.. _specphot_standards:

Spectrophotometric Standards
============================

Introduction
------------

Instrument sensitivity as a function of wavelength is calibrated using observations of
spectrophotometric standard stars. `specreduce <https://specreduce.readthedocs.io/en/stable/index.html>`_ offers some
convenience functions for accessing some databases of commonly used standard stars and loading the data into `~specutils.Spectrum1D`
instances.

Supported Databases
-------------------

Probably the most well-curated database of spectrophotometric calibration data is the
`CALSPEC <https://www.stsci.edu/hst/instrumentation/reference-data-for-calibration-and-tools/astronomical-catalogs/calspec>`_
database at `MAST <https://archive.stsci.edu/>`_ (Ref.: `Bohlin, Gordon, & Tremblay 2014 <https://iopscience.iop.org/article/10.1086/677655>`_).
It also has the advantage of including data that extends well into both the UV and the IR. The `~specreduce.calibration_data.load_MAST_calspec`
function provides a way to easily load CALSPEC data either directly from `MAST <https://archive.stsci.edu/>`_
(specifically, https://archive.stsci.edu/hlsps/reference-atlases/cdbs/calspec/) or from a previously downloaded local file.
Here is an example of how to use it and of a CALSPEC standard that has both UV and IR coverage:

.. plot::
    :include-source:

    import matplotlib.pyplot as plt
    from specreduce.calibration_data import load_MAST_calspec

    spec = load_MAST_calspec("agk_81d266_stisnic_007.fits")

    fig, ax = plt.subplots()
    ax.step(spec.spectral_axis, spec.flux, where="mid")
    ax.set_yscale('log')
    ax.set_xlabel(f"Wavelength ({spec.spectral_axis.unit})")
    ax.set_ylabel(f"Flux ({spec.flux.unit})")
    ax.set_title("AGK+81 266")
    fig.show()

The `specreduce_data <https://github.com/astropy/specreduce-data/>`_ package provides several datasets of spectrophotometric standard spectra.
The bulk of them are inherited from IRAF's `onedstds <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds>`_ datasets, but
some more recently curated datasets from `ESO <https://www.eso.org/sci/observing/tools/standards/spectra/stanlis.html>`_, the
`Nearby Supernova Factory <https://snfactory.lbl.gov/>`_, and `Gemini
<https://github.com/GeminiDRSoftware/DRAGONS/tree/master/geminidr/gemini/lookups/spectrophotometric_standards>`_ are included as well. The
`~specreduce.calibration_data.load_onedstds` function is provided to load these data into `~specutils.Spectrum1D`
instances. If `specreduce_data <https://github.com/astropy/specreduce-data/>`_ is not installed, the data will be downloaded from the GitHub
`repository <https://github.com/astropy/specreduce-data/tree/main/specreduce_data/reference_data/onedstds>`_. The available
database names and their descriptions are listed here. Please refer to the `specreduce-data repository
<https://github.com/astropy/specreduce-data/tree/main/specreduce_data/reference_data/onedstds>`_ for details on the
specific data files that are available:

- `bstdscal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/bstdscal>`_: Directory of the brighter KPNO IRS
  standards (i.e., those with HR numbers) at 29 bandpasses, data from various sources transformed to the Hayes and Latham system, unpublished.

- `ctiocal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/ctiocal>`_:
  Directory containing fluxes for the southern tertiary standards as published by
  `Baldwin & Stone, 1984, MNRAS, 206, 241 <https://ui.adsabs.harvard.edu/abs/1984MNRAS.206..241B/abstract>`_
  and `Stone and Baldwin, 1983, MNRAS, 204, 347 <https://ui.adsabs.harvard.edu/abs/1983MNRAS.204..347S/abstract>`_.

- `ctionewcal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/ctionewcal>`_:
  Directory containing fluxes at 50 Å steps in the blue range 3300-7550 Å for the
  tertiary standards of Baldwin and Stone derived from the revised calibration of
  `Hamuy et al., 1992, PASP, 104, 533
  <https://ui.adsabs.harvard.edu/abs/1992PASP..104..533H/abstract>`_. This
  directory also contains the fluxes of the tertiaries in the red (6050-10000 Å) at
  50 Å steps as will be published in PASP (Hamuy et al 1994).  The combined fluxes
  are obtained by gray shifting the blue fluxes to match the red fluxes in the
  overlap region of 6500-7500 Å and averaging the red and blue fluxes in the
  overlap.  The separate red and blue fluxes may be selected by following the star
  name with "red" or "blue"; i.e. CD 32 blue.

- `iidscal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/iidscal>`_:
  Directory of the KPNO IIDS standards at 29 bandpasses,
  data from various sources transformed to the Hayes and Latham
  system, unpublished.

- `irscal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/irscal>`_:
  Directory of the KPNO IRS standards at 78 bandpasses,
  data from various sources transformed to the Hayes and
  Latham system, unpublished (note that in this directory the
  brighter standards have no values - the `bstdscal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/bstdscal>`_
  directory must be used for these standards).

- `oke1990 <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/oke1990>`_:
  Directory of spectrophotometric standards observed for use with the HST, Table
  VII, `Oke 1990, AJ, 99, 1621
  <https://ui.adsabs.harvard.edu/abs/1990AJ.....99.1621O/abstract>`_ (no
  correction was applied).  An arbitrary 1 Å bandpass is specified for these
  smoothed and interpolated flux "points".

