Identifying stars in a STEREO/SECCHI COR2 coronagraph image¶

Since the field of view in SECCHI COR2 images can span 2 to 15 solar radii, we often observe stars in these image data. In this example, we will use the Astroquery package to query the VizieR star catalog for stars observed by the Gaia satellite within the SECCHI COR2 field of view. Then we will use the coordinates framework in SunPy and AstroPy to transform the coordinates returned by VizieR into SECCHI COR2 image coordinates. As a bonus, we’ll also identify Mars.”

This requires the installation of the astroquery package, which can be installed on top of the existing sunpy conda environment: conda install -c astropy astroquery and an active internet connection.

import matplotlib.pyplot as plt
from astroquery.vizier import Vizier

import astropy.units as u
from astropy.coordinates import SkyCoord

import sunpy.map
from sunpy.coordinates import frames, get_body_heliographic_stonyhurst
from sunpy.net import helioviewer

Let’s download a STEREO-A SECCHI COR2 image from Helioviewer which provide pre-processed images and load it into a Map.

hv = helioviewer.HelioviewerClient()
f = hv.download_jp2('2014/05/15 07:54', observatory='STEREO_A',
                    instrument='SECCHI', detector='COR2')
cor2 = sunpy.map.Map(f)

Out:

/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/parfive/downloader.py:86: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
  self.http_queue = asyncio.Queue(loop=self.loop)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/asyncio/queues.py:48: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
  self._finished = locks.Event(loop=loop)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/parfive/downloader.py:87: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
  self.http_tokens = asyncio.Queue(maxsize=self.max_conn, loop=self.loop)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/asyncio/queues.py:48: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
  self._finished = locks.Event(loop=loop)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/parfive/downloader.py:88: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
  self.ftp_queue = asyncio.Queue(loop=self.loop)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/asyncio/queues.py:48: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
  self._finished = locks.Event(loop=loop)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/parfive/downloader.py:89: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
  self.ftp_tokens = asyncio.Queue(maxsize=self.max_conn, loop=self.loop)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/asyncio/queues.py:48: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
  self._finished = locks.Event(loop=loop)

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  hosts = await asyncio.shield(self._resolve_host(
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/aiohttp/locks.py:21: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
  self._event = asyncio.Event(loop=loop)


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/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/asyncio/base_events.py:654: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=False>
  _warn(f"unclosed event loop {self!r}", ResourceWarning, source=self)
<frozen importlib._bootstrap_external>:580: ResourceWarning: unclosed <socket.socket fd=16, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('172.17.0.2', 38094), raddr=('129.164.137.18', 443)>
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/asyncio/selector_events.py:694: ResourceWarning: unclosed transport <_SelectorSocketTransport fd=16>
  _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/glymur/jp2k.py:1140: DeprecationWarning: Use array-style slicing instead.
  warnings.warn("Use array-style slicing instead.", DeprecationWarning)

To efficiently search the star field we need to know what stars are near the Sun as observed by STEREO. We need the vector that points from STEREO to the Sun. By converting to HCRS we get the vector from the Sun to STEREO

sun_to_stereo = cor2.observer_coordinate.transform_to('hcrs')

We next reflect the vector to get our search vector which points from STEREO to the Sun

stereo_to_sun = SkyCoord(-sun_to_stereo.data, obstime=sun_to_stereo.obstime, frame='hcrs')

Let’s look up bright stars using the Vizier search capability provided by astroquery (note that this is not a required package of SunPy so you will likely need to install it). We will search the GAIA2 star catalog for stars with magnitude brighter than 7.

vv = Vizier(columns=['**'], row_limit=-1, column_filters={'Gmag': '<7'}, timeout=1200)
vv.ROW_LIMIT = -1
result = vv.query_region(stereo_to_sun, radius=4 * u.deg, catalog='I/345/gaia2')

Let’s see how many stars we’ve found.

print(len(result[0]))

Out:

Now we load each star into a coordinate and transform it into the COR2 image coordinates. Since we don’t know the distance to each of these stars we will just put them very far away.

hpc_coords = []
for this_object in result[0]:
    tbl_crds = SkyCoord(this_object['RA_ICRS'] * u.deg, this_object['DE_ICRS'] * u.deg,
                        1e12 * u.km, frame='icrs', obstime=cor2.date)
    hpc_coords.append(tbl_crds.transform_to(cor2.coordinate_frame))

Out:

/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
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One of the bright features is actually Mars so let’s also get that coordinate. get the location of Mars.

mars = get_body_heliographic_stonyhurst('mars', cor2.date, observer=cor2.observer_coordinate)
mars_hpc = mars.transform_to(frames.Helioprojective(observer=cor2.observer_coordinate))

Out:

INFO: Apparent body location accounts for 1269.96 seconds of light travel time [sunpy.coordinates.ephemeris]

Let’s plot the results.

ax = plt.subplot(projection=cor2)

# Let's tweak the axis to show in degrees instead of arcsec
lon, lat = ax.coords
lon.set_major_formatter('d.dd')
lat.set_major_formatter('d.dd')

cor2.plot(axes=ax, vmin=0, vmax=600)
cor2.draw_limb()

# Plot the position of Mars
ax.plot_coord(mars_hpc, 's', color='white', fillstyle='none', markersize=12, label='Mars')
# Plot all of the stars
for this_coord in hpc_coords:
    ax.plot_coord(this_coord, 'o', color='white', fillstyle='none')
plt.legend()
plt.show()

../../../_images/sphx_glr_STEREO_SECCHI_starfield_001.png

Out:

/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
keyword looks very much like CROTAn but isn't.
  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/wcs/wcs.py:464: FITSFixedWarning: CROTA = -2.4904523E-12
keyword looks very much like CROTAn but isn't.
  wcsprm = _wcs.Wcsprm(header=header_bytes, key=key,
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/sunpy/map/mapbase.py:588: SunpyUserWarning: Missing metadata for solar radius: assuming photospheric limb as seen from Earth.
  warnings.warn("Missing metadata for solar radius: assuming photospheric limb as seen from Earth.",

Total running time of the script: ( 0 minutes 45.433 seconds)

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