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Stars and Catalogs

In addition to images, GCX handles lists of astronomical objects generally referred to as stars. There are several types of stars, and the various types have different kinds of information attached. They can be grouped in two classes:

  1. Frame stars only have positions within a frame (pixel coordinates). They come into existence by detection, either automatic, or user-directed. There are three sub-types of frame stars:
  2. Catalog stars have world coordinates (right ascension and declination) attached to them, and can also hold photometric information. Catalog stars are either read from a catalog or recipy file, or created interactively by editing another star. There are four sub-types of catalog stars:
Stars are drawn on top of the displayed images. The apearance of the various types of stars can be changed using the options under Star Display Options.

Star Detection

A relatively straight-forward star detection algorithm is implemented in GCX. It will search for local intensity peaks that satisfy the following conditions:

  1. The peak is higher than the local background plus a specified number of standard deviations (usually 6-9). The number of standard deviations is specified in the Star Detection SNR option;
  2. There are at least 4 pixels adjacent to the peak which are above the threshold;
  3. The peak is not too close to another, higher peak;
  4. The star radius (are in which the star is above the background) isn't too large;
The Maximum Detected Stars parameter limits the number of stars that are detected. The whole frame is searched and the bightest stars are kept.

The star detection routine is called by selecting Stars/Detect Sources, or automatically by other operations (WCS fitting and frame alignment). The detection routine will produce detected stars, except when called on an alignment reference frame, in which case it will produce alignment stars.

Another way of creating frame stars is to control-click on or near an otherwise unmarked star image. This will initiate a spiral star search in a region around the the cursor, with the SNR parameter set to a low value (3.0). In this way, even very faint stars that are located near the cursor can be marked. This procedure creates user stars.

Loading Catalog Stars

As their name implies, catalog stars are generally loaded from catalog files. GCX supports three kinds of catalogs: object catalogs, from which the stars are loaded by name (like GCVS, NGC, Messier, IC); field star catalogs, from which the stars are loaded by region of interest; and star files (including recipy files), which are loaded in their entirety, as they presumably contain stars in a region of interest.

As catalog stars are located by world coordinates, the program cannot display any of them if it doesn't have at least an approximate idea of what the image's coordinates are in the real world. The frame must have at least an initial WCS.4.1

If the frame WCS is unset, the program will refuse to load field stars; it will set the initial WCS so that the loaded object is at the center of the frame if a single object is loaded from an object catalog; and it will refuse to load a star file, unless the star file is a recipy that contains target coordinates, in which case the initial WCS will be set from those coordinates.

Object Catalogs

Two formats for object catalogs are currently supported: the .edb format, also used by XEphem, and the .gcx format. The two are treated differently: .gcx files can be loaded in memory and the whole set searched by name; .edb files are searched directly (without loading). The particular edb file to search is selected depending on the object name.

There are two ways of loading stars from object catalogs: Selecting Stars/Add From Catalog will prompt for an object name and load it. If the frame has the OBJECT FITS field set, selecting Stars/Show Target will try to load the object specified in that field. Stars loaded from object catalogs are of the catalog object type.

Field Star Catalogs

Currently, the program supports two field star catalogs directly: GSC (and GSC-ACT) and TYCHO2. It can also load GSC2 objects in the form of star files (see below).

To load stars from GSC or Tycho, the frame has to have at least an initial WCS; then select File/Load Field Stars/From GSC Catalog or File/Load Field Stars/From Tycho2 Catalog.

Options under Star Detection and Search Options control the maximum number of stars loaded and the limiting magnitude. When more stars than the specified limit are found in the catalog, only the brightest ones are loaded. The region searched is slightly larger than the frame's size in world coordinates.

Star Files

GSC-2 files

A file containing GSC2 stars in a specified region can be obtained from
It can be loaded into GCX by selecting File/Load Field Stars/From GSC-2 file. Stars loaded this way are marked as field stars.

Native format files

GCX defines a native format for star files. It is used for object catalogs, photometry recipy and report files, among other things. Stars from any of these files can be read by selecting File/Load Recipy. The native format contains star type information, which is maintained on loads.

Other star files

Many star files are available from various sources, usually in some form of tabular format. GCX has a command-line only import function to convert these to recipy files (see gcx -help for more details). While it can read a limited range of formats, it is relativey easy to either convert a given file to a supported format, or modify an existing conversion routine.4.2

Setting up Catalogs

The program expects the various catalogs to be set up in a certain way. Here are the requirements for the various catalogs.

Setting up GSC

The program reads the GSC or GSC-ACT4.3 catalogs in the compact (binary) form. It can be downloaded in this format from:
Place the downloaded files in a directory (for instance /usr/share/gcx/gsc-act) and set File and Device Options/GSC Location to point to that directory. Make sure all the directory and file names under the gsc dir use all lower case letters.

Setting up Tycho2

The Tycho2 catalog can be downloaded from
We need to download all the tyc2.dat.nn.gz files, unzip them and concatenate together to create one (big) tycho2.dat file, perhaps like this:
zcat tyc2.dat.??.gz >tycho2.dat
After that, set File and Device Options/Tycho2 location to the full path of the file, like for instance:
GCX must have write permissions to the directory where the tycho2.dat file is located, as it needs to create some indexes when the catalog is first accessed.

Setting up Object Catalog Files

To tell the program what native format catalog files to load, set the File and Device Options/Catalog files option to a comma-delimited list of files. The list can contain wildcards and is tilde-expanded. An example entry would be:


All .edb files that are searched must be located in the same directory, specified in File and Device Options/EDB files. Depending on the searched object's name, GCX will look in a specific file:4.4

Objects with names starting in ``M''&Messier.edb  
Objects with names starting in ``NGC''&NGC.edb  
Objects with names starting in ``UGC'' or ``UGCA''&UGC.edb  
Objects with names starting in ``IC''&IC.edb  
Objects with names starting in ``SAO''&sao.edb  
Objects with ending in a constellation name gcvs.edb
Other objects YBS.edb

next up previous contents
Next: World Coordinates Up: GCX User's Manual Previous: Image Files   Contents
Radu Corlan 2004-12-07