en:news:nomad

# The NOMAD catalog

The Naval Observatory Merged Astrometric Dataset (NOMAD) contains astrometric and photometric data for over 1 billion stars derived from the Hipparcos , Tycho-2 , UCAC2 , and USNO-B1.0 catalogs for astrometry and optical photometry, supplemented by 2MASS near-infrared photometry.

Thank to the effort of Oleg Maliy we can now use this catalog with Cartes du Ciel!

The catalog is available to download by Bittorrent. It is presented in the form of text file for each zone area (South Polar Distance).
You need to run Catgen to use this data with Skychart but this as the advantage to let you select the data you need for the catalog size you want.
This also make it easy to use this data outside of Skychart.

For each star the files contain:

- NOMAD identifiers.
- A flag for each source catalog.
- Coordinates ICRS, with mean error and source flag.
- Mean epoch of the position.
- Proper motion, with mean error.
- Magnitude B, V, R with indication of origin.
- Magnitude J, H, K from 2MASS
- A flag if the star is a recommended astrometric standard.

## Build a catalog for Skychart

Update June 9 2018: A new torrent with working tracker is now available. This version is also fixed for the two data files that where previously in error.

1. Get the zip file containing the torrent and a sample Catgen project.
2. Download all the torrent, or just the declination zone you need. In this last case beware the zone number are South Polar distance.
3. Extract every .rar file. (Use 7zip or on Linux: ls -1 *.rar | xargs -n1 unrar e )
4. Copy every .txt file to a single directory.
5. This result in a directory with 1800 .txt files for a total size of 152G.
6. On Linux/Mac set “ulimit -S -n 20000” see Catgen documentation
7. Launch Skychart
8. Setup / Catalog / Catgen
9. Load the sample project: Load Project → nomad2.prj
10. Update the input files with your location folder: Input catalog files, click the folder button → select the 1800 .txt files (be patient until it update the list)
11. Click the Next button.
12. On General option page click the Next button.
13. Review the field list. You can add or remove information here. Click the Next button.
14. Output directory → select where to put the catalog.
15. Click the “Build catalog” button.
16. Wait a few hours … (three hours for me). Using the default options the result is 48GB in size.
17. You can now add the catalog to Skychart.

And a final note: Please let the torrents active after you finish to download! We need your upload capacity to make the files available to all on the best condition.