def compute_paral_angles(header, latitude, ra_key, dec_key, lst_key,
acqtime_key, date_key='DATE-OBS'):
"""Calculates the parallactic angle for a frame, taking coordinates and
local sidereal time from fits-headers (frames taken in an alt-az telescope
with the image rotator off).
The coordinates in the header are assumed to be J2000 FK5 coordinates.
The spherical trigonometry formula for calculating the parallactic angle
is taken from Astronomical Algorithms (Meeus, 1998).
Parameters
----------
header : dictionary
Header of current frame.
latitude : float
Latitude of the observatory in degrees. The dictionaries in
vip_hci/conf/param.py can be used like: latitude=LBT['latitude'].
ra_key, dec_key, lst_key, acqtime_key, date_key : strings
Keywords where the values are stored in the header.
Returns
-------
pa.value : float
Parallactic angle in degrees for current header (frame).
"""
obs_epoch = Time(header[date_key], format='iso', scale='utc')
# equatorial coordinates in J2000
ra = header[ra_key]
dec = header[dec_key]
coor = SkyCoord(ra=ra, dec=dec, unit=(hourangle,degree), frame=FK5,
equinox='J2000.0')
# recalculate for DATE-OBS (precession)
coor_curr = coor.transform_to(FK5(equinox=obs_epoch))
# new ra and dec in radians
ra_curr = coor_curr.ra
dec_curr = coor_curr.dec
lst_split = header[lst_key].split(':')
lst = float(lst_split[0])+float(lst_split[1])/60+float(lst_split[2])/3600
exp_delay = (header[acqtime_key] * 0.5) / 3600
# solar to sidereal time
exp_delay = exp_delay*1.0027
# hour angle in degrees
hour_angle = (lst + exp_delay) * 15 - ra_curr.deg
hour_angle = np.deg2rad(hour_angle)
latitude = np.deg2rad(latitude)
# PA formula from Astronomical Algorithms
pa = -np.rad2deg(np.arctan2(-np.sin(hour_angle), np.cos(dec_curr) * \
np.tan(latitude) - np.sin(dec_curr) * np.cos(hour_angle)))
#if dec_curr.value > latitude: pa = (pa.value + 360) % 360
return pa.value
def compute_paral_angles(header, latitude, ra_key, dec_key, lst_key,
acqtime_key, date_key='DATE-OBS'):
"""Calculates the parallactic angle for a frame, taking coordinates and
local sidereal time from fits-headers (frames taken in an alt-az telescope
with the image rotator off).
The coordinates in the header are assumed to be J2000 FK5 coordinates.
The spherical trigonometry formula for calculating the parallactic angle
is taken from Astronomical Algorithms (Meeus, 1998).
Parameters
----------
header : dictionary
Header of current frame.
latitude : float
Latitude of the observatory in degrees. The dictionaries in
vip/conf/param.py can be used like: latitude=LBT['latitude'].
ra_key, dec_key, lst_key, acqtime_key, date_key : strings
Keywords where the values are stored in the header.
Returns
-------
pa.value : float
Parallactic angle in degrees for current header (frame).
"""
obs_epoch = Time(header[date_key], format='iso', scale='utc')
# equatorial coordinates in J2000
ra = header[ra_key]
dec = header[dec_key]
coor = SkyCoord(ra=ra, dec=dec, unit=(hourangle,degree), frame=FK5,
equinox='J2000.0')
# recalculate for DATE-OBS (precession)
coor_curr = coor.transform_to(FK5(equinox=obs_epoch))
# new ra and dec in radians
ra_curr = coor_curr.ra
dec_curr = coor_curr.dec
lst_split = header[lst_key].split(':')
lst = float(lst_split[0])+float(lst_split[1])/60+float(lst_split[2])/3600
exp_delay = (header[acqtime_key] * 0.5) / 3600
# solar to sidereal time
exp_delay = exp_delay*1.0027
# hour angle in degrees
hour_angle = (lst + exp_delay) * 15 - ra_curr.deg
hour_angle = np.deg2rad(hour_angle)
latitude = np.deg2rad(latitude)
# PA formula from Astronomical Algorithms
pa = -np.rad2deg(np.arctan2(-np.sin(hour_angle), np.cos(dec_curr) * \
np.tan(latitude) - np.sin(dec_curr) * np.cos(hour_angle)))
#if dec_curr.value > latitude: pa = (pa.value + 360) % 360
return pa.value
def skycoord_to_lmn(pos: SkyCoord, phasecentre: SkyCoord):
"""
Convert astropy sky coordinates into the l,m,n coordinate system
relative to a phase centre.
The l,m,n is a RHS coordinate system with
* its origin on the sky sphere
* m,n and the celestial north on the same plane
* l,m a tangential plane of the sky sphere
Note that this means that l increases east-wards
"""
# Determine relative sky position
todc = pos.transform_to(phasecentre.skyoffset_frame())
dc = todc.represent_as(CartesianRepresentation)
# Do coordinate transformation - astropy's relative coordinates do
# not quite follow imaging conventions
return dc.y.value, dc.z.value, dc.x.value - 1
