The UCAC4 star catalog was published by USNO in 2012. It include 113 millions stars to magnitude 16.

UCAC4 is largely based on UCAC3 however, has a number of critical improvements:

• bug fixes (e.g. missing stars, multiple entries, mag.eq. corrections)
• use NPM data to derive proper motions of faint stars north of -20 Dec
• final tweak of systematic error corrections brings it closer to UCAC2
• photometry in the B, V, g, r and i bands from APASS for 50 mill.stars
• use APASS photometry to calibrate (“flatten”) instrumental mags
• removal of photometric bias as function of CCD x-coordinate
• add brightest stars from FK6, Hipparcos and Tycho-2 catalogs
• link to Hipparcos star numbers and inclusion of Hipparcos 2007 release data like parallaxes
• cross reference to Tycho-2 star numbers

For the use with Skychart the principal improvement are:

• Inclusion of the bright stars, no more need to use Tycho2 with.
• Better photometry and color index.
• About 14% more stars.

The screen shot to the right is for the same field as the one in the UCAC3 page. You can open the two picture in full for a better comparison.

A catalog is build with this data is available for use with Skychart software.
The information for each star include: identifier, RA, Dec, proper motion, magnitude, color index and a flag (see below).

To use this catalog start by getting the part you need from the download page.
It is split in three part so you can get only the zone visible from your area, but you need all for a full sky coverage.

• ucac4-catalog-south.zip : South zone -90 to -30
• ucac4-catalog-equator.zip : Equatorial zone -30 to +22.5
• ucac4-catalog-north.zip : North zone +22.5 to +90

A fourth file ucac4-index.zip contain the optional search index for all the zones. Beware you need a skychart version more recent that 3.7 to use the index.

Each file is about 600 MB is size.

Extract all this zip in the same directory (Program Files\Ciel\cat, share/skychart/cat). As each file is autonomous there is two common file on each, when prompted for replacement you can respond yes or no without effect.

Then from Skychart open the menu Setup -> Catalog -> Catalog, click the Add button and select the file 4uc.hdr. Click on the first column to make it green.
Do not forget to save the default options.

My preferred star catalog setting for general use is now:

The first magnitude available is selected by order of preference:

• APASS V
• UCAC aperture
• UCAC fit model
• APASS g
• APASS B
• APASS r
• APASS i
• 2MASS J
• 2MASS H
• 2MASS K

A color index is indicated only if APASS B and V are available.

Flags are the following:

• 0 OK
• 1 double (UCAC4 combined double star flag is non zero)
• 2 streak, magnitude set to 99 (UCAC4 object type flag = 2 and no magnitude from 2MASS or APASS)
• 4 non APASS V or UCAC magnitude
• 8 DSO match ( UCAC4 gc flag >= 5 or leda>0 or x2m>0)

If you have a copy of the UCAC4 DVD, or if you can download the files from Vizier, you can use Catgen to build your own catalog from the original data files. This allow you to include more or less information for each star depending on your needs.

You must first convert the binary data on the DVD to ASCII text files using the include u4dump program or my u4conv program.
The advantage of u4conv is it process all the files at a time, add the star identifier, a selected magnitude and the color index after the other data.
You can download u4conv for Windows and Linux or get the source code. Run the program in a console to show the syntax help.

Then use Catgen to convert the text files to a binary catalog optimized for chart display. You can start by loading the project file used for the standard catalog.
Use a snapshot more recent than skychart-3.7-2252 to build an index more than 2GB in size.