The Third USNO CCD Astrograph Catalog
Readme File

The third U.S. Naval Observatory CCD Astrograph Catalog (UCAC3)

Table of contents

1) Introduction

2) Summary of the project
  a) Observations and instrumentation
  b) Reductions of UCAC observations
  c) Computation of proper motions

3) Properties of the catalog and important notes for the user
  a) Sky coverage
  b) Completeness
  c) Reference frame 
  d) Magnitudes
  e) Additional photometry
  f) Non-stellar data
  g) Provided utility software
  h) Arrangement of the data files
  i) Star identification numbers

4) Files contained on the DVD
  a) Main catalog data
  b) Other files 

5) Data formats
  a) Main catalog zone files 
  b) Other files 

6) Description of utility software

7) Acknowledgments

8) References


1) Introduction
This is the "readme" file of the third U.S. Naval Observatory CCD
Astrograph Catalog, UCAC3.  This data release is available free of
charge on a double sided DVD or through astronomical data centers.

UCAC3 is a compiled, all-sky star catalog covering mainly the 8 to 
16 magnitude range in a single bandpass between V and R.  Positional 
errors are about 15 to 20 mas for stars in the 10 to 14 mag range.  
It is supplemented by proper motions and SuperCosmos and 2MASS
photometric data, as well as various flags.

The proper motions of bright stars are based on about 140 catalogs,
including Hipparcos and Tycho, as well as all catalogs used for the 
Tycho-2 proper motion construction.  Proper motions of faint stars are 
based on a re-reduction of early epoch SPM data (-90 to -10 deg Dec) 
plus Schmidt plate data from the SuperCosmos project (down weighted due
to systematic errors of order 100 mas).  The proper motions of faint 
stars (R >= 13.5) therefore should be used with caution. The unpublished
plate measure data from the AGK2, the Hamburg Zone Astrograph, the USNO
Black Birch Astrograph, and the Lick Astrograph have considerably
contributed to improve proper motions for stars mainly in the 10 to
14 mag range (down to the UCAC3 limit for Lick data); however, these
data do not cover all sky.

UCAC3 features a number of major differences with respect to UCAC2:
 - complete sky coverage
 - re-reduction of the pixel data with better modeling
 - double stars are resolved to the limit of the data
 - significantly improved photometry from CCD data
 - slightly deeper limiting magnitude with larger number of stars/area
 - reduced systematic errors of CCD observations
 - the addition of several new catalogs for improved proper motions
 - photometry in the B, R, and I bands from the SuperCosmos project
 - minor planet observations have been sorted out
 - identification of more high proper motion stars 
 - match with 2MASS extended sources and LEDA galaxies

A UCAC4 release is anticipated in the future. The plan is to utilize
a new reduction of the NPM plate material and benefit from global
iterative solutions to further reduce possible systematic (zonal) 
errors and provide a strong link to the extragalactic reference
frame.  The UCAC4 release also might make individual positions at
observational epochs public, and may serve pixel data as well.

Additional details will be published in the upcoming release paper
(Zacharias et al. 2009) and in several technical papers describing
details of the reduction procedures and results.  For the latest 
updates see .  The release
paper should be sited whenever UCAC3 data is utilized.  Requests for
the data DVD should be sent to ucac3*at*, technical questions
can be addressed to nz*at* .  The DVD will be sent to all
addresses on the UCAC2 distribution list as soon as our resources allow.

2) Summary of the Project

2a) Observations and Instrumentation
The UCAC is an observational program, using the U.S. Naval Observatory
Twin Astrograph and a 4k by 4k CCD camera, covering just over one square
degree per frame with a scale of 0.9"/pixel.  The red-corrected, 20 cm
aperture, 5-element lens of the astrograph provides a 9 degree (diameter)
field of view (designed for photographic plates), thus only a fraction is
utilized with our CCD camera, centered on the optical axis.

The CCD is a thick, Kodak device with 9 micrometer pixel size.  The
camera, made by Spectral Instruments, is Peltier cooled to -18 C.
The raw data are severely affected by a low charge transfer efficiency
(CTE) of our otherwise cosmetically excellent CCD chip.  To mitigate 
this problem, a relatively warm operating temperature is used, causing 
a significant dark current.

Observations started in January 1998 at Cerro Tololo Interamerican
Observatory (CTIO) in Chile, where the entire southern sky and about 
half of the northern sky were observed.  In October 2001 the instrument
was moved to the Naval Observatory Flagstaff Station (NOFS) in Arizona
where it completed the northern sky in 2004.  

A 2-fold, center-in-corner overlap pattern was adopted on a 0.5 degree
grid, starting at the South Celestial Pole.  Each field was observed
with both a long (100 to 150 sec) and a short (20 to 30 sec) exposure.
Extensive quality control routines led to the rejection of over 15% of
the frames taken.  All raw (pixel) data were saved and continue to be

Observations were made in a single bandpass (579-642 nm), thus the UCAC
magnitudes are between Johnson V and R.  No attempt has been made to
obtain high quality, photometric data from the CCD observations.  In
fact, observations were made during nights with thin cirrus clouds.

