU1
----

EXTNAME = ATERM

Atmospheric extinction term

Required Header Keywords
~~~~~~~~~~~~~~~~~~~~~~~~

.. collapse:: Required Header Keywords Table

    .. rst-class:: keywords

    ======== ================= ===== ==============================================
    KEY      Example Value     Type  Comment
    ======== ================= ===== ==============================================
    NAXIS1   2380              int
    PAIRMASS 1.132032318244172 float pivot airmass (airmass of average calib.)
    CHECKSUM Xd3dYd0bXd0bXd0b  str   HDU checksum updated 2018-11-27T11:46:41
    DATASUM  1519080666        str   data unit checksum updated 2018-11-27T11:46:41
    ======== ================= ===== ==============================================

Data: FITS image [float32, 2380]

HDU2
----

EXTNAME = STERM

Seeing term

Required Header Keywords
~~~~~~~~~~~~~~~~~~~~~~~~

.. collapse:: Required Header Keywords Table

    .. rst-class:: keywords

    ======== ================= ===== ==============================================
    KEY      Example Value     Type  Comment
    ======== ================= ===== ==============================================
    NAXIS1   2380              int
    PSEEING  1.075342118740081 float pivot seeing (seeing of average calib.), FWHM arcsec
    CHECKSUM IOTaKNRTINRZINRZ  str   HDU checksum updated 2018-11-27T11:46:41
    DATASUM  1211437529        str   data unit checksum updated 2018-11-27T11:46:41
    ======== ================= ===== ==============================================

Data: FITS image [float32, 2380]

HDU3
----

EXTNAME = WAVELENGTH

Wavelengths at which the flux calibration model is evaluated.

Required Header Keywords
~~~~~~~~~~~~~~~~~~~~~~~~

.. collapse:: Required Header Keywords Table

    .. rst-class:: keywords

    ======== ================ ==== ==============================================
    KEY      Example Value    Type Comment
    ======== ================ ==== ==============================================
    NAXIS1   2380             int
    BUNIT    Angstrom         str
    CHECKSUM CbCQFZ9OCaAOCY9O str  HDU checksum updated 2018-11-27T11:46:41
    DATASUM  3517056679       str  data unit checksum updated 2018-11-27T11:46:41
    ======== ================ ==== ==============================================

Data: FITS image [float32, 2380]

HDU4
----

EXTNAME = ATERM_ERR

Uncertainty on atmospheric extinction term

Required Header Keywords
~~~~~~~~~~~~~~~~~~~~~~~~

.. collapse:: Required Header Keywords Table

    .. rst-class:: keywords

    ======== ================ ==== ==============================================
    KEY      Example Value    Type Comment
    ======== ================ ==== ==============================================
    NAXIS1   2380             int
    CHECKSUM fCmKhBkHfBkHfBkH str  HDU checksum updated 2018-11-27T11:46:41
    DATASUM  984665518        str  data unit checksum updated 2018-11-27T11:46:41
    ======== ================ ==== ==============================================

Data: FITS image [float32, 2380]

HDU5
----

EXTNAME = STERM_ERR

Uncertainty on seeing dependent term

Required Header Keywords
~~~~~~~~~~~~~~~~~~~~~~~~

.. collapse:: Required Header Keywords Table

    .. rst-class:: keywords

    ======== ================ ==== ==============================================
    KEY      Example Value    Type Comment
    ======== ================ ==== ==============================================
    NAXIS1   2380             int
    CHECKSUM LaNJNZNJLaNJLYNJ str  HDU checksum updated 2018-11-27T11:46:41
    DATASUM  803867299        str  data unit checksum updated 2018-11-27T11:46:41
    ======== ================ ==== ==============================================

Data: FITS image [float32, 2380]

HDU6
----

EXTNAME = FFRACFLUX_WAVE

TODO: *Description needed.*

Required Header Keywords
~~~~~~~~~~~~~~~~~~~~~~~~

.. collapse:: Required Header Keywords Table

    .. rst-class:: keywords

    ======== ================ ===== ==============================================
    KEY      Example Value    Type  Comment
    ======== ================ ===== ==============================================
    NAXIS1   2751             int
    MDSEEING 0.8553508780470  float TODO: *Description needed.*
    MDFFRACF 0.7200142212446  float TODO: *Description needed.*
    FACWPOW  -0.25            float TODO: *Description needed.*
    FSTNIGHT 20201214         int   TODO: *Description needed.*
    CHECKSUM LaNJNZNJLaNJLYNJ str   HDU checksum updated 2018-11-27T11:46:41
    DATASUM  803867299        str   data unit checksum updated 2018-11-27T11:46:41
    ======== ================ ===== ==============================================

Data: FITS image [float32, 2751]

Notes and Examples
==================

File produced by the desispec script desi_average_flux_calibration.
Used by QuickLook and the off-line calibration.