SkyCoord.transform_to(other) ignores other.equinox if default

Transforming from one frame to another acts differently depending on whether the destination frame's equinox was its default value or explicitly passed to the constructor.

from astropy.coordinates import SkyCoord,FK5
import astropy.units as u
cfk5_default = SkyCoord(1 * u.deg, 0 * u.deg, frame='fk5')
cfk5_J2000 = SkyCoord(1 * u.deg, 0 * u.deg, frame='fk5', equinox='J2000')
cfk5_B1950 = SkyCoord(1 * u.deg, 0 * u.deg, frame='fk5', equinox='B1950')
print(cfk5_default,cfk5_J2000,cfk5_B1950,sep='\n')

The print claims that both cfk5_default and cfk5_J2000 are equinox J2000

<SkyCoord (FK5: equinox=J2000.000): (ra, dec) in deg
    ( 1.,  0.)>
<SkyCoord (FK5: equinox=J2000.000): (ra, dec) in deg
    ( 1.,  0.)>
<SkyCoord (FK5: equinox=B1950.000): (ra, dec) in deg
    ( 1.,  0.)>

But when an equinox=1950 coordinate is transformed to the cfk5_default frame, the equinox remains 1950.

print("Transformed:")
print(cfk5_B1950.transform_to(cfk5_default))
print(cfk5_B1950.transform_to(cfk5_J2000))

Transformed:
<SkyCoord (FK5: equinox=B1950.000): (ra, dec) in deg
    ( 1.,  -2.32615580e-17)>
<SkyCoord (FK5: equinox=J2000.000): (ra, dec) in deg
    ( 1.64050073,  0.27831554)>

@eteq

This seems to be related to #3106 where @eteq @taldcroft @cdeil @astrofrog agreed that this behavior is what it should be. It is in the transform_to() documentation.

The frame attributes (e.g. equinox or obstime) for the returned object depend on the corresponding attributes of SkyCoord object and the supplied frame, with the following precedence:

Non-default value in the supplied frame
Non-default value in the SkyCoord instance
Default value in the supplied frame

So how can I deal with this and transform from original to other and get something in the other frame? Is there a function that yields a frame with default values made explicit?

Should we add a flag to transform_to(...,taking_destination_defaults=) or make a new function transform_to_taking_destination_defaults() (names to be discussed)?

As it is now:

c0x = c0.transform_to(c1)
assert c0x.is_equivalent_frame(c1)

will sometimes fail.

Also, the documentation block from the SkyCoord.transform_to() documentation should be replicated in the BaseCoordinateFrame.transform_to() documentation.

SkyCoord.position_angle bug fix

Merge pull request astropy#5715 from larrybradley/sigclip

So how can I deal with this and transform from original to other and get something in the other frame? Is there a function that yields a frame with default values made explicit?

The simple solution is to explicitly specify the attributes you care about. But doing this is a generic way is not possible with an existing method, but is straightforward with access to private attributes:

In [48]: c = SkyCoord(1 * u.deg, 0 * u.deg, frame='fk5')

In [49]: print(c._equinox)
None

In [50]: for attr in c.frame_attributes:
    ...:     setattr(c, '_' + attr, getattr(c, attr))
    ...:     

In [51]: print(c._equinox)
J2000.000

In [52]: print(c.equinox)
J2000.000

So maybe we could add a method like harden_attributes which does the simple loop above and has the effect of making "soft attributes" that derive from defaults turn into "hard attributes" which were explicitly set by the user.

SkyCoord.position_angle bug fix

fixed by #5723

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SkyCoord.transform_to(other) ignores other.equinox if default #5722

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SkyCoord.transform_to(other) ignores other.equinox if default #5722

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dmopalmer commented on Jan 22, 2017

dmopalmer commented on Jan 22, 2017

taldcroft commented on Jan 23, 2017

mhvk commented on Mar 4, 2017

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