The telescope was actively guided with an ST-4 autoguider mounted behind
the second, visually-corrected lens of the twin-astrograph.  Operation
was automated using a PC and a single board computer.  An HP-Unix
workstation was used for on-line reductions to obtain quality control
statistics in near real-time.

2b) Reductions of UCAC Observations
The UCAC3 positions of the CCD observations are based on the Tycho-2
reference stars, similar to the UCAC2 catalog.  However, the 2MASS
was utilized to probe for systematic errors in the CCD data, 
particularly magnitude equations and coma-like errors caused by
the poor CTE of the detector.  Image centroiding is based on a modified
Lorentz profile model which matches the observed PSF better than a
Gaussian function.  For details, please see upcoming journal papers.  

Pixel processing included double star fits and real aperture photometry.
"Flip" observations (telescope on West and East side of pier for same
calibration field) were utilized to fix the overall magnitude equations.

Saturated images of stars were propagated through the output catalog,
so UCAC3 contains many more bright stars than UCAC2.  However positional
results are not as reliable as for unsaturated images and the user is
urged to pay attention to flags and the meaning of various data columns.

2c) Computation of Proper Motions
Proper motions of bright stars (R ~8 to ~12.5) were derived using a
combination of ground-based photographic and transit circle catalogs,
and included satellite observations from the Hipparcos and Tycho-2 
catalogs.  In addition, the U.S. Naval Observatory measured about 
5000 astrograph plates on the StarScan machine to derive about 9 million 
positions for stars mainly in the 10 to 14 magnitude range, including 
the complete set of AGK2 (Bonn and Hamburg zones, +90 to -2.5 deg Dec).

For the faint stars (~12.5 to ~16.5), data from the first epoch plates
of the Yale Southern Proper Motions (SPM, van Altena et al. 1999) are 
utilized.  These plates were measured on the Precision Measuring Machine 
at USNO Flagstaff Station by D. Monet.  A complete re-reduction of the
data were performed in a joint USNO - Yale University effort utilizing
the StarScan pipeline for the pixel reductions and the Yale software
to obtain RA,Dec coordinates.  Because the re-reduction of the NPM data 
is still in progress, the SuperCosmos data (Hambly et al. 2001a,b,c) 
were included in UCAC3 to provide proper motion coverage of faint stars,
particularly in the north.  For this purpose stars with magnitude R <= 17
and all sky were extracted from a copy of the SuperCosmos "source" data.
Due to zonal (plate pattern) systematic errors in the range of 50 to 
200 mas, as compared to 2MASS data, the SuperCosmos positions at their 
mean epoch were downweighted with an additional 100 mas RMS error 
contribution.  Other catalogs were handled similarly, with somewhat 
lower additional error contributions.  The number of stars in UCAC3
which are also in major other catalogs used for the proper motions
is as follows:

  numb.stars cat ID and name
      65392  1  Hipparcos
    2386607  2  Tycho-2
    4098873  3  AC2000
     270823  4  AGK2 Bonn
     960074  5  AGK2 Hamb
    4320925  6  ZA
    2970383  7  BY
    1043857  8  Lick
   85563642  9  SuperCosmos
   51112855 10  SPM YSJ1
      51297  -  high proper motion stars from external catalogs
   98114307  -  2MASS
  100766420  -  all stars in UCAC3

For more details please see the upcoming journal papers.

It is hoped that this approach reduced systematic errors in UCAC3
proper motions of faint stars as compared to the procedures adopted
for UCAC2.

There are 51297 tagged previously known high proper motion stars in
the UCAC3 release.  While we have made an effort to identify
previously known high proper motion stars, the list is not comprehensive.
The high proper motion stars were identified by utilizing known proper
motion catalogs to match with our UCAC3 positions. In all cases the proper
motion given for each star come from the source catalog.  In the North we
used the LSPM-North Catalog (Lepine+ 2005) of 61977 new and previously
known high proper motion stars having proper motions greater than
0.15"/yr.  In the South we used many smaller surveys along with the
Revised NLTT Catalog (Salim+, 2003), which produced 17730 unique high
proper motion stars greater than 0.15"/yr. In chronological order, the 
smaller southern surveys used include: seven papers covering various 
portions of the southern sky by Wroblewski and collaborators (Wroblewski 
& Torres 1989, 1991, 1994, 1996, 1997; Wroblewski & Costa 1999, 2001),
(2) UK Schmidt Telescope survey plates of 40 survey fields by Scholz and 
collaborators (Scholz et al. 2000), (3) The Calan-ESO survey (Ruiz et al.
2001) (4) SuperCOSMOS-RECONS proper-motion survey of the entire southern 
sky (Henry et al. 2004; Subasavage et al. 2005a, 2005b; Finch et
al. 2007), (5) the Southern Infrared Proper-Motion Survey (SIPS;
Deacon et al. 2005) and (7) Lepine's SUPERBLINK survey of a portion of
the southern sky (Lépine 2008).)
Identified images of high proper motion stars in UCAC data were
cut from the remaining pipeline and reduced separately.  Results
for those stars were then added in a later stage of the reductions.
This avoids creating duplicate entries in UCAC3.

