Observing
=========

Not written yet.

.. If you need to dither manually,
.. you need to do the following from a python session running on the DRPC:

.. >>> from hcam_widgets.gtc.corba import get_telescope_server
.. >>> server = get_telescope_server()
.. >>> server.requestTelescopeOffset(20,20)

.. Where here the telescope offsets entered are absolute and are in arcseconds,
.. as entered in the json file during phase II.

.. Another way is to import from hcam_drivers.utils.gtc a function called
.. calculate_sky_offset:

.. ngclin7 insuser:~/hcam-drivers/hcam_drivers/utils 1036 > more gtc.py
.. # This is open-source software licensed under a BSD license.
.. # Please see the file LICENSE.txt for details.
.. #
.. from __future__ import print_function, absolute_import, unicode_literals, divisi
.. on

.. import numpy as np
.. from astropy.coordinates.matrix_utilities import rotation_matrix


.. def calculate_sky_offset(xoff, yoff, sky_pa):
..     """
..     Convert pixel offset to sky offset in arcseconds

..     Arguments

..     xoff, yoff : float
..         desired pixel offsets
..     sky_pa : float
..         current instrument PA

..     px_scale = 0.081  # arcseconds per pixel
..     flipEW = True  # is E to the right in the image?
..     EofN = True  # does increasing rotator PA move us to E?
..     paOff = 74.0  # pa when rotator = 0

..     if EofN:
..         theta = sky_pa - paOff
..     else:
..         theta = -sky_pa - paOff
.. # only take 3d of 3d rotation matrix
..     rmat = rotation_matrix(theta)[:2, :2]

.. # +ve shifts should move stars right and up
.. # offset for skyPA = 0 are:
..     ra_shift_arcsecs = -xoff*px_scale if flipEW else xoff*px_scale
..     dec_shift_arcsecs = -yoff*px_scale
..     pix_shift = np.array([ra_shift_arcsecs, dec_shift_arcsecs])
..     return rmat.dot(pix_shift)
.. ngclin7 insuser:~/hcam-drivers/hcam_drivers/utils 1037 >