=========
DESI-0347
=========

:Summary: Focal plane distortion maps. ECSV_ files may also be present.
:Naming Convention: ``DESI-0347_random_offset_{N}.fits``, where ``{N}`` is
                    the (arbitrary) number of the random realization.
:Regex: ``DESI-0347_random_offset_[0-9]+\.fits``
:File Type: FITS, 525 KB

.. _ECSV: https://github.com/astropy/astropy-APEs/blob/master/APE6.rst

Contents
========

====== ======= ===== ========================================
Number EXTNAME Type  Contents
====== ======= ===== ========================================
HDU0_  PRIMARY IMAGE Empty
HDU1_  XOFFSET IMAGE 2D image of x-offsets on the focal plane
HDU2_  YOFFSET IMAGE 2D image of y-offsets on the focal plane
====== ======= ===== ========================================

TODO: expand description with links to documentation about how this is used.

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

HDU0
----

EXTNAME = PRIMARY

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

==== ============= ==== =======
KEY  Example Value Type Comment
==== ============= ==== =======
SEED 1             int
==== ============= ==== =======

Empty HDU.

HDU1
----

EXTNAME = XOFFSET

2D image of x-offsets on the focal plane.

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

======= ============= ===== =======================================
KEY     Example Value Type  Comment
======= ============= ===== =======================================
NAXIS1  256           int
NAXIS2  256           int
WCSAXES 2             int   Number of coordinate axes
CRPIX1  128.5         float Pixel coordinate of reference point
CRPIX2  128.5         float Pixel coordinate of reference point
CDELT1  0.0125        float Coordinate increment at reference point
CDELT2  0.0125        float Coordinate increment at reference point
CTYPE1  x             str   Coordinate type code
CTYPE2  y             str   Coordinate type code
CRVAL1  0.0           float Coordinate value at reference point
CRVAL2  0.0           float Coordinate value at reference point
LATPOLE 90.0          float [deg] Native latitude of celestial pole
BUNIT   um            str
EXTNAME XOFFSET       str   extension name
======= ============= ===== =======================================

Data: FITS image [float32, 256x256]

HDU2
----

EXTNAME = YOFFSET

2D image of x-offsets on the focal plane.

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

======= ============= ===== =======================================
KEY     Example Value Type  Comment
======= ============= ===== =======================================
NAXIS1  256           int
NAXIS2  256           int
WCSAXES 2             int   Number of coordinate axes
CRPIX1  128.5         float Pixel coordinate of reference point
CRPIX2  128.5         float Pixel coordinate of reference point
CDELT1  0.0125        float Coordinate increment at reference point
CDELT2  0.0125        float Coordinate increment at reference point
CTYPE1  x             str   Coordinate type code
CTYPE2  y             str   Coordinate type code
CRVAL1  0.0           float Coordinate value at reference point
CRVAL2  0.0           float Coordinate value at reference point
LATPOLE 90.0          float [deg] Native latitude of celestial pole
BUNIT   um            str
EXTNAME YOFFSET       str   extension name
======= ============= ===== =======================================

Data: FITS image [float32, 256x256]


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

The ECSV_ files express blur and offset calculated from ray-tracing (?)
the DESI optical path.