The computation of proper motions is performed similarly to the procedure
used for the UCAC2 and the Tycho-2 catalogs.  All input catalogs were
reduced to the ICRF utilizing Hipparcos data or some denser, interim
catalog that follows the system of Hipparcos.  Standard errors for
each position are estimated. These error estimates are used as weights
to compute a mean position and proper motion, which utilizes a weighted,
least-squares adjustment procedure.  Error estimates of UCAC3 astrometry
are provided.

Errors in proper motions of the bright stars (to R ~12) run from about
1 to 3 mas/yr mainly due to the large epoch spans involved.  For the
fainter stars using SPM data, typical errors are 2 to 3 mas/yr, while
with SuperCosmos early epoch data alone proper motion errors are about
6 to 8 mas/yr (as a conservative estimate).

Contrary to UCAC2, not all stars in UCAC3 have proper motions.  Stars 
made it into the UCAC3 release catalog if any one of the following applies:
 - at least 2 matching positions from different CCD frames were obtained,
 - star was matched with a 2MASS entry,
 - star was matched with a SupeCosmos entry,
 - proper motions could be obtained in combination with any other
  early epoch data.

A master list of exactly 139,134,800 mean positions from CCD data was
matched against the various other catalogs, resulting in exactly
100,766,420 objects actually provided in this UCAC3 data release. 
Of these 51,297 are identified, known high proper motions stars.

3) Properties of the catalog and important notes for the user

3a) Sky coverage
UCAC3 is an all-sky catalog with at least about 40 stars per square 
deg anywhere on the sky.  The average density of this catalog is over
2000 stars per square deg. 

3b) Completeness
UCAC3 is more complete than UCAC2, including previously omitted "problem"
stars and double stars, many of which could be new discoveries.  A paper 
is in preparation about a sample of new double stars found in UCAC3 and 
the rate of confirmation by speckle observations.  The separation limit 
for double stars in UCAC3 varies as function of brightness of the 
components and brightness difference.  However, some doubles with 
separations of only 1 arcsec in UCAC3 have been confirmed as real.  
Stars fainter than about R = 10 and separations larger than 2 arcsec 
are well covered.

UCAC3 also contains observations of some bright stars, as they happened
to make it through the pipeline.  However, UCAC3 is not supplemented
by a bright star catalog and is incomplete at around magnitude R=7.5
and brighter.  Some positions are based on a center-of-mass centroiding
when the least-squares fit did not succeed.  The positions of those
stars (number of used images, nu = 0) are not reliable and are given
only to indicate a problem area in this area of the sky.

If a star's proper motion was computed to be larger than 300 mas/yr
in either coordinate, but it was not found in a high proper motion star
catalog, the proper motion was set to zero and only the mean observed
CCD position is given in UCAC3.

Over 95 million stars in UCAC3 have a computed proper motion;
for 45.4 million of these the proper motion is based on only 2 epochs,
while for 49.5 million stars at least 3 epochs were available.

If the computed position error of a star exceeds 500 mas in either
coordinate it was set to 500 mas but the star was kept, if at least
2 observations from different CCD observations were matched or the
star is either in the 2MASS, SPM or SuperCosmos data files.
Similarly, the error in proper motion was truncated to 50 mas/yr
but respective stars kept in UCAC3 by the same criteria as for large
positional errors.  Obviously all large error objects need to be
handled with caution, and some of these will just be fake.
This approach, different to UCAC2, was taken in order to drive up the
completeness of UCAC3 at the expense of slightly greater contamination.
In particular, the larger limits in errors deemed acceptable for UCAC3
accommodate the fainter limiting magnitude, which was possible mainly
due to superior handling of dark subtraction in the pixel data.
This allowed many low signal-to-noise, real stars to enter the
catalog, although with expected larger random errors.

3c) Reference frame
The astrometry provided in UCAC3 is on the Hipparcos system, i.e.
the International Celestial Reference System (ICRS), as represented
by the Tycho-2 catalog.  Any possible deviations of the Tycho system
with respect to Hipparcos or the extragalactic radio reference frame
are under investigation for UCAC4. 

Positions in UCAC3 are given at the standard epoch of Julian date 2000.0,
thus the UCAC3 is a compiled catalog.  In order to be able to calculate 
positional errors at any epoch, the central epoch, i.e. the weighted mean 
epoch of the data (UCAC + early epoch) is given.  At the central epoch 
(which varies from star to star and is also different for RA and Dec) 
the positional error has its smallest value, the one given in the catalog
for "sigma position".  In most cases this central epoch will be close to 
the UCAC observational epoch due to the relatively large weight given to 
the UCAC observations.  However, a fair number of stars have a vastly
different mean epoch, ranging back to about 1900.

