Can't create an AltAz frame with a fixed set of scalar coordinates and an array of obstimes.

Description

A colleague has data from a telescope that just points at the zenith and records the data that it sees along with the time. He asked me how he could find the ICRS coordinates that this telescope was seeing as a function of time. I gave him an example script that I thought should do this, but it failed. Perhaps I’m just doing something wrong here, but I couldn’t see anything in the documentation that said that this can’t be done.

Expected behavior

I expected an array of ICRS positions in RA/Dec.

Actual behavior

It bombed. The only way I could get it to work was to create arrays of alt and az with the same dimension as obstime.

Steps to Reproduce

Here’s the script.

import numpy as np
from astropy.time import Time
from astropy.coordinates import  EarthLocation, AltAz, ICRS
import astropy.units as u

jd = 2459205.5 + np.arange(0.0, 1.0, 0.1)
my_times = Time(jd, format='jd')
print (my_times)

class_lat = -22.959748*u.deg
class_lon = -67.787260*u.deg
class_elev= 5186*u.m
class_loc =  EarthLocation.from_geodetic(class_lon,class_lat,class_elev)

aa = AltAz(az=0.0 * u.deg, alt=90.0 * u.deg, obstime=my_times, location=class_loc)
radec = aa.transform_to(ICRS)
print (radec)

And here’s the output.

