feat(hfbcat): Move get_doppler_shifted_freq from ephemeris (#73)

This isn't going to go with the rest of the ephemeris
code to the new repository, so let's put is somewhere else.

Weirdly, the original docstring for this function seems to
indicate it was meant to go in `tools`, but since its hardcoded
to work work only on the `HFBCatalog` putting it in `hfbcat`
directly seems most intuitive.

The re-constituted stub in `ephermis` avoids the circular import.

ch_util/ephemeris.py

@@ -820,90 +820,18 @@ def peak_RA(body, date=None, deg=False):
     return ra
 
 
-def get_doppler_shifted_freq(
-    source: Union[skyfield.starlib.Star, str],
-    date: Union[float, list],
-    freq_rest: Union[float, list] = None,
-    obs: Observer = chime,
-) -> np.array:
-    """Calculate Doppler shifted frequency of spectral feature with rest
-    frequency `freq_rest`, seen towards source `source` at time `date`, due to
-    Earth's motion and rotation, following the relativistic Doppler effect.
-
-    Parameters
-    ----------
-    source
-        Position(s) on the sky. If the input is a `str`, attempt to resolve this
-        from `ch_util.hfbcat.HFBCatalog`.
-    date
-        Unix time(s) for which to calculate Doppler shift.
-    freq_rest
-        Rest frequency(ies) in MHz. If None, attempt to obtain rest frequency
-        of absorption feature from `ch_util.hfbcat.HFBCatalog.freq_abs`.
-    obs
-        An Observer instance to use. If not supplied use `chime`. For many
-        calculations changing from this default will make little difference.
-
-    Returns
-    -------
-    freq_obs
-        Doppler shifted frequencies in MHz. Array where rows correspond to the
-        different input rest frequencies and columns correspond either to input
-        times or to input sky positions (whichever contains multiple entries).
-
-    Notes
-    -----
-    Only one of `source` and `date` can contain multiple entries.
-
-    Example
-    -------
-    To get the Doppler shifted frequencies of a feature with a rest frequency
-    of 600 MHz for two positions on the sky at a single point in time (Unix
-    time 1717809534 = 2024-06-07T21:18:54+00:00), run:
-
-    >>> from skyfield.starlib import Star
-    >>> from skyfield.positionlib import Angle
-    >>> from ch_util.tools import get_doppler_shifted_freq
-    >>> coord = Star(ra=Angle(degrees=[100, 110]), dec=Angle(degrees=[45, 50]))
-    >>> get_doppler_shifted_freq(coord, 1717809534, 600)
-    """
-
-    from scipy.constants import c as speed_of_light
-
-    from ch_util.fluxcat import _ensure_list
-    from ch_util.hfbcat import HFBCatalog
-
-    # For source string inputs, get skyfield Star object from HFB catalog
-    if isinstance(source, str):
-        try:
-            source = HFBCatalog[source].skyfield
-        except KeyError:
-            raise ValueError(f"Could not find source {source} in HFB catalog.")
-
-    # Get rest frequency from HFB catalog
-    if freq_rest is None:
-        if not source.names or source.names not in HFBCatalog:
-            raise ValueError(
-                "Rest frequencies must be supplied unless source can be found "
-                "in ch_util.hfbcat.HFBCatalog. "
-                f"Got source {source} with names {source.names}"
-            )
-        else:
-            freq_rest = HFBCatalog[source.names].freq_abs
-
-    # Prepare rest frequencies for broadcasting
-    freq_rest = np.asarray(_ensure_list(freq_rest))[:, np.newaxis]
+# For backwards compatibility
+def get_doppler_shifted_freq(*args, **kwargs):
+    """Deprecated.  Use `ch_util.hfbcat.get_doppler_shifted_freq`."""
+    from . import hfbcat
 
-    # Get rate at which the distance between the observer and source changes
-    # (positive for observer and source moving appart)
-    range_rate = get_range_rate(source, date, obs)
+    import warnings
 
-    # Compute observed frequencies from rest frequencies
-    # using relativistic Doppler effect
-    beta = range_rate / speed_of_light
-    freq_obs = freq_rest * np.sqrt((1.0 - beta) / (1.0 + beta))
+    warnings.warn(
+        "Use `ch_util.hfbcat.get_doppler_shifted_freq` instead.", DeprecationWarning
+    )
 
