tess-asteroids

tess-asteroids allows you to make TPFs and LCs for any object that moves through the TESS field of view, for example solar system asteroids, comets or minor planets.

Example use

Making a TPF

You can create a TPF that tracks a moving object by providing an ephemeris, as follows:

import numpy as np
import pandas as pd
from tess_asteroids import MovingTPF

# Create an artificial ephemeris
time = np.linspace(1790.5, 1795.5, 100)
ephem = pd.DataFrame({
            "time": time,
            "sector": np.full(len(time), 18),
            "camera": np.full(len(time), 3),
            "ccd": np.full(len(time), 2),
            "column": np.linspace(500, 600, len(time)),
            "row": np.linspace(1000, 900, len(time)),
        })

# Initialise
target = MovingTPF("example", ephem)

# Make TPF and save to file
target.make_tpf()

The make_tpf() function is retrieving and reshaping the FFI data, performing a background correction, computing the aperture and saving a SPOC-like TPF as a fits file. A few things to note about the format of the ephemeris:

• time must have units BTJD = BJD - 2457000.
• sector, camera, ccd must each have one unique value.
• column, row must be one-indexed, where the lower left pixel of the FFI has value (1,1).

There are a few optional parameters in the make_tpf() function. This includes:

• shape controls the shape of the TPF. Default : (11,11).
• file_name is the name the TPF will be saved with. If one is not given, a default name will be generated. In the above example, the default name was tess-example-s0018-3-2-shape11x11-moving_tp.fits.
• save_loc is the directory where the TPF will be saved. Note, the directory is not automatically created.

These settings can be changed as follows:

# Make TPF and save to file - changed default settings
target.make_tpf(shape=(20,10), file_name="test.fits", save_loc="movingTPF")

Instead of inputting an ephemeris, you can also create a TPF using the name of an object from the JPL/Horizons database and the TESS sector. This will use tess-ephem to compute the ephemeris for you.

# Initialise for asteroid 1998 YT6 from TESS sector 6.
target, ephem = MovingTPF.from_name("1998 YT6", sector=6)

# Make TPF and save to file (tess-1998YT6-s0006-1-1-shape11x11-moving_tp.fits)
target.make_tpf()

Understanding the TPF

The TPF has four HDUs:

• "PRIMARY" - a primary HDU containing only a header.
• "PIXELS" - a table with the same columns as a SPOC TPF. Note that "POS_CORR1" and "POS_CORR2" are defined as the offset between the center of the TPF and the expected position of the moving object given the input ephemeris.
• "APERTURE" - an image HDU containing the average aperture across all times.
• "EXTRAS" - a table HDU containing columns not found in a SPOC TPF. This includes "CORNER1"/"CORNER2" (original FFI column/row of the lower-left pixel in the TPF), "PIXEL_QUALITY" (3D pixel quality mask identifying e.g. strap columns, non-science pixels and saturation) and "APERTURE" (aperture as a function of time).

Plotting the TPF with lightkurve

The TPFs that get created by tess-asteroids can be plotted using lightkurve, as follows:

import lightkurve as lk
import matplotlib.pyplot as plt

# Read in TPF without removing bad cadences
tpf = lk.read("tess-1998YT6-s0006-1-1-shape11x11-moving_tp.fits", quality_bitmask="none")

# Plot TPF and aperture for a single frame
aperture_mask = tpf.hdu[3].data["APERTURE"][200]
fig, ax = plt.subplots(1, figsize=(7,7))
tpf.plot(ax=ax, aperture_mask=aperture_mask, frame=200, title="Target: 1998 YT6")
plt.show()

# You can also animate the TPF and save as a gif.
fig, ax = plt.subplots(1, figsize=(7,7))
tpf._to_matplotlib_animation(ax=ax).save("tess-1998YT6-s0006-1-1-shape11x11-moving_tp.gif", writer="pillow")