API Reference

CHIME ephemeris routines.

Instrument Observrers and General Ephemeris Routines

Any ephemeris routine which needs to know the location of the observer on the Earth are accessible via instrument Observer instances importable from ch_ephem.observers():

>>> from ch_ephem.observers import pathfinder
>>> pathfinder.solar_transit(...)

The Observer instances provided by this module are subclassed from caput.time.Observers and can be used as normal caput observers. Location and geometry data for the instrument observers are defined in the data file instruments.yaml provided as part of ch_ephem.

Celestial Intermediate Reference System

The precession of the Earth’s axis gives noticeable shifts in object positions over the life time of CHIME. To minimise the effects of this we need to be careful and consistent with our ephemeris calculations. Historically Right Ascension has been given with respect to the Vernal Equinox which has a significant (and unnecessary) precession in the origin of the RA axis. To avoid this we use the new Celestial Intermediate Reference System which does not suffer from this issue.

Practically this means that when calculating RA, DEC coordinates for a source position at a given time you must be careful to obtain CIRS coordinates (and not equinox based ones). Internally using ch_ephem.coord.object_coords does exactly that for you, so for any lookup of coordinates you should use that on your requested body.

Note that the actual coordinate positions of sources must be specified using RA, DEC coordinates in ICRS (which is roughly equivalent to J2000). The purpose of object_coords is to transform into new RA, DEC coordinates taking into account the precession and nutation of the Earth’s polar axis since then.

These kind of coordinate issues are tricky, confusing and hard to debug years later, so if you’re unsure you are recommended to seek some advice.

Radio Source Catalogs

This package provides several radio source catalogues used by CHIME. The standard radio source catalogue used by CHIME is available as ch_ephem.sources.source_dictionary:

>>> from ch_ephem.sources import source_dictionary
>>> source_dictionary['CAS_A'].ra
<Angle 23h 23m 27.94s>

The four standard CHIME cailbrators are also available by name:

>>> from ch_ephem.sources import CasA, CygA, VirA, TauA
>>> CasA.ra
<Angle 23h 23m 27.94s>

Additional catalogues which are not part of the standard radio source catalogue are also available through the ch_ephem.catalogs module. Any catalog can be accessed using ch_ephem.catalogs.load(), which takes a catalogue name and returns a dict containing the parsed JSON representation of the catalog:

>>> import ch_ephem.catalogs
>>> ch_ephem.catalogs.list()
['atnf_psrcat', 'hfb_target_list', 'primary_calibrators_perley2016',
 'specfind_v2_5Jy_vollmer2009']
>>> perley2016 = ch_ephem.catalogs.load('primary_calibrators_perley2016')
>>> perley2016["CAS_A"]["ra"]
350.86642

Submodules

Note: the submodules observers and sources read data from disk at import time.

`catalogs`	CHIME ephemeris source catalogues.
`coord`	CHIME co-ordinate transformations.
`observers`	Observers for CHIME instruments.
`pointing`	CHIME ephemeris pointing routines.
`sources`	CHIME source objects.
`time`	CHIME ephemeris time routines.