- `redcal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/redcal>`_:
  Directory of standard stars with flux data beyond 8370 Å.
  These stars are from the IRS or the IIDS directory but
  have data extending as far out into the red as the
  literature permits.  Data from various sources.

- `spechayescal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/spechayescal>`_:
  The KPNO spectrophotometric standards at the Hayes flux
  points, Table IV, Spectrophotometric Standards, `Massey
  et al., 1988, ApJ 328, p. 315 <https://ui.adsabs.harvard.edu/abs/1988ApJ...328..315M/abstract>`_.

- `spec16cal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/spec16cal>`_:
  Directory containing fluxes at 16 Å steps in the blue
  range 3300-7550 Å for the secondary standards, published in `Hamuy et al., 1992,
  PASP, 104, 533
  <https://ui.adsabs.harvard.edu/abs/1992PASP..104..533H/abstract>`_.  This
  directory also contains the fluxes of the secondaries in the red (6020-10300 Å)
  at 16 Å steps as will be published in PASP (`Hamuy et al 1994
  <https://ui.adsabs.harvard.edu/abs/1994PASP..106..566H/abstract>`_).  The
  combined fluxes are obtained by gray shifting the blue fluxes to match the red
  fluxes in the overlap region of 6500-7500 Å and averaging the blue and red
  fluxes in the overlap. The separate red and blue fluxes may be selected by
  following the star name with "red" or "blue"; i.e. HR 1544 blue.

- `spec50cal <https://github.com/iraf-community/iraf/tree/main/noao/lib/onedstds/spec50cal>`_:
  The KPNO spectrophotometric standards at 50 Å intervals. The data
  are from (1) Table V, Spectrophotometric Standards, `Massey et al., 1988, ApJ
  328, p. 315 <https://ui.adsabs.harvard.edu/abs/1988ApJ...328..315M/abstract>`_
  and (2) Table 3, The Kitt Peak Spectrophotometric Standards: Extension to 1
  micron, `Massey and Gronwall, 1990, ApJ 358, p. 344
  <https://ui.adsabs.harvard.edu/abs/1990ApJ...358..344M/abstract>`_.

- `snfactory <https://snfactory.lbl.gov/>`_:
  Preferred standard stars from the LBL Nearby Supernova Factory project:
  https://ui.adsabs.harvard.edu/abs/2002SPIE.4836...61A/abstract
  Data compiled from https://snfactory.lbl.gov/snf/snf-specstars.html.
  See notes there for details and references.

- `eso`_:
  Directories of spectrophotometric standards copied from
  ftp://ftp.eso.org/pub/stecf/standards/. See
  https://www.eso.org/sci/observing/tools/standards/spectra/stanlis.html
  for links, notes, and details.

- `gemini <https://github.com/GeminiDRSoftware/DRAGONS/tree/master/geminidr/gemini/lookups/spectrophotometric_standards>`_:
  Directory of spectrophotometric standards used by Gemini. Originally copied from
  https://github.com/GeminiDRSoftware/DRAGONS/tree/master/geminidr/gemini/lookups/spectrophotometric_standards.


Selecting Spectrophotometric Standard Stars
-------------------------------------------

Many commonly used standard stars have spectra in multiple datasets, but the quality and systematics can differ.
The `~specreduce.calibration_data.load_MAST_calspec` and `~specreduce.calibration_data.load_onedstds` functions can be
useful tools for exploring and comparing spectra from the various databases. An example is shown here for `LTT 9491
<http://simbad.u-strasbg.fr/simbad/sim-id?Ident=LTT+9491&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id>`_ which has
spectra available from MAST, ESO, and the Nearby Supernova factory:

.. plot::
    :include-source:

    import matplotlib.pyplot as plt
    from specreduce.calibration_data import load_MAST_calspec, load_onedstds

    s1 = load_MAST_calspec("ltt9491_002.fits")
    s2 = load_onedstds("snfactory", "LTT9491.dat")
    s3 = load_onedstds("eso", "ctiostan/ltt9491.dat")

    fig, ax = plt.subplots()
    ax.step(s1.spectral_axis, s1.flux, label="MAST", where="mid")
    ax.step(s2.spectral_axis, s2.flux, label="SNFactory", where="mid")
    ax.step(s3.spectral_axis, s3.flux, label="ESO", where="mid")
    ax.set_yscale('log')
    ax.set_xlabel(f"Wavelength ({s1.spectral_axis.unit})")
    ax.set_ylabel(f"Flux ({s1.flux.unit})")
    ax.set_title("LTT 9491")
    ax.legend()
    fig.show()

The `MAST`_ data have the best UV coverage, but that's not useful from the ground and
they only extend to 0.9 microns in the red in this case. The other data extend to 1.0 microns, but both spectra show
systematics due to telluric absorption. The `SNFactory`_ data extend well past the atmospheric
cutoff with no correction applied for atmospheric transmission. The `ESO`_ data, on the other hand,
are not corrected for the telluric features in the near-IR while the `SNFactory`_ data are.
Regions affected by such telluric systematics should be masked out before these spectra are used for calibration purposes.
