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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.
Let’s download a STEREO-A SECCHI COR2 image from Helioviewer which provide pre-processed images and load it into a Map.
Out:
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/parfive/downloader.py:100: 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:101: 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:102: 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:103: 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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done, _ = await asyncio.wait(futures, loop=self.loop)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/aiohttp/connector.py:964: DeprecationWarning: The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.
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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hosts = await asyncio.shield(self._resolve_host(
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<frozen importlib._bootstrap_external>:580: ResourceWarning: unclosed <socket.socket fd=15, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('172.17.0.2', 47890), 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=15 read=idle write=<idle, bufsize=0>>
_warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
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. The location of STEREO in HCRS provides the Sun-to-STEREO vector.
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.spherical, obstime=sun_to_stereo.obstime, frame='hcrs')
Let’s look up bright stars using the Vizier search capability provided by astroquery. 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:
23
Now we load all stars into an array coordinate and transform it into the COR2
image coordinates. The reference epoch for the star positions is J2015.5,
so we update these positions to the date of the COR2 observation using
astropy.coordinates.SkyCoord.apply_space_motion().
tbl_crds = SkyCoord(ra=result[0]['RA_ICRS'],
dec=result[0]['DE_ICRS'],
distance=Distance(parallax=u.Quantity(result[0]['Plx'])),
pm_ra_cosdec=result[0]['pmRA'],
pm_dec=result[0]['pmDE'],
radial_velocity=result[0]['RV'],
frame='icrs',
obstime=Time(result[0]['Epoch'], format='jyear'))
tbl_crds = tbl_crds.apply_space_motion(new_obstime=cor2.date)
hpc_coords = tbl_crds.transform_to(cor2.coordinate_frame)
Out:
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/_erfa/core.py:104: ErfaWarning: ERFA function "starpm" yielded 7 of "solution didn't converge (Note 8)"
warnings.warn('ERFA function "' + func_name + '" yielded ' + wmsg, ErfaWarning)
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/units/equivalencies.py:91: RuntimeWarning: invalid value encountered in less
d[d < 0] = np.nan
/home/docs/checkouts/readthedocs.org/user_builds/sunpy/conda/stable/lib/python3.8/site-packages/astropy/units/quantity.py:477: RuntimeWarning: invalid value encountered in less
result = super().__array_ufunc__(function, method, *arrays, **kwargs)
/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,
One of the bright features is actually Mars, so let’s also get that coordinate.
mars = get_body_heliographic_stonyhurst('mars', cor2.date, observer=cor2.observer_coordinate)
mars_hpc = mars.transform_to(cor2.coordinate_frame)
Out:
INFO: Apparent body location accounts for 1269.95 seconds of light travel time [sunpy.coordinates.ephemeris]
/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,
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
ax.plot_coord(hpc_coords, 'o', color='white', fillstyle='none')
plt.legend()
plt.show()
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:748: 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 15.487 seconds)