-    return freq_obs
+    return hfbcat.get_doppler_shifted_freq(*args, **kwargs)
 
 
 def get_range_rate(

ch_util/hfbcat.py

@@ -2,9 +2,16 @@
 Catalog of HFB test targets
 """
 
+from __future__ import annotations
 import os
+import numpy as np
+from typing import TYPE_CHECKING, Union
 
-from .fluxcat import FluxCatalog
+from . import ephemeris
+from .fluxcat import FluxCatalog, _ensure_list
+
+if TYPE_CHECKING:
+    import skyfield.starlib.Star
 
 # Define the source collection that should be loaded when this module is imported.
 HFB_COLLECTION = os.path.join(
@@ -79,5 +86,88 @@ def __init__(
         self.freq_abs = freq_abs
 
 
+def get_doppler_shifted_freq(
+    source: Union[skyfield.starlib.Star, str],
+    date: Union[float, list],
+    freq_rest: Union[float, list] = None,
+    obs: ephemeris.Observer = ephemeris.chime,
+) -> np.array:
+    """Calculate Doppler shifted frequency of spectral feature with rest
+    frequency `freq_rest`, seen towards source `source` at time `date`, due to
+    Earth's motion and rotation, following the relativistic Doppler effect.
+
+    Parameters
+    ----------
+    source
+        Position(s) on the sky. If the input is a `str`, attempt to resolve this
+        from `ch_util.hfbcat.HFBCatalog`.
+    date
+        Unix time(s) for which to calculate Doppler shift.
+    freq_rest
+        Rest frequency(ies) in MHz. If None, attempt to obtain rest frequency
+        of absorption feature from `ch_util.hfbcat.HFBCatalog.freq_abs`.
+    obs
+        An Observer instance to use. If not supplied use `chime`. For many
+        calculations changing from this default will make little difference.
+
+    Returns
+    -------
+    freq_obs
+        Doppler shifted frequencies in MHz. Array where rows correspond to the
+        different input rest frequencies and columns correspond either to input
+        times or to input sky positions (whichever contains multiple entries).
+
+    Notes
+    -----
+    Only one of `source` and `date` can contain multiple entries.
+
+    Example
+    -------
+    To get the Doppler shifted frequencies of a feature with a rest frequency
+    of 600 MHz for two positions on the sky at a single point in time (Unix
+    time 1717809534 = 2024-06-07T21:18:54+00:00), run:
+
+    >>> from skyfield.starlib import Star
+    >>> from skyfield.units import Angle
+    >>> from ch_util.hfbcat import get_doppler_shifted_freq
+    >>> coord = Star(ra=Angle(degrees=[100, 110]), dec=Angle(degrees=[45, 50]))
+    >>> get_doppler_shifted_freq(coord, 1717809534, 600)
+    """
+
+    from scipy.constants import c as speed_of_light
+
+    # For source string inputs, get skyfield Star object from HFB catalog
+    if isinstance(source, str):
+        try:
+            source = HFBCatalog[source].skyfield
+        except KeyError:
+            raise ValueError(f"Could not find source {source} in HFB catalog.")
+
+    # Get rest frequency from HFB catalog
+    if freq_rest is None:
+        if not source.names or source.names not in HFBCatalog:
+            raise ValueError(
+                "Rest frequencies must be supplied unless source can be found "
+                "in ch_util.hfbcat.HFBCatalog. "
+                f"Got source {source} with names {source.names}"
+            )
+        else:
+            freq_rest = HFBCatalog[source.names].freq_abs
+
+    # Prepare rest frequencies for broadcasting
+    freq_rest = np.asarray(_ensure_list(freq_rest))[:, np.newaxis]
+
+    # Get rate at which the distance between the observer and source changes
+    # (positive for observer and source moving appart)
+    range_rate = ephemeris.get_range_rate(source, date, obs)
+
+    # Compute observed frequencies from rest frequencies
+    # using relativistic Doppler effect
+    beta = range_rate / speed_of_light
+    freq_obs = freq_rest * np.sqrt((1.0 - beta) / (1.0 + beta))
+
+    return freq_obs
+
+
 # Load the HFB target list
 HFBCatalog.load(HFB_COLLECTION)