# Helioprojective/AltAz Coordinate Conversions¶

This is a very simple example to show how to convert Helioprojective Coordiantes to Altitude/Azimuth Coordinates. In this example, we use balloons observations to include elevation as a factor.

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


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, frame=frames.Helioprojective)


Now we establish our location on the Earth, in this case Fort Sumner, NM. We use the balloon’s observational altitude as ‘height’. Accuracy of ‘height’ is far less of a concern than Lon/Lat accuracy.

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('Altitude is {0} and Azimuth is {1}'.format(sun_altaz.T.alt, sun_altaz.T.az))


Out:

Altitude is 37.78295995856715 deg and Azimuth is 121.34217338830759 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 = get_sunearth_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)
print('The helioprojective point is {0}, {1}'.format(sun_helio.T.Tx, sun_helio.T.Ty))


Out:

The helioprojective point is -0.02185434793773311 arcsec, -0.007955831399695407 arcsec


The output is within a radius of 0.02 arcseccs.

Total running time of the script: ( 0 minutes 1.902 seconds)

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