The proper motions are given at the central epoch.  Positional errors
of stars increase according to the errors in the proper motions when
going forward or backward in time from the central epoch.

3d) Magnitudes
UCAC3 covers the magnitude range of about R = 7.5 to 16.3 in a 
579-642 nm bandpass.  This is between visual (V) and red (R).  The 
limiting magnitude can vary by about +-0.3 mag.  The bright magnitude
cut-off varies even more, depending on sky conditions at the time 
of observation.

UCAC3 gives centroid fit-model magnitudes as well as aperture photometry
derived from the new pixel data reductions.  Systematic errors in these
magnitudes are believed to be below 0.1 mag, which is a significant
improvement over the UCAC2 release.  Tycho-2 stars, except for the
faint end, were used to perform differential photometric reductions,
CCD frame by frame, including a zero-point and a linear color term.
However, UCAC observations were performed in non-photometric sky
conditions.  Mean magnitudes were derived from CCD frames with
indications of "good photometric quality", which is about 50% of the 

3e) Additional photometry
For convenience to the user, the UCAC3 observational data are 
supplemented with plate photometry (B, R, I) from the SuperCosmos
project (Hambly et al 2001), as well as with new, IR photometry from 
the Two Micron All Sky Survey, 2MASS (Scrutsky 2006).  In addition 
to the J, H, and K_s magnitudes, rounded errors and some flags are 
provided as well.
For more details see  and .

3f) Non-stellar data
The UCAC3 contains some galaxies, particularly at the faint end.
No flag indicating a galaxy or star has been derived from the CCD
pixel data; however, a cross reference to the LEDA galaxies (Paturel
et al. 2005) and the 2MASS extended source catalog (Skrutskie et al.
2006) has been made and corresponding information is put into the 
UCAC3 release.

Observations of asteroids have been identified and taken out of the
main UCAC3 catalog.  Individual epoch observations of about 1000
asteroids at multiple epochs have been prepared for publication, but 
are not part of the UCAC3 release. The observing schedule explicitly 
avoided all minor planets brighter than about R = 12, except for 
special observing campaigns to derive masses of asteroids.

3g) Provided utility software
Only some basic access code is included with this release to 
illustrate the data format and allow conversion of the main
binary files to ASCII tables.  It is anticipated that 3rd party
companies specialized in providing interfaces between various
star catalogs and the user will integrate UCAC3 data into their
products, as has been the case for UCAC2.

3h) Arrangement of the data files
The main catalog data are arranged in declination zones of 0.5 degree 
width, with zone number 1 starting at the South Pole and increasing  
toward north.  There is a separate file for each of the 360 zones.
Records in the individual data files are sorted by right ascension. 
Each record contains the data for one star, 84 bytes of binary integers.

3i) Star identification numbers
Official UCAC3 star ID numbers should be utilized for identification
purposes and for communication with the UCAC team.  The main identifier
is the running star number in the CCD mean position data (MPOS number)
which is provided on the UCAC3 individual star records as the last
4-byte integer item.  This number spans the range of 3 to 140051297
but includes gaps for stars which did not make it into the release.
MPOS numbers over 140 million identify high proper motion stars,
which have been handled differently in the pipeline and were merged
at a later stage into the release.

Alternatively, a unique running star ID number without any gaps can
be generated on the fly, counting stars along zones from south to
north and along RA within each zone, similarly to what was recommended
for UCAC2.  

  ID = n0 + running star number in zone z

where z is zone number where the star in question is located
(1 to 360; zone 1: -90 to -89.5 decl., ...), n0 is the largest star
ID number of the previous zone and ID is this official UCAC3 star
ID number.  A table is provided listing some statistics of zones,
which include this n0 number.

If needed, this 9-digit star ID number should be given in connection
with the IAU sanctioned acronym of this project and this data release
number, such as "3UCAC100200300". The star ID number runs from 000000001
to 100766420.  Leading zeros are to be used to fill 9 digits in the
star name.  

4) Files contained on the DVD
The DVD does not contain sub-directories; all information is provided
in files on the top level for each side of the double-sided DVD.

4a) Main catalog
The main catalog data are contained in zone files "z001" to "z360".
Each zone is 0.5 degree wide, beginning at the south celestial pole.
The format of these binary data files is explained below.

