Add AltAzCoords

Project.toml

@@ -5,6 +5,7 @@ version = "1.0.0"
 
 [deps]
 AstroAngles = "5c4adb95-c1fc-4c53-b4ea-2a94080c53d2"
+AstroLib = "c7932e45-9af1-51e7-9da9-f004cd3a462b"
 StaticArrays = "90137ffa-7385-5640-81b9-e52037218182"
 
 [compat]

examples/observation_planning.jl

@@ -0,0 +1,22 @@
+using SkyCoords, AstroTime, AstroAngles, AstroLib
+
+# First we create the instance of the object in the sky we want to observe
+M13 = ICRSCoords(250.423475 |> deg2rad, 36.4613194 |> deg2rad)
+
+# Then we define the observation time and location
+# We will use the top of Mount Wilson at 4AM UTC at Nov 8, 2021 which corresponds
+# to 8PM the night before local time
+mt_wilson = Observatory("Mount Wilson",32.2264,-118.0642,1693.25,-8)
+time = from_utc("2021-11-08T04:00")
+# We will need the julian date for the conversions, which we can do now
+jd = julian(time) |> value
+
+# Now we perform the conversion
+altaz = AltAzCoords(M13,jd,mt_wilson)
+
+# We may want to see this result in degrees, which is easy with rad2deg
+# For this result, M13 is visible at an elevation of 12 degrees off the horizon
+# and at a (true north) compass heading of 305.8, which is WNW.
+
+# FIXME plot over time
+# This is a few tenths of a degree off the astropy solution - why the difference?

src/SkyCoords.jl

@@ -1,12 +1,13 @@
 __precompile__()
 
 module SkyCoords
-using StaticArrays
+using StaticArrays, AstroLib
 
 export AbstractSkyCoords, 
        ICRSCoords,
        GalCoords,
        FK5Coords,
+       AltAzCoords,
        separation,
        position_angle,
        offset
@@ -95,6 +96,39 @@
     zrotmat(-z) * yrotmat(theta) * zrotmat(-zeta)
 end
 
+# AltAz --> ICRS
+
+# Conversion functions
+# FIXME these should be matrix operations?
+function AltAzCoords(icrs::ICRSCoords,jd::AbstractFloat,location::Observatory)
+    hour_angle = 360*ct2lst(location.longitude,jd)/24 - rad2deg(icrs.ra)
+    if hour_angle < 0
+        hour_angle = (hour_angle + 360) % 360
+    end
+    alt = asin(sin(icrs.dec)*sind(location.latitude) +
+               cos(icrs.dec)*cosd(location.latitude)*cosd(hour_angle))
+    a = acos((sin(icrs.dec) - sin(alt)*sind(location.latitude)) /
+             (cos(alt)*cosd(location.latitude)))
+    if sind(hour_angle) <= 0
+        az = a
+    else
+        az = 2π - a
+    end
+    return AltAzCoords(alt,az)
+end
+
+AltAzCoords(icrs::ICRSCoords,jd::T,lat::T,long::T) where T <: AbstractFloat = AltAzCoords(icrs,jd,Observatory("",lat,long,0,0))
+
+# This isn't working yet
+function ICRSCoords(altaz::AltAzCoords,time,location)
+    dec = asind(sin(altaz.alt)*sind(location.lat) +
+                cos(altaz.alt)*cosd(location.lat)*cos(altaz.az))
+    hour_angle = acosd((sin(altaz.alt)-sind(location.lat)*sind(dec)) /
+                       (cosd(location.lat)*cos(dec)))
+    ra = local_sidereal_time(location,time) - hour_angle
+    return ICRSCoords(dec |> deg2rad,ra |> deg2rad)
+end
+
 # -----------------------------------------------------------------------------
 # Type-dependent methods
 

src/types.jl

@@ -70,6 +70,23 @@
 FK5Coords{e}(c::T) where {e,T<:AbstractSkyCoords} = convert(FK5Coords{e}, c)
 FK5Coords{e,F}(c::T) where {e,F,T<:AbstractSkyCoords} = convert(FK5Coords{e,F}, c)
 
+"""
+    AltAzCoords(alt, az)
+
+[Horizontal Coordinate System](https://en.wikipedia.org/wiki/Horizontal_coordinate_system)
+
+This coordinate system uses the observer's local horizon as the fundamental plane to define an object in the local sky. The mapping to non-local coordinates requires a location and time. This is useful for planning observations with azimuthal mount telescopes.
+
+# Coordinates
+- `alt` - Angle between object and the observer's local horizon in radians (0, π/2)
+- `az` - Angle of objectaround horizon, increasing eastward from true north (0, 2π)
+"""
+struct AltAzCoords{T<:AbstractFloat} <: AbstractSkyCoords
+    alt::T
+    az::T
+end
+AltAzCoords(alt::Real, az::Real) = AltAzCoords(promote(float(alt), float(az))...)
+# FIXME constructor from other coordinates
 
 # Scalar coordinate conversions
 Base.convert(::Type{T}, c::T) where {T<:AbstractSkyCoords} = c