2 Operating Instructions

To run COCO interactively, on either VAX/VMS platforms or the Sun SPARCstation and DECstation Unix-based platforms, simply type:

The program then accepts commands and outputs appropriate replies.

The primary commands are as follows:

where the parameter p, specifying the coordinate system, is as follows:

Coordinates type 4 and B default to equinox B1950. Coordinates type 5 default to equinox J2000. In all three of these coordinate systems the epoch defaults to the equinox. For coordinate type G the epoch defaults to B1950.

The following commands are also available:

Input is free-format, with spaces separating the fields (comma is also acceptable as a field separator within coordinates). Both upper and lower case letters are acceptable. Blank lines can be input freely, and a comment can be appended to any line by preceding it with an asterisk.

The $\alpha$ format command, F, selects either hours or degrees as the units for $\alpha$, affecting both the input formats which are accepted and the format of the outputs. The command “F D" selects degrees as the $\alpha$ unit, limits the input formats to a single number in degrees for both $\alpha$ and $\delta$, and causes the output formats to be a single number in degrees for both $\alpha$ and $\delta$. The command “F H" switches to the hours format, enabling a variety of sexagesimal input formats as well as plain hours and degrees, and causing the output formats to be h,m,s,d,${}^{\prime }$,${}^{″}$.

On startup, COCO is set to input FK4 B1950 and output FK5 J2000, with $\alpha$ in hours and medium report resolution.

In a typical COCO run, the first step would be to specify the input and output coordinate systems by means of the I and O commands, and then to enter the coordinates to be transformed. For example, suppose that we wish to convert a QSO position measured from a plate taken in mid-1976 and using reference stars from the B1950.0 SAO catalogue to coordinates in the new J2000.0 system. The following commands could be used:

Some additional operating modes are described in a later section.

Notes:

(1)

The results output by COCO are believed to be of more than adequate accuracy for all practical purposes at present and are far more precise than any available star coordinates. Geocentric apparent place is the least accurate form, and is limited (in the worst case) to about 0.3 milliarcsec by the model for Earth velocity and position that is used. The more straightforward conversions are, as implementations of the accepted algorithms, several orders of magnitude better than this figure. It should be noted, however, that there is a lingering debate about the precise formulation of the conversion between FK4 data and the new FK5 system. COCO uses the algorithm published in the 1985 Astronomical Almanac. The differences are far too small to pose problems for non-specialists.

(2)

The three report resolutions provided are referred to simply as L (low), M (medium), and H (high). At resolutions “L" and “M", all the figures output are trustworthy. Resolution “H" is provided mainly to allow comparison with other predictions and to decrease rounding errors where differences are taken.

(3)

COCO is for use only with sources well outside the solar system. Where appropriate, stellar parallax and aberration are allowed for, but the corrections for gravitational deflection assume that the source is distant. In particular, COCO is not suitable for predicting apparent places for the Sun.

(4)

COCO is not intended for the conversion of catalogue data, and reports positions only; updated proper motions etc. are not reported. Full conversion of catalogue data is best done by writing ad hoc programs, using the subprograms in the SLALIB library (see SUN/67).

(5)

COCO accepts both commands and data from up to two sources, called the primary and secondary input files. To switch to a secondary input file called STARS.DAT, for example, would require the command /STARS.DAT. (The switch command itself is available only from the primary input file.) When an E command, or end of file, is detected during input from a secondary file, control reverts to the primary file. The same secondary input file may be processed several times in one COCO run, as in the example of producing the apparent places of a fixed list of stars for a series of dates – the secondary file could be the list of stars and the primary file a series of pairs of commands, each pair specifying a new epoch and then switching again to the secondary file to process all the stars anew.

(6)

All of COCO’s coordinate systems except for geocentric apparent are barycentric, i.e. unaffected by parallax. Mean places which include displacements due to parallax can be handled by working via the intermediary of geocentric apparent place. For example, the following procedure takes a J2000 barycentric place and adds the (small) effect of parallax for a given date:

(a)

Set the report resolution to high.

(b)

Set the input system to J2000.

(c)

Set the output system to geocentric apparent place for the required date.

(d)

Enter the mean place with proper motions and parallax (and radial velocity if available).

(e)

Note the apparent place.

(f)

Set the input system to geocentric apparent, specifying the same date as in step (c).

(g)

Set the output system to J2000.

(h)

Type in the apparent place from step (e). The result will be close to the position entered in step (d), but with the effects of parallax added.