4b) Other files
readme_u3   = this file
table_zones = table listing info for each zone
u3index.asc = index file for each 0.1 hour in RA, ASCII text file
u3index.unf = index file for each 0.1 hour in RA, unformatted binary 
u3read.f    = Fortran code to read UCAC3 zone files
u3read.dat  = sample data ASCII output file generated with u3read.f

5) Data formats

5a) Main catalog zone data files

sample code to illustrate data format (Fortran 77)

* items to read from input files
      INTEGER*4 i, ra,spd, pmrac,pmdc, id2m, rn
      INTEGER*2 im1,im2,sigmag, sigra,sigdc, cepra,cepdc,sigpmr,sigpmd
     .         ,jmag,hmag,kmag, smB,smR2,smI
      INTEGER*1 objt,dsf, na1,nu1,us1,cn1, clbl,qfB,qfR2,qfI
      INTEGER*1 icqflg(3), e2mpho(3), catflg(10), g1,c1,leda,x2m

      OPEN (20,FILE=fnu3,ACCESS='direct',RECL=84) ! 1 record per star
      i = 0
        i = i + 1
        READ (20,REC=i,ERR=99)                    ! number of bytes
     .     ra,spd, im1,im2,sigmag, objt,dsf       !  8 + 6 +  2  = 16
     .    ,sigra,sigdc, na1,nu1,us1,cn1           !  4 + 4       =  8
     .    ,cepra,cepdc, pmrac,pmdc, sigpmr,sigpmd !  4 + 8 +  4  = 16
     .    ,id2m, jmag,hmag,kmag, icqflg, e2mpho   !  4 + 6 +  6  = 16
     .    ,smB,smR2,smI, clbl,qfB,qfR2,qfI        !  6 + 4       = 10
     .    ,catflg, g1,c1, leda, x2m, rn           ! 10 + 4 +  4  = 18
 99    WRITE (*,'(a,i9)') 'last record number on file = ',i-1

num item   fmt unit        explanation                            remark
 1  ra     I*4 mas         right ascension at  epoch J2000.0 (ICRS)  (1)
 2  spd    I*4 mas         south pole distance epoch J2000.0 (ICRS)  (1)
 3  im1    I*2 millimag    UCAC fit model magnitude                  (2)
 4  im2    I*2 millimag    UCAC aperture  magnitude                  (2)
 5  sigmag I*2 millimag    UCAC error on magnitude (larger of sc.mod)(3)
 6  objt   I*1             object type                               (4)   
 7  dsf    I*1             double star flag                          (5)   
 8  sigra  I*2 mas         s.e. at central epoch in RA (*cos Dec)      
 9  sigdc  I*2 mas         s.e. at central epoch in Dec                 
10  na1    I*1             total # of CCD images of this star
11  nu1    I*1             # of CCD images used for this star        (6)
12  us1    I*1             # catalogs (epochs) used for proper motions
13  cn1    I*1             total numb. catalogs (epochs) initial match
14  cepra  I*2 0.01 yr     central epoch for mean RA, minus 1900     
15  cepdc  I*2 0.01 yr     central epoch for mean Dec,minus 1900  
16  pmrac  I*4 0.1 mas/yr  proper motion in RA*cos(Dec)           
17  pmdc   I*4 0.1 mas/yr  proper motion in Dec                    
18  sigpmr I*2 0.1 mas/yr  s.e. of pmRA * cos Dec                   
19  sigpmd I*2 0.1 mas/yr  s.e. of pmDec                            
20  id2m   I*4             2MASS pts_key star identifier          
21  jmag   I*2 millimag    2MASS J  magnitude                     
22  hmag   I*2 millimag    2MASS H  magnitude                       
23  kmag   I*2 millimag    2MASS K_s magnitude                     
24  icqflg I*1 * 3         2MASS cc_flg*10 + phot.qual.flag          (7)
25  e2mpho I*1 * 3         2MASS error photom. (1/100 mag)           (8)
26  smB    I*2 millimag    SuperCosmos Bmag
27  smR2   I*2 millimag    SC R2mag                                  (9)
28  smI    I*2 millimag    SC Imag
29  clbl   I*1             SC star/galaxy classif./quality flag     (10)
30  qfB    I*1             SC quality flag Bmag                     (11)
31  qfR2   I*1             SC quality flag R2mag                    (11)
32  qfI    I*1             SC quality flag Imag                     (11)
33  catflg I*1 * 10        mmf flag for 10 major catalogs matched   (12)
34  g1     I*1             Yale SPM object type (g-flag)            (13)
35  c1     I*1             Yale SPM input cat.  (c-flag)            (14)
36  leda   I*1             LEDA galaxy match flag                   (15)
37  x2m    I*1             2MASS extend.source flag                 (16)
38  rn     I*4             MPOS star number; identifies HPM stars   (17)
         84 = total number of bytes per star record

Note  (1):  Positions are on the International Celestial Reference
    System (ICRS) as represented by the Hipparcos / Tycho-2  catalogs.
    The epoch for the positions of all stars is J2000.0; the weighted
    mean catalog position was updated using the provided proper
    motions.  The observational UCAC position is but one of several
    going into these values and is not given in the UCAC3, thus the
    original UCAC observation cannot be recovered from these data. 
    The declination is given in south pole distance (spd) and can be
    converted back to a true declination by subtracting 324000000 mas.