[2459205.5 2459205.6 2459205.7 2459205.8 2459205.9 2459206.  2459206.1
 2459206.2 2459206.3 2459206.4]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/attributes.py in __get__(self, instance, frame_cls)
    111                         out = out._apply(np.broadcast_to, shape=instance_shape,
--> 112                                          subok=True)
    113                     else:~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/time/core.py in _apply(self, method, format, cls, *args, **kwargs)
   1055         if apply_method:
-> 1056             jd1 = apply_method(jd1)
   1057             jd2 = apply_method(jd2)~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/time/core.py in <lambda>(array)
   1045         if callable(method):
-> 1046             apply_method = lambda array: method(array, *args, **kwargs)
   1047 <__array_function__ internals> in broadcast_to(*args, **kwargs)~/anaconda3/envs/hera/lib/python3.7/site-packages/numpy/lib/stride_tricks.py in broadcast_to(array, shape, subok)
    179     """
--> 180     return _broadcast_to(array, shape, subok=subok, readonly=True)
    181 ~/anaconda3/envs/hera/lib/python3.7/site-packages/numpy/lib/stride_tricks.py in _broadcast_to(array, shape, subok, readonly)
    117     if not shape and array.shape:
--> 118         raise ValueError('cannot broadcast a non-scalar to a scalar array')
    119     if any(size < 0 for size in shape):ValueError: cannot broadcast a non-scalar to a scalar arrayDuring handling of the above exception, another exception occurred:ValueError                                Traceback (most recent call last)
<ipython-input-6-3711448b41bf> in <module>
     13 class_loc =  EarthLocation.from_geodetic(class_lon,class_lat,class_elev)
     14 
---> 15 aa = AltAz(az=0.0 * u.deg, alt=90.0 * u.deg, obstime=my_times, location=class_loc)
     16 radec = aa.transform_to(ICRS)
     17 print (radec)~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/builtin_frames/altaz.py in __init__(self, *args, **kwargs)
    105 
    106     def __init__(self, *args, **kwargs):
--> 107         super().__init__(*args, **kwargs)
    108 
    109     @property~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/baseframe.py in __init__(self, copy, representation_type, differential_type, *args, **kwargs)
    599                 # Validate attribute b
[2459205.5 2459205.6 2459205.7 2459205.8 2459205.9 2459206.  2459206.1
 2459206.2 2459206.3 2459206.4]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/attributes.py in __get__(self, instance, frame_cls)
    111                         out = out._apply(np.broadcast_to, shape=instance_shape,
--> 112                                          subok=T
[2459205.5 2459205.6 2459205.7 2459205.8 2459205.9 2459206.  2459206.1
 2459206.2 2459206.3 2459206.4]
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/attributes.py in __get__(self, instance, frame_cls)
    111                         out = out._apply(np.broadcast_to, shape=instance_shape,
--> 112                                          subok=True)
    113                     else:~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/time/core.py in _apply(self, method, format, cls, *args, **kwargs)
   1055         if apply_method:
-> 1056             jd1 = apply_method(jd1)
   1057             jd2 = apply_method(jd2)~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/time/core.py in <lambda>(array)
   1045         if callable(method):
-> 1046             apply_method = lambda array: method(array, *args, **kwargs)
   1047 <__array_function__ internals> in broadcast_to(*args, **kwargs)~/anaconda3/envs/hera/lib/python3.7/site-packages/numpy/lib/stride_tricks.py in broadcast_to(array, shape, subok)
    179     """
--> 180     return _broadcast_to(array, shape, subok=subok, readonly=True)
    181 ~/anaconda3/envs/hera/lib/python3.7/site-packages/numpy/lib/stride_tricks.py in _broadcast_to(array, shape, subok, readonly)
    117     if not shape and array.shape:
--> 118         raise ValueError('cannot broadcast a non-scalar to a scalar array')
    119     if any(size < 0 for size in shape):ValueError: cannot broadcast a non-scalar to a scalar arrayDuring handling of the above exception, another exception occurred:ValueError                                Traceback (most recent call last)
<ipython-input-6-3711448b41bf> in <module>
     13 class_loc =  EarthLocation.from_geodetic(class_lon,class_lat,class_elev)
     14 
---> 15 aa = AltAz(az=0.0 * u.deg, alt=90.0 * u.deg, obstime=my_times, location=class_loc)
     16 radec = aa.transform_to(ICRS)
     17 print (radec)~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/builtin_frames/altaz.py in __init__(self, *args, **kwargs)
    105 
    106     def __init__(self, *args, **kwargs):
--> 107         super().__init__(*args, **kwargs)
    108 
    109     @property~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/baseframe.py in __init__(self, copy, representation_type, differential_type, *args, **kwargs)
    599                 # Validate attribute by getting it.  If the instance has data,
    600                 # this also checks its shape is OK.  If not, we do it below.
--> 601                 values[fnm] = getattr(self, fnm)
    602             else:
    603                 setattr(self, '_' + fnm, fdefault)~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/attributes.py in __get__(self, instance, frame_cls)
    118                         "attribute {} should be scalar or have shape {}, "
    119                         "but is has shape {} and could not be broadcast."
--> 120                         .format(self.name, instance_shape, out.shape))
    121 
    122                 converted = TrueValueError: attribute obstime should be scalar or have shape (), but is has shape (10,) and could not be broadcast. rue)
    113                     else:~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/time/core.py in _apply(self, method, format, cls, *args, **kwargs)
   1055         if apply_method:
-> 1056             jd1 = apply_method(jd1)
   1057             jd2 = apply_method(jd2)~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/time/core.py in <lambda>(array)
   1045         if callable(method):
-> 1046             apply_method = lambda array: method(array, *args, **kwargs)
   1047 <__array_function__ internals> in broadcast_to(*args, **kwargs)~/anaconda3/envs/hera/lib/python3.7/site-packages/numpy/lib/stride_tricks.py in broadcast_to(array, shape, subok)
    179     """
--> 180     return _broadcast_to(array, shape, subok=subok, readonly=True)
    181 ~/anaconda3/envs/hera/lib/python3.7/site-packages/numpy/lib/stride_tricks.py in _broadcast_to(array, shape, subok, readonly)
    117     if not shape and array.shape:
--> 118         raise ValueError('cannot broadcast a non-scalar to a scalar array')
    119     if any(size < 0 for size in shape):ValueError: cannot broadcast a non-scalar to a scalar arrayDuring handling of the above exception, another exception occurred:ValueError                                Traceback (most recent call last)
<ipython-input-6-3711448b41bf> in <module>
     13 class_loc =  EarthLocation.from_geodetic(class_lon,class_lat,class_elev)
     14 
---> 15 aa = AltAz(az=0.0 * u.deg, alt=90.0 * u.deg, obstime=my_times, location=class_loc)
     16 radec = aa.transform_to(ICRS)
     17 print (radec)~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/builtin_frames/altaz.py in __init__(self, *args, **kwargs)
    105 
    106     def __init__(self, *args, **kwargs):
--> 107         super().__init__(*args, **kwargs)
    108 
    109     @property~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/baseframe.py in __init__(self, copy, representation_type, differential_type, *args, **kwargs)
    599                 # Validate attribute by getting it.  If the instance has data,
    600                 # this also checks its shape is OK.  If not, we do it below.
--> 601                 values[fnm] = getattr(self, fnm)
    602             else:
    603                 setattr(self, '_' + fnm, fdefault)~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/attributes.py in __get__(self, instance, frame_cls)
    118                         "attribute {} should be scalar or have shape {}, "
    119                         "but is has shape {} and could not be broadcast."
--> 120                         .format(self.name, instance_shape, out.shape))
    121 
    122                 converted = TrueValueError: attribute obstime should be scalar or have shape (), but is has shape (10,) and could not be broadcast. y getting it.  If the instance has data,
    600                 # this also checks its shape is OK.  If not, we do it below.
--> 601                 values[fnm] = getattr(self, fnm)
    602             else:
    603                 setattr(self, '_' + fnm, fdefault)~/anaconda3/envs/hera/lib/python3.7/site-packages/astropy/coordinates/attributes.py in __get__(self, instance, frame_cls)
    118                         "attribute {} should be scalar or have shape {}, "
    119                         "but is has shape {} and could not be broadcast."
--> 120                         .format(self.name, instance_shape, out.shape))
    121 
    122                 converted = TrueValueError: attribute obstime should be scalar or have shape (), but is has shape (10,) and could not be broadcast. 

System Details

Astropy 4.2

[mkbrewer]

Yes. As I stated under “Actual behavior”, that’s what I had to do to get this to work.

az = np.zeros(len(jd))
alt = np.ones(len(jd)) * 90.0
aa = AltAz(az=az * u.deg, alt=alt * u.deg, obstime=my_times, location=class_loc)

I raised this issue as it is my understanding that Astropy should be able to evaluate a single position at multiple times. Perhaps that is a misunderstanding on my part, but I was unable to find anything in the documentation that stated otherwise.

[adrn]

The warning is expected and a result of #10475 - that line just needs to be changed to aa.transform_to(ICRS()).

On the specific issue here, while it’s hard to make broadcasting assumptions when both the coordinate data and the obstime have arbitrary shapes, in this case (with scalar coordinate data) I think it is unambiguous, so I think this is a bug and we should support it! Thoughts @mhvk?