"The unit 'ppm' is unrecognized" when stitching TASOC light curves

Sorry, me again…

Problem description

I’m getting the error ValueError: The unit 'ppm' is unrecognized. It can not be converted to other units. when trying to stitch a collection of TASOC lightcurves.

$ python3 mwe.py
/home/wball/.local/lib/python3.9/site-packages/lightkurve/lightcurve.py:965: LightkurveWarning: The light curve already appears to be in relative units; `normalize()` will convert the light curve into relative units for a second time, which is probably not what you want.
  warnings.warn(
Traceback (most recent call last):
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/units/quantity.py", line 738, in to_value
    scale = self.unit._to(unit)
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/units/core.py", line 950, in _to
    raise UnitConversionError(
astropy.units.core.UnitConversionError: 'UnrecognizedUnit(ppm)' is not a scaled version of 'UnrecognizedUnit(ppm)'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/wball/work/steve/mwe.py", line 5, in <module>
    lc = lcs.stitch()
  File "/home/wball/.local/lib/python3.9/site-packages/lightkurve/collections.py", line 228, in stitch
    return vstack(lcs, join_type="inner", metadata_conflicts="silent")
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/table/operations.py", line 638, in vstack
    out = _vstack(tables, join_type, col_name_map, metadata_conflicts)
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/table/operations.py", line 1299, in _vstack
    col[idx0:idx1] = array[name]
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/units/quantity.py", line 1061, in __setitem__
    self.view(np.ndarray).__setitem__(i, self._to_own_unit(value))
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/units/quantity.py", line 1339, in _to_own_unit
    _value = value.to_value(self.unit)
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/units/quantity.py", line 741, in to_value
    value = self._to_value(unit, equivalencies)
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/units/quantity.py", line 660, in _to_value
    return self.unit.to(unit, self.view(np.ndarray),
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/units/core.py", line 987, in to
    return self._get_converter(other, equivalencies=equivalencies)(value)
  File "/home/wball/.local/lib/python3.9/site-packages/astropy/units/core.py", line 1782, in _get_converter
    raise ValueError(
ValueError: The unit 'ppm' is unrecognized.  It can not be converted to other units.

Example

import lightkurve as lk

s = lk.search_lightcurve('HV 2112', mission='TESS', author='TASOC', exptime=1800)
lcs = s.download_all()
lc = lcs.stitch()

In case it’s relevant, I get the same result if I call lc = lcs.stitch(corrector_func=None).

I’m a bit surprised that ppm wouldn’t be recognised because a quick search turned up this list of AstroPy units, which includes ppm.

Expected behavior

I’d get a stitched lightcurve (which has so far worked fine with other data HLSPs).

Environment

• Fedora 33
• Python 3.9.1
• lightkurve 2.0.0 & astropy 4.2, both via pip

[scfleming]

Hi Warrick, the sequence number (Sector) is mislabeled for all the QLP Sector 2 entries in Portal as being from Sector 1 by mistake. We are actively working on fixing that now, should be cleared up soon. The timestamps are correct, and the sequence number will be fixed imminently.

[barentsen]

Yes, it would be worth discussing on an AstroPy issue!

• astropy.units.cds.ppm exists but is not an “enabled” unit by default.
• ppt does not exist at all.

If you are interested, feel free to make the case on an astropy issue!

(Now, regardless of whether or not we can add these to astropy, we’ll have to work around it in lightkurve for the time being.)