Note  (2):  Unknown, or unrealistic photometric results are set to 
    magnitude = 18 (18000 mmag entry in catalog).  Systematic errors
    are expected to be below 0.1 mag for im1, im2 photometric results
    obtained from the UCAC CCD pixel data.  The aperture photometry
    is considered more robust, particularly for "odd" cases, while the
    model fit magnitude is expected to be more accurate for "well behaved"

Note  (3):  A value of -1 for error in magnitude means "unknown".
    For many stars a photometric error based on the scatter from 
    individual observations of that star on different CCD frames
    could be obtained.  A model error was also attempted to be
    assigned, based on the S/N ratio.  The error quoted here is
    the larger of the 2.  If that error exceeds 0.9 mag the error
    was set to 0.9 mag (900 mmag entry in catalog data).

Note  (4):  The object type flag is used to identify possible problems 
    with a star and has the following meaning:

    -2 = warning: object could be from possible streak
    -1 = warning: object is near overexposed star
     0 = good star
     1 = good star (data copied from another entry)
     2 = warning: contains at least 1 overexposed image
     3 = warning: all images are overexposed or "bad"

Note  (5):  The dsf flag is a double star flag used to indicate the
    type/quality of double star fit with the following meaning:

     0 = single star
     1 = primary of pair with unreal secondary = single
     2 = forced separation, on same frame
     3 = blended image, some CCD frames show single star, some double
     4 = forced separation, 2 objects on same frame number
     5 = primary   component of real double
     6 = secondary component of real double
     7 = other "odd" case  

Note  (6):  A zero for the number of used images indicates that all images
    have some "problem" (such as overexposure).  In that case an unweighted
    mean over all available images (na) is taken to derive the mean
    position, while normally a weighted mean was calculated based on
    the "good" images, excluding possible problem images (nu <= na).

Note  (7):  For each 2MASS bandpass a combined flag was created
    (cc_flg*10 + ph_qual) consisting of the contamination flag (0 to 5) 
    and the photometric quality flag (0 to 8).  

    0 =  cc_flg  2MASS 0, no artifacts or contamination
    1 =  cc_flg  2MASS p, source may be contaminated by a latent image
    2 =  cc_flg  2MASS c, photometric confusion
    3 =  cc_flg  2MASS d, diffraction spike confusion
    4 =  cc_flg  2MASS s, electronic stripe
    5 =  cc_flg  2MASS b, bandmerge confusion

    0 =  no ph_qual flag
    1 =  ph_qual 2MASS X, no valid brightness estimate
    2 =  ph_qual 2MASS U, upper limit on magnitude
    3 =  ph_qual 2MASS F, no reliable estimate of the photometric error
    4 =  ph_qual 2MASS E, goodness-of-fit quality of profile-fit poor
    5 =  ph_qual 2MASS A, valid measurement, [jhk]snr>10 AND [jhk]cmsig<0.10857
    6 =  ph_qual 2MASS B, valid measurement, [jhk]snr> 7 AND [jhk]cmsig<0.15510
    7 =  ph_qual 2MASS C, valid measurement, [jhk]snr> 5 AND [jhk]cmsig<0.21714
    8 =  ph_qual 2MASS D, valid measurement, no [jhk]snr OR [jhk]cmsig req.

    For example icqflg = 05 is decoded to be cc_flg=0, and ph_qual=5, meaning
    no artifacts or contamination from cc_flg and 2MASS qual flag = "A" .

Note  (8):  The photometric errors from 2MASS were rounded by 1 digit
    here to fit into fewer bytes (1/100 mag instead of millimag).

Note  (9):  The R magnitude from the second epoch Schmidt plates (R2) is 
    used for UCAC3, as provided in the SuperCosmos source catalog.

Note (10):  clbl is a combination of the SuperCOSMOS "meanclass" and
    modified "blend" flag (meanclass + 10*blend) originally for each
    magnitude (B, R1, R2, I).  The flag provided here is the maximum
    value over all bands for each of the 2 flags. The SuperCOSMOS mean 
    class flag is an estimate of image class based on unit-weighted mean 
    of individual classes from (B, R1, R2, I).  The SuperCOSMOS modified 
    blend flag indicates if blending is detected.

    The "meanclass" has the following meaning from SuperCOSMOS:
    1 = galaxy
    2 = star
    3 = unclassifiable
    4 = noise

    The modified "blend" flag has the following meaning:
    0 = no blending
    1 = possible blending detected

Note (11):  This is a modified quality flag from the "qualB", "qualR2",
    and "qualI" quality flag from SuperCOSMOS, which gives an indication
    of the quality of the image from the three bands (B, R2, I).  The
    modified quality flag qfB, qfR2, and qfI have the following meaning:

    -1 = qual blank    in SuperCOSMOS, no flag given
     0 = qual zero     in SuperCOSMOS, no problems detected
     1 = qual < 128    in SuperCOSMOS, reliable image
     2 = qual < 65535  in SuperCOSMOS, problems detected
     3 = qual >= 65535 in SuperCOSMOS, spurious detection

Note (12):  The catflg provides reference to 10 major catalogs used
     in the computation of the proper motions and catalog matching.
     Each of the 10 numbers range from 0 to 6 and are the "mmf" 
     (multiple match flag) with respect to each of the 10 following

