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SSDataLab / tess-ephem

Where are Solar System objects located in TESS data?
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`interpolation_step="7D"` raises a ValueError #2

Closed barentsen closed 3 years ago

barentsen commented 3 years ago

@jessie-dotson pointed me to the following failure for "2004 QX2", which occurs when interpolation_step="7D" but not when interpolation_step="12H".

>>> from tess_ephem import ephem
>>> ephem("2004 QX2", interpolation_step="7D")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/gb/dev/tess-ephem/src/tess_ephem/ephem.py", line 180, in ephem
    return te.predict(time=time, verbose=verbose)
  File "/Users/gb/dev/tess-ephem/src/tess_ephem/ephem.py", line 100, in predict
    crd = SkyCoord(sky.ra, sky.dec, unit="deg")
  File "/Users/gb/Library/Caches/pypoetry/virtualenvs/tess-ephem-_eKBvW1X-py3.8/lib/python3.8/site-packages/astropy/coordinates/sky_coordinate.py", line 315, in __init__
    skycoord_kwargs, components, info = _parse_coordinate_data(
  File "/Users/gb/Library/Caches/pypoetry/virtualenvs/tess-ephem-_eKBvW1X-py3.8/lib/python3.8/site-packages/astropy/coordinates/sky_coordinate_parsers.py", line 301, in _parse_coordinate_data
    _components[frame_attr_name] = attr_class(arg, unit=unit)
  File "/Users/gb/Library/Caches/pypoetry/virtualenvs/tess-ephem-_eKBvW1X-py3.8/lib/python3.8/site-packages/astropy/coordinates/angles.py", line 536, in __new__
    self._validate_angles()
  File "/Users/gb/Library/Caches/pypoetry/virtualenvs/tess-ephem-_eKBvW1X-py3.8/lib/python3.8/site-packages/astropy/coordinates/angles.py", line 557, in _validate_angles
    raise ValueError('Latitude angle(s) must be within -90 deg <= angle <= 90 deg, '
ValueError: Latitude angle(s) must be within -90 deg <= angle <= 90 deg, got [ 2.57963712e+01  2.59729000e+01  2.61491591e+01  2.63251927e+01
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  2.32037011e+01  2.49610172e+01  2.72586700e+01  3.02267270e+01
  3.40158149e+01  3.87912231e+01  4.47100920e+01  5.18693126e+01
  6.02196925e+01  6.94768300e+01  7.91100857e+01  8.84771241e+01
  9.70402895e+01  1.04505716e+02  1.10816309e+02  1.16063376e+02] deg
>>> ephem("2004 QX2", interpolation_step="12H")
Empty DataFrame
Columns: [sector, camera, ccd, column, row]
Index: []
barentsen commented 3 years ago

I determined the cause of this bug to be as follows:

  1. 2004 QX2 had a close approach with Earth/TESS. During this time, its declination changed very rapidly (>degrees/day).
  2. As a result, the object ephemeris which was obtained with resolution step="7D" was too coarse, which in turn caused the interpolation performed by TessEphem.predict_sky to be inaccurate.
  3. The inaccurate interpolation included illegal values for declination (>90 degrees), which caused SkyCoord to raise the ValueError seen in the exception message above.

Graphical illustration of the ephemeris of 2004 QX2, including its close encounter:

Screen Shot 2021-01-05 at 09 42 15
barentsen commented 3 years ago

I've addressed this in cb2af02873b8efda79dca83ca31fef16c5598ce9 by setting the default interpolation step to 12H.

This is an incomplete fix because there are likely extremely fast-moving objects for which this resolution is insufficient as well.

A better fix would be to properly detect and address periods affected by extreme motion. I opened issue #3 to address this.