Coordinate Strings

The ephemeris programs make use of what I call Coordinate strings which have the following keyword=value space delimited format: An example is:

ra=12:29:06.7 dec=2:03:09 equinox=2000 name=3c273

The keywords are the following:

ra- the right ascension in hours:minutes:seconds or decimal degrees (range: 0-360 degrees)
dec- the declination in degrees either degrees:minutes:seconds or decimal degrees (range: +/- 90 degrees)
equinox- the equinox in Julian decimal years (or J2000,B1950, or "now") of the coordinate system (default: J2000)
epoch- the time that the coordinate refers to in Julian decimal years (default: current time)
name- a string describing the object without spaces (default:none)
mag- a string describing the magnitude (system not defined)(default:none)
over- whether over the pole (0-no 1-yes)(default:0)
cat- a string describing the name of the catalog that the coordinate is derived from. (default:none)

pmra- the proper motion of the star in Right Ascension expressed in milliarcseconds per year (def:0)
pmdec- the proper motion of the star in declination expressed in milliarcseconds per year (def:0)
par- the parallax of the star in milli arc-seconds (def:0)
rv- the radial velocity in km/sec (def:0)

mode- modifier specifying the mode of the coordinates (see below). Optional as it is guessed from other parameters
ha- the hour angle in hours, minutes and seconds or decimal degrees (range: +/- 180 degrees)
basis - RA origin, the default is equinox, but "cio" is acceptable if ERA is used instead of LST

alt- the altitude in decimal degrees
az- the azimuth in decimal degrees

Conventions and Defaults

The coordinates can be entered in three styles:

ra and dec - if alt, az or ha appears after it is ignored
ha and dec - if alt or az appears after it is ignored
alt and az - no ra and decs can appear in the string

The defaults may depend on the program and the style (sorry).

equinox - is usually 2000.0 if not specified (this differs from my earlier use)
epoch - the current date and time expressed as a decimal (Julian) year
mode- depends on program but is often astrometric unless one of the below
mode=observed if ha/dec or alt/az style is used

Important: HA and RA are assumed to be hour:minutes:seconds unless a period appears before the first colon at which point they are assumed to be in degrees. Declination is assumed to be degrees:minutes:seconds unless a period appears before the first colon at which point it is assumed to be decimal degrees. Minutes and second fields may be omitted for all HA,RA and Dec. Names and other identifiers cannot have spaces, use www conventions if needed.

Modes:

In the following I treat ICRS as the same a J2000 ignoring the 20mas difference between the two.

icrs: uses the ICRS system. In this software it does not treat the very small difference between ICRS and J2000. It may include pmra,pmdec,rv and par with epoch.

astrometric: similar to icrs, but the epoch may be adjusted to the current time and the proper motions have been included. The equator and equinox are of J2000 / ICRS. Aberration and refraction are NOT included.

observed: the coordinate that the object would be observed with a perfect telescope mounted at the site. It has the equinox and epoch of the current time. It has been corrected for everything except telescope pointing errors, i.e. proper motion, parallax, aberration (diurnal too) and refraction. This coordinate is referred to the true equator and equinox of the moment. This is similar to topocentric but with refraction added. The basis maybe referred to the equinox or cio (I do not use the latter).

geocentric: adjusted to a different epoch and equinox as viewed from the center of the earth. Corrections for aberration, precession and nutation have been made. None for refraction. The basis maybe referred to the equinox or cio.

topocentric: similar to geocentric, but with diurnal aberration included. The basis maybe referred to the equinox or cio.

null: this means the mode has not been determined or specified.

Discussion

Many of the distinctions here are irrelevant from a telescope pointing standpoint as the angular changes are far less than the pointing errors.

Contrary to my earlier usage, these strings are much more pedantic in the use of the word epoch. It is never used to specify the year of the coordinate system, that is the job of the equinox keyword.

For most programs the default mode is astrometric A minimal string would be:

ra=10 dec=20

Which formally would be (in mid 2019)

ra=10:00:00.00 dec=+22:00:00.00 equinox=2000.000 epoch=2019.612

Note the equinox=2000 and epoch is the current time, the mode is not specified.

Note: the distinction between decimal and Babylonian is subtle and depends on a decimal point in the first field and no colons. Therefore:

ra=10. dec=20  -->  ra=00:40:00.00 dec=20:00:00.00 equinox=2000.000 epoch=2019.662

The over keyword forces the telescope to go over (=1) or under (=0) the pole.

Other keyword value pairs such as mag= dist= are ignored as input but printed out when displayed the end of the program. This is not of commands like the telescope pointing that use other special keyords along with the coordinate string.

Examples

ra=10:23 dec=34:44 equinox=J2000     #J2000 is converted to 2000.0 (not really needed)
ra=10:55 dec=-55:22 equinox=now      #uses todays equinox
ra=103.3 dec=44                      #decimal degrees (usually ICRS)
ha=10.   dec=44.                     #points to hour angle=10 degrees dec=44 degrees (observed)
ha=10    dec=44.                     #points to hour angle 150 degrees dec=44
alt=23.3  az=55.0        
alt=90                               #point straight up (observed) special case az not needed
dec=90                               #point to pole (observed) special case ha not needed

Calling index