           Hip, Tycho, AC2000, AGK2B, AGK2H, ZA, BY, Lick, SC, SPM
     catflg 1     2       3      4      5     6   7   8     9   10

     The value for each byte, the mmf flag, has the following meaning:

     0 = star not matched with this catalog
     1 = unique match,  not involving a double star
     2 = closest match, not involving a double, likely o.k. 
     3 = unique match,  and involving a double star
     4 = closest match, and involving a double, likely o.k.
     5 = maybe o.k. smallest sep. match in both directions
     6 = same as 5, but involving a double star

Note (13):  The g-flag from the Yale San Juan first epoch Southern
    Proper Motion data (YSJ1, SPM) has the following meaning:

     0 = no info
     1 = matched with 2MASS extended source list
     2 = LEDA  galaxy
     3 = known QSO

Note (14):  The c-flag from the Yale San Juan first epoch Southern
    Proper Motion data (YSJ1, SPM) indicates which input catalog
    has been used to identify stars for pipeline processing:

     1 = Hipparcos
     2 = Tycho2
     3 = UCAC2
     4 = 2MASS psc
     5 = 2MASS xsc (extended sources, largely (but not all!) galaxies)
     6 = LEDA  (confirmed galaxies, Paturel et al. 2005)
     7 = QSO   (Veron-Cetty & Veron 2006)

Note (15):  This flag is either 0 (no match) or contains the log of
    the apparent total diameter for I-band (object size) information
    copied from the LEDA catalog (galaxies).  A size value of less
    than 1 has been rounded up to 1.

Note (16):  This flag is either 0 (no match) or contains the length of
    the semi-major axis of the fiducial ellipse at the K-band 
    (object size) information copied from the 2MASS extended source
    catalog.  If the size is larger than 127, the flag was set to 127.

Note (17):  MPOS running star numbers over 140 million indicate high
    proper motion stars which were identified in UCAC pixel data from
    matches with known HPM stars.  The position given for those HPM
    stars is the unweighted mean of the CCD observations and the
    proper motion is copied from the literature catalog.

The following table gives the total range of values present these data.

 # explanation                               unit   minimum    maximum
 1 right ascension     epoch J2000.0 (ICRS) [mas]         2 1295999990
 2 south pole distance epoch J2000.0 (ICRS) [mas]     57990  647908484
 3 UCAC fit model magnitude                [mmag]      2329      18000
 4 UCAC aperture  magnitude                [mmag]      2797      18000
 5 error on magnitude (larger of sc.model) [mmag]        -1        900
 6 object type                                           -2          3
 7 double star flag                                       0          7
 8 s.e. at central epoch in RA (*cos Dec)   [mas]         5        500
 9 s.e. at central epoch in Dec             [mas]         5        500
10 total numb. of CCD images per star                     1         70
11 number of CCD images used per star                     0         68
12 numb. catalogs (epochs) used for proper motion         1         31
13 total numb. of catalogs (epochs) initial match         0        113
14 central epoch - 1900 for mean RA    [1/100 yr]      -477      10438
15 central epoch - 1900 for mean Dec   [1/100 yr]      -938      10438
16 proper motion in RA*cos(Dec)      [0.1 mas/yr]    -37060      67670
17 proper motion in Dec              [0.1 mas/yr]    -58130     102770
18 s.e. of pmRA * cos Dec            [0.1 mas/yr]       -10        500
19 s.e. of pmDE                      [0.1 mas/yr]       -10        500
20 2MASS pts_key star identifier                          0 1340037129
21 2MASS J  magnitude                      [mmag]     -2652      30000
22 2MASS H  magnitude                      [mmag]     -3732      30000
23 2MASS K_s magnitude                     [mmag]     -4227      30000
24 2MASS cc_flg*10 + phot.qual.flag J band                0         58
25 2MASS cc_flg*10 + phot.qual.flag H band                0         58
26 2MASS cc_flg*10 + phot.qual.flag K band                0         58
27 2MASS error photom. J band         [1/100 mag]         0        127
28 2MASS error photom. H band         [1/100 mag]         0        127
29 2MASS error photom. K band         [1/100 mag]         0        127
30 SC Bmag                                 [mmag]         0      30000
31 SC R2mag                                [mmag]         0      30000
32 SC Imag                                 [mmag]         0      30000
33 SC star/galaxy classif./quality flag                   0         14
34 SC quality flag Bmag                                  -1          2
35 SC quality flag R2mag                                  0          2
36 SC quality flag Imag                                  -1          2
37 mmf flag Hipparcos catalog                             0          1
38 mmf flag Tycho-2 catalog                               0          1
39 mmf flag AC2000 catalog                                0          1
40 mmf flag AGK Bonn catalog                              0          6
41 mmf flag AGK Hamburg catalog                           0          6
42 mmf flag ZA catalog                                    0          6
43 mmf flag BY catalog                                    0          6
44 mmf flag Lick catalog                                  0          6
45 mmf flag SuperCosmos catalog                           0          6
46 mmf flag SPM catalog                                   0          6
47 Yale SPM object type (g-flag)                          0          3
48 Yale SPM input cat.  (c-flag)                          0          7
49 LEDA galaxy match flag    (size info)                  0        118
50 2MASS extend.source flag  (size info)                  0        127
51 MPOS star number                                       3  140051297

