.. include:: ../include/links.rst

====
Bias
====

Overview
========

This file describes the data model for the ``Bias`` image.
This is written to disk as a multi-extension FITS file prefixed by
``Bias`` in the ``Calibrations/`` folder.

This is generally a simple combination of all input bias frames.

Inspecting
==========

The first extension is the stacked image.  You can view it with
any standard image viewer, e.g.:

.. code-block:: console

    ginga Calibrations/Bias_A_1_01.fits

It should resemble any one of your input bias frames aside from:

- Having been overscan subtracted

- Being only a trimmed portion of the image

- Re-oriented so that vertical is the spectral dimension with blue at the bottom

Here is an example for the :ref:`keck-lris-red` spectrograph.

.. image:: ../figures/bias_image.png

Pretty boring, as expected.
Note that this is only 1 of the 2 detectors for this spectrograph.

Trouble Shooting
================

If your image appears to be in err, here are the things to consider:

 - Is one or more of your input bias frames junk?

 - Is one or more of your input bias frames mis-labeled?

Current BiasImage Data Model
============================

Internally, the image is held in
:class:`~pypeit.images.buildimage.BiasImage`,
which subclasses from :class:`~pypeit.images.pypeitimage.PypeItImage` and
:class:`~pypeit.datamodel.DataContainer`.
The datamodel written to disk is:

.. include:: ../include/datamodel_biasimage.rst