obs_strategies
==============

ChopNodCombiner
+++++++++++++++


Creates and combines 4 images for each of the chop/nod positions.

- AA : original position ``(dx, dy) = (0, 0)``
- AB : chop position ``(dx, dy) = chop_offsets``
- BA : nod position ``(dx, dy) = nod_offsets``
- BB : chop-nod position ``(dx, dy) = nod_offsets + chop_offsets``

Images are combined using::

    im_combined = (AA - AB) - (BA - BB)

If no ``nod_offset`` is given, it is set to the inverse of ``chop_offset``.

``ChopNodCombiner`` is a detector effect and should be placed last in the
 detector yaml (after the noise effects).

Keyword Arguments
-----------------
chop_offsets : tuple, optional
    [arcsec] (dx, dy) offset of chop position relative to AA
nod_offsets : tuple, optional
    [arcsec] (dx, dy) offset of nod position relative to AA

Example yaml entry
------------------
::

    name: perpendicular_chop_nod_slanted_pattern
    description: chop throw to (+5, 0) and nod throw to (-7, +10) arcsec
    class: ChopNodCombiner
    include: True
    kwargs:
        pixel_scale : "!INST.pixel_scale"
        chop_offsets : (5, 0)
        nod_offsets : (-7, 10)