5b) Other files 
table_zones = table listing info for each zone

  This is a text file (ASCII) giving summary information about
  stars in each zone.  There are 360 lines, 1 each for a 0.5 deg
  wide zone in declination, beginning at the South Celestial Pole.
  The meaning of the columns are:

  1  zn   = zone number (1 to 360)
  2  n1   = running star ID number of 1st star in this zone
  3  ns   = total number of stars in this zone
  4  dec  = upper limit of declination of this zone

  sample data:
   zn         n1         ns    dec
    1          1       1258  -89.5
    2       1259       3828  -89.0
    3       5087       6485  -88.5
  358  100759267       4104   89.0
  359  100763371       2132   89.5
  360  100765503        918   90.0  

u3index.asc = index file for each 0.1 hour in RA, ASCII

  This is a text file (ASCII).  Each line contains data for an area
  of sky (bin) 0.5 deg wide in declination (indexed by zone number from
  1 to 360), and 0.1 h wide in RA (indexed from 1 to 240).  So there are
  a total of 360 * 240 = 86400 lines on this file.
  The meaning of the columns are:

  1  n0   = running star number (index along the main data file)
              of the star before the first one in this bin,
              the sequence stars out with 0 at the beginning of
              each new declination zone
  2  nn   = number of stars in this bin (which can be zero)
  3  zn   = zone number (1 to 360)
  4  j    = index for bins along RA (1 to 240) (divide by 10 = RA hour)
  5  dec  = declination corresponding to zn,
              printed out only at the beginning of a new zone

  sample data:
         0         3   1   1 -89.5
         3         3   1   2
         6         6   1   3
        12         4   1   4
    304461       305 180 239
    304766       310 180 240
         0       318 181   1   0.5
       318       315 181   2
       913         1 360 238
       914         1 360 239
       915         3 360 240

u3index.unf = index file for each 0.1 hour in RA, unformatted binary 

  This file contains the same data as the u3index.asc, however as
  binary with the following access explanation (Fortran):

      INTEGER*4 n0(360,240), nn(360,240)  

      OPEN (15,FILE=fnidxu,ACCESS='direct',RECL=345600)  ! 360 * 240 * 4
      READ (15,REC=1) n0
      READ (15,REC=2) nn

  This is the output of a run of the u3read code on zone z042 showing
the first 20 records of that binary file.  Exactly the same data as
the corresponding binary file data are shown with columns in the same
order as outlined above.  However, the 3 items of "e2mpho" (error in
2MASS photometry) are separate columns, while the SPM g1 and c1 items
are merged into a single column.  Thus the ASCII table output of the
sample data has 39 columns.

6) Description of utility software
u3read.f  = sample Fortran code to read UCAC3 zone files

This code reads a file from the main UCAC3 data (binary zone data)
and outputs all the information into an ASCII table, star by star,
1 star per line.  The user can select a range in RA, Dec and magnitude.

7) Acknowledgments
In a project such as this that has spanned a decade and two continents,
many people have been involved.  Here we list people and their main 
contributions to the project.

Norbert Zacharias - Principle Investigator, observer, responsible for
  UCAC position reductions (from pixels to positions on the sky),
  observing schedule, and external position comparisons, re-processing
  of pixel data and development of reduction pipeline, Star-Scan plate
  measure reductions.

Ted Rafferty - former project lead, instrumentation specialist, observer.

Charlie Finch - re-reduction of CCD x,y to RA,Dec, including systematic
   error control.

Gary Wycoff - proper motions data preparation, star identifications.

Marion Zacharias - quality control, observer, position reductions,
   extragalactic link program.

Terry Girard - SPM data reductions, from x,y plate data to RA,Dec,
   incl. systematic error handling (magnitude equations) and all
   related reduction pipeline development.

Bill van Altena - for many years of collaboration in astrometry.

Nigel Hambly - sharing of SuperCosmos source data for UACA proper motions.

Dave Monet - measurements of SPM and NPM plates on PPM at NOFS.

Sean Urban - proper motions of UCAC2 release, preparation of
  other position catalogs including systematic error removal.

Valeri Makarov - reduction of AGK2 and other StarScan plate measures
   from global x,y to individual RA,Dec positions.

Brian Mason, Bill Hartkopf - new double star speckle observations with
   USNO 26in and data reductions.

David Hall - verification of tape archive, data handling.

Marvin Germain - telescope control system software development,
  software-to-hardware interface including electronics.

Ellis Holdenried - telescope control system software maintenance
  and extension of user interface for telescope operation.

Lars