Last modified: December 2013

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AHELP for CIAO 4.16


Context: proposaltools


PRECESS is an interactive astronomical coordinate conversion program. It provides precession of equatorial coordinates and conversion between equatorial, ecliptic, galactic, and supergalactic coordinates.


PRECESS can be run over the web using a browser or run locally using the command-line interface (CLI). The CLI version will accept a text file with a list of coordinates as input.

PRECESS is part of the Proposal toolkit. The toolkit is distributed with CIAO.

For details about available coordinate and time systems, please see the prop-time and prop-coords ahelp documents.

Available Coordinate Systems

The available coordinate systems are: Equatorial or Ecliptic with Besselian Epoch (default is B1950, or specify as 'BXXXX'), Equatorial with Julian Epoch (default is J2000, or specify as 'JXXXX'), or Galactic. The CLI also version supports conversions using supergalactic coordinates. PRECESS can be used to identify the Constellation for a given input position.

Running PRECESS Online

PRECESS can be accessed via the URL

General help information is available by clicking the 'HELP' button.

Running PRECESS from the Command Line

The syntax for processing a file of coordinates is:

prop_precess commands:infile:outfile

See the EXAMPLES section for more information.

To run PRECESS in the interactive CLI mode, type 'prop_precess' at the ciao prompt. This enters an interactive command mode for setting program parameters. The command level prompt is Precess[Setup]>:. Within the command level, type '?' or 'help' to see a list of the available commands and coordinate systems, and type 'list' or 'l' to display the current program settings.

The eval option

Running PRECESS with the "eval" option returns you to the CIAO command line when done, instead of remaining within the precess environment:

ciao> prop_precess from j/deg to j/hms p0 eval 12.138 -05.218
  default assignment to config/jcm_data has been made.
ASCDS_PROP_PREC_DATA (for constellations) = config/jcm_data
00 48 33.12 -05 13 04.80

Input Commands

The most usual command sequence is 'from', 'to', and 'convert', which can be abbreviated as 'f', 't', and 'c', respectively. 'from' and 'to' define the Input and Output coordinate systems. The default conversion is from B1950 to J2000. Commands are not case-sensitive.

The 'convert' command enters the processing level. Within the processing level, enter the Input coordinates in the appropriate format, either both coordinates on the same line separated by spaces or a comma, or the x-coordinate on one line and the y-coordinate on the next. The default format for RA and DEC is hh mm and dd mm The default format for the other coordinate systems is decimal degrees.

Other Commands

You can change the format of the Input coordinate system with the commands 'f [system]/DEG' for decimal degrees and 'f [system]/HMS' for hh mm (such as "f B1950/DEG"). You can change the format of the Output coordinate system with the commands 't [system]/DEG' and `t [system]/HMS'.

The 'p' command sets the output display mode: use 'p0' for terse, 'p1' for normal, or 'p2' for verbose display. The default output display mode is p1.

To leave the processing level and return to the command level, type 'q' or 'quit'. Type 'q' or 'quit' at the command level to exit the program.


Example 1

Precess equatorial coordinates from 1950 to 1986.

unix prompt> prop_precess 

-------------------- Precess -------------------- 
You are now in setup mode. Type "c" to enter conversion mode,
"?" to list  setup mode commands, or "q" to quit the program. The
default conversion is from B1950 to J2000.
Precess[Setup]>: f B1950 t B1986
Precess[Setup]>: c

Precess[Conversion mode] 
Enter "q" to return to setup mode

RA (B1950): 16 26 36.7 37 27 54.3

RA,Dec B1950.0    16 26 36.70     +37 27 54.30
RA,Dec B1986.0    16 27 53.51     +37 23 10.23

Example 2

Convert galactic coordinates to J2000 equatorial coordinates. Use the syntax 'f G t J' to set up the conversion.

Precess[Setup]>: f G t J; c 

Precess[Conversion mode] 
Enter coords in degrees (q to quit)
Example: 293.14 -20.14 
Enter "q" to return to setup mode

L: 97.74 -60.18 

Galactic l,b       97.740000    -60.180000 
RA,Dec J2000.0    00 02 33.43  +00 16 44.19 

Example 3

Convert from B1987 decimal degrees to J2000 decimal degrees. Set the printing mode for terse program display.

Precess[Setup]>: f B1987/DEG 
Precess[Setup]>: t J2000/DEG 
Precess[Setup]>: list   

From: RA,Dec B1987.0   Format:  Degrees
To  : RA,Dec J2000.0   Format:  Degrees

Precess[Setup]>: p0
Precess[Setup]>: c

RA  (B1987): 182.663 14.175 

182.828930    14.102684

Example 4

Convert a file of coordinates,, using the syntax 'prop_precess commands:infile:outfile'.

The file contains the following.

00 01 12.3 -00 02 23.4
02 59 59.9 23 12 22.1 
02 23 0 11 23 12 
23 48 48.23 -2 11 14.123 

Convert from B1950 to galactic coordinates, and store the results in the file

unix prompt> prop_precess f B1950 t G:
Opened input file
Opened output file
unix prompt> 

The output file,, contains the following.

RA,Dec B1950.0                  Galactic, l,b
00 01 12.30  -00 02 23.40        98.274959   -60.330944
02 59 59.90  +23 12 22.10       158.171584   -30.327495
02 23 00.00  +11 23 12.00       156.570708   -45.015506
23 48 48.23  -02 11 14.12        90.686763   -60.96294

See Also

colden, dates, obsvis, pimms, prop-coords, prop-time, prop-tools