Converting between Helioprojective and AltAz Coordinate

How to find the Sun in the sky as viewed from a particular location.

import astropy.units as u
from astropy.coordinates import AltAz, EarthLocation, SkyCoord
from astropy.time import Time

from sunpy.coordinates import frames, sun

We use SkyCoord to define the center of the Sun.

obstime = "2013-09-21 16:00:00"
c = SkyCoord(0 * u.arcsec, 0 * u.arcsec, obstime=obstime,
             observer="earth", frame=frames.Helioprojective)

Now we establish our location on the Earth, in this case let’s consider a high altitude balloon launched from Fort Sumner, NM.

Fort_Sumner = EarthLocation(lat=34.4900*u.deg, lon=-104.221800*u.deg, height=40*u.km)

Now lets convert this to a local measurement of Altitude and Azimuth.

frame_altaz = AltAz(obstime=Time(obstime), location=Fort_Sumner)
sun_altaz = c.transform_to(frame_altaz)
print(f'Altitude is {sun_altaz.T.alt} and Azimuth is {sun_altaz.T.az}')

Altitude is 37.78295990255345 deg and Azimuth is 121.34217342122582 deg

Next let’s check this calculation by converting it back to helioprojective. We should get our original input which was the center of the Sun. To go from Altitude/Azimuth to Helioprojective, you will need the distance to the Sun. solar distance. Define distance with sunpy’s almanac.

distance = sun.earth_distance(obstime)
b = SkyCoord(az=sun_altaz.T.az, alt=sun_altaz.T.alt, distance=distance, frame=frame_altaz)
sun_helio = b.transform_to(frames.Helioprojective(observer="earth"))
print(f'The helioprojective point is {sun_helio.T.Tx}, {sun_helio.T.Ty}')

The helioprojective point is 0.37367192486297096 arcsec, 0.13601753460962557 arcsec