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Add Frame objects without data to a Table #16823
Description
What is the problem this feature will solve?
Please make it possible to store Frame objects that do not have data in Astropy tables.
I am working on an Astropy-based observation planning and scheduling toolkit for space missions, primarily for tiled imaging observations with wide-field cameras. A natural way for my code to represent the fields to observe is as SkyOffsetICRS objects without any attached data: these objects represent the RA, Dec, and roll of the spacecraft for the observation.
I would like my code to be able to save the planned timeline of observations as a Table which has a column of type SkyOffsetICRS. However, I find that Astropy does not allow me to do this. Here's a minimal reproducer:
from astropy.coordinates import SkyCoord, SkyOffsetFrame
from astropy.table import Table
from astropy import units as u
import numpy as np
ra = [1, 2, 3] * u.deg
de = [4, 5, 6] * u.deg
pa = [7, 8, 9] * u.deg
# SkyOffsetFrame with dummy values is happy, but it's not what I want.
dummy = np.zeros(3) * u.deg
frame_with_dummy_data = SkyOffsetFrame(
dummy, dummy, origin=SkyCoord(ra, de), rotation=pa)
Table({'frame': frame_with_dummy_data})
# SkyCoord with dummy values is happy, but it's not what I want either.
dummy_coord_with_frame = SkyCoord(
dummy, dummy, frame=SkyOffsetFrame(origin=SkyCoord(ra, de), rotation=pa))
Table({'coord': dummy_coord_with_frame})
# This is what I want: a bare frame with no data. But the Table constructor
# raises the following exception: `ValueError: The frame object
# "<SkyOffsetICRS ...>" does not have associated data`
frame = SkyOffsetFrame(origin=SkyCoord(ra, de), rotation=pa)
Table({'frame': frame})Describe the desired outcome
Add support for adding Frame objects that do not have data as columns in tables.
Additional context
No response
Activity
eerovaher commentedon Aug 13, 2024
Why does the
SkyOffsetFrameneed to be stored in a column? Is there some reason you can't store it in metadata?lpsinger commentedon Aug 14, 2024
Because my data is a series of many SkyOffsetFrames. Each planned observation is a row in the table, with a SkyOffsetFrame that represents the attitude of the telescope, a Time for the start of the observation, an exposure time, an integer to represent the observing mode, etc.
mhvk commentedon Aug 14, 2024
@lpsinger - I agree that should be possible. Looking at the traceback from what you actually want, a problem is that
len(frame)does not work, but really it breaks down earlier:frameshould be recognized as a mixin column, but isn't:And the reason for that is that
SkyOffsetFramedoes not have aninfoattribute. That seems a pure oversight. But oddly, the oversight actually starts withBaseCoordinateFramenot havinginfoeither. As a result,This is clearly wrong: it should have been an ICRS column.
mhvk commentedon Aug 14, 2024
A fix is in
#16381#16831 - was much more painful than I had hoped...lpsinger commentedon Aug 14, 2024
Do you mean #16831?
mhvk commentedon Aug 14, 2024
Indeed, silly, now corrected.