Coordinate Converter Help

Help for the Coordinate Converter

Single targets
Lists
Separations
Access from programs
Coordinate systems
Acknowledgements
Further assistance

The coordinate converters page provides two simple coordinate conversion utilities: a single target form for individual names/coordinates and a list converter for files that may contrain many positions.

Single targets

Use the form at the top of the page and just enter coordinates or a target name in the input box. Click on the Find Target/Convert Coordinates button. You should get a table giving the coordinates in the most popular coordinate systems. If you use this page for a series of positions, the great-circle distance from the previous target/position will also be displayed.

When you enter coordinates these are assumed to be in the coordinate system specified in the Input Coordinate Type box. This box is ignored when a target name is specified.

A target name is converted to coordinates using the NED and Simbad name-resolver services. The HEASARC keeps a local cache of previously requested names to speed up requests. If your target name is successfully resolved you will get a message indicating the resolver used. When the target name was found in the local cache that will be noted.

You can control the order in which the caches are searched using the name resolver selection. To speed up requests to the name resolvers, the HEASARC keeps a cache of previous requests which it updates periodically. To ensure that the name resolvers are actually queried, uncheck the local caches option. The query may take a second or two longer.

If you would like to see data in a special coordinate system select the desired epoch of the equinox in the Special Equinox box. Data will then be displayed in both Besselian and Julian coordinates for this equinox.

If you want to convert coordinates from a non-standard equinox, select the Special Equinox and also specify Special Equinox as your Input Coordinate Type selection.

Lists

The lists converter is below the single target form. Use it to read a file on your machine and convert the position for each line. Unlike the single target converter only a single output coordinate system and format are displayed.

Each line of your input file should be single target name or coordinate string or a comment. The file may be as long as you like. Blank lines and lines beginning with # are treated as comments and just copied to the output. You can use them to help label the results. Any of the coordinate formats supported in the Web forms may be used. Different lines may use different formats, but all coordinate strings are assumed to be in the input coordinate system.

There are several output options. You can choose sexagesimal or decimal output formats. The former are the default for equatorial coordinates and that later are used for ecliptic and galactic systems. You can choose to display the data in HTML or text formats. By default the program displays the inputs, which resolver was used for names, and any error messages, but you may select to just show the output coordinates. In the later case you'll get a blank line if the program couldn't do the conversion. If you are planning on plugging the converted coordinates into some other document, then using the text format with only the output coordinates displayed may be convenient.

If you have a lot of names in your list, then suppressing the local name caches may dramatically slow your request and will rarely alter the results.

Separations

When you get back the results in the single target converter you can enter a new target or coordinate for conversion. When you enter a second and subsequent positions, the separation from the previous position is displayed after the conversions of the current position. You can also use this to compute the distance between the positions returned by different name resolvers. E.g., as of March 2006, the NED and SIMBAD positions for Abell 1000 differ by about 0.1'.

The lists converter gives the separations between the first position found and all subsequent postions.

Access from programs

The single target converter may be used by software and has an output mode specifically designed to be simple to parse by programs. The CGI script uses the following parameters:
CoordVal
The name of an object or a set of coordinates. Coordinates can be specified in a variety of formats. E.g., '10 00 00', '10:00:00', '10h00m00.0s' '150.0', and '150d 00m 00s' can all be used (without the quotes). You may need to use a comma, or specify the sign of the declination to help the parser distinguish the RA and declination (or galactic latitude and longitude). Where the angle unit is not given explicitly RA's given in a sexagesimal format are assumed to be in hours. All other coordinates are assumed to be in degrees. Other examples are given on the web form directly under the CoordVal entry box.
CoordType
The type of the coordinate system (J2000, B1950, Galactic, Ecliptic or Special Epoch)
Epoch
The epoch of the equinox to be used as the special coordinate system. If an Epoch is specified the coordinates will be displayed in the Julian and Besselian coordinates of that epoch. If "Special Epoch" is chosen as the CoordType, then this epoch will be the input epoch for the data. The epoch may be preceded by a B or J to indicate a Besselian or Julian input epoch.
Output
If this is specified as Batch the results are displayed using a text/plain mime type as series of simple pipe-delimited strings.

Coordinate systems

J2000
Eulerian coordinates in Equinox 2000 using the FK5 reference frame.
B1950
Besselian coordinates in Equinox 1950 using the FK4 reference frame. This is not a fixed frame and no corrections are applied for proper or radial motion. The epoch of observation is assumed to be 1950. Errors introduced by these assumptions are typically less than 0.5".
Galactic
Galactic coordinates.
Ecliptic
Ecliptic coordinates. These assume an equinox and epoch of 2000.
Special Epoch
If the Special Epoch box is filled with a valid epoch a second table giving the Julian and Besselian coordinates for the given epoch year is displayed.

Acknowledgements

The coordinate converter uses a Perl translation of the StarLink Astrometry library (SLALIB) developed by Pat Wallace. Errors in the Perl transcription are the responsbility of the HEASARC. Errors in the mathematical transformations among coordinate systems should be small compared to the uncertainties in the definitions of those coordinate systems.

This program uses the FK4/FK5-compatible precession models rather than the latest (IAU 2000 and IAU 2006) models. As a consequence, accuracy falls off slightly for epochs remote from 2000. Errors are less than 0.1 arcsecond within 40 years and less than 1 arcsecond within 360 years of 2000.

Errors in the resolution of names to coordinates will vary considerably -- especially for extended objects where the meaning of a precise position may be unclear.

Further assistance.

For further assistance, to report bugs or request enhancements please use our Feedback form and select "Other web tools" from the pull-down menu.
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