add example for tangential projection

examples/tangential_projection.py

@@ -0,0 +1,99 @@
+# SPDX-FileCopyrightText: 2024 the i3astropy contributors
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+"""example to show how to make plots in a tangential projection using astropy's wcs."""
+
+import matplotlib.pyplot as plt
+import numpy as np
+from astropy import units as u
+from astropy.coordinates import SkyCoord
+from astropy.time import Time
+from astropy.visualization.wcsaxes import SphericalCircle
+from astropy.wcs import WCS
+from matplotlib.colors import LogNorm
+
+from i3astropy import I3Dir
+
+# this is were we will center our plot
+center = SkyCoord(90 * u.deg, 80 * u.deg)
+# the is the size across the plot in degree
+SIZE = 10
+
+wcs_helix_list = WCS(naxis=2)
+# for some reason the code is 1 = bottom left, 2= bottom right 1.5 = center
+wcs_helix_list.wcs.crpix = [1.5, 1.5]
+# set the center of the plot
+wcs_helix_list.wcs.crval = [center.ra.deg, center.dec.deg]
+# use degrees
+wcs_helix_list.wcs.cunit = ["deg", "deg"]
+wcs_helix_list.wcs.ctype = ["RA---TAN", "DEC--TAN"]
+# make the plot approx 10 degree across
+wcs_helix_list.wcs.cdelt = [SIZE, SIZE]
+
+# load the datafile but cut it to 30000 events so we can see what is happening
+f = np.load("IC86_2019_exp.npy")[:30000]
+
+# create a figure
+fig = plt.figure(figsize=(12, 10))
+ax = plt.subplot(projection=wcs_helix_list)
+
+# save the limits of the plot for later
+xlim = ax.get_xlim()
+ylim = ax.get_ylim()
+
+# set the labels and make a grid
+ax.set_xlabel("RA")
+ax.set_ylabel("Dec")
+ax.grid(color="grey", ls="dotted")
+
+
+time = Time(f["time"], scale="ut1", format="mjd")
+i3dir = I3Dir(zen=f["zen"] * u.rad, az=f["azi"] * u.rad, obstime=time)
+icrs = SkyCoord(ra=f["ra"] * u.rad, dec=f["dec"] * u.rad)
+
+MAX_ANG_ERR = 2
+# Cut out events which are too far from the center of our plot or have big errors
+cut = (icrs.separation(center) < SIZE / 1.5 * u.deg) * (np.rad2deg(f["angErr"]) < MAX_ANG_ERR)
+
+# make a color scale based on energy
+cmap = plt.cm.rainbow
+norm = LogNorm(vmin=1e2, vmax=1e4)
+colors = cmap(norm(10 ** (f["logE"][cut])))
+
+# plot our events based using coordinate transform from zenith-azimuth with a plut sign
+icrs2 = SkyCoord(i3dir[cut], frame="i3dir").transform_to("icrs")
+ax.scatter(
+    icrs2.ra.deg, icrs2.dec.deg, transform=ax.get_transform("icrs"), s=100, facecolor=colors, marker="+"
+)
+
+# plot our events based on the RA-Dec in the file with a cross overlaying the plus signs we just made
+icrs = icrs[cut]
+ax.scatter(
+    icrs.ra.deg, icrs.dec.deg, transform=ax.get_transform("icrs"), s=100, facecolor=colors, marker="x"
+)
+
+# Plot a Circle around each event with radius of its angErr
+for i, coord in enumerate(icrs):
+    ax.add_patch(
+        SphericalCircle(
+            SkyCoord(coord),
+            f["angErr"][cut][i] * u.rad,
+            edgecolor=colors[i],
+            facecolor="none",
+            transform=ax.get_transform("icrs"),
+        )
+    )
+
+# set the limits back to what they were before we plotted stuff off screen
+ax.set_xlim(*xlim)
+ax.set_ylim(*ylim)
+
+# add color bar to the side of the plot
+sm = plt.cm.ScalarMappable(cmap=cmap, norm=norm)
+cbar = fig.colorbar(sm, ax=ax)
+cbar.ax.get_yaxis().labelpad = 15
+cbar.ax.set_ylabel("Energy [GeV]", rotation=270)
+
+fig.tight_layout()
+plt.show()