# UCAC 4

## A new version of UCAC catalog is available

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.

## Use with Skychart

A new version is available since March 4 2013 with correction for the following problems with the original data:

- Correction for the high proper motion stars as describe here.
- Removal of streak stars specifically present in the galaxy center area. You can read all the detail about streak removal on this page. In the case you need the full UCAC4 data, including the “streak” you can activate a second catalog in the ucac4_streak folder which contain the removed data.

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-v2-south.zip : South zone -90 to -30
- ucac4-catalog-v2-equator.zip : Equatorial zone -30 to +22.5
- ucac4-catalog-v2-north.zip : North zone +22.5 to +90

A fourth file ucac4-index-v2.zip contain the optional search index for all the zones. Note that you need to use skychart version 3.7-svn2252 or later in order to use the index.

Each file is about 600 MB is size.

Extract every zip in the same directory (Windows: Program Files\Ciel\cat\, Linux: /usr/share/skychart/cat/, Mac: Applications/Cartes du Ciel/cat/). As each zip can be used without the other there is two common files on each, when prompted for replacement by your zip software you can respond yes or no to overwrite without effect.

Then from Skychart open the menu Setup -> Catalog -> Catalog, click the Add button and select the file ucac4\4uc.hdr. Click on the first column to make it green.
Do not forget to save the default options.

Then zoom to a one degree FOV and you must now see the magnitude 16 stars.

My preferred star catalog setting for general use is now:

Catalog | Field number | Field degrees
--- | --- | ---
Extended Hipparcos | 0 - 10 | 0 - 360
4UC | 0 - 5 | 0 - 20

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, the star is put on a separate directory
- 4 non APASS V or UCAC magnitude
- 8 DSO match ( UCAC4 gc flag >= 5 or leda>0 or x2m>0)

## Build your own catalog

If you have a copy of the UCAC4 DVD, or if you download the full u4b folder from the CDS or a web mirror, 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 copy the DVD content to your disk to correct for the error with the high proper motion stars. See USNO UCAC page for more information and to get the script required to correct the binary data.

Then 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. It also remove the streak by using the file streaklist.txt.
You can download u4conv for Windows and Linux or get the source code. Run the program in a console to show the syntax help.
The --cddir option must point to the folder containing u4b/, not the individual data files.

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.
If you use Linux or Mac OS, be sure to read the specific notes in the Catgen documentation page.
Use a snapshot more recent than skychart-3.7-2252 to build an index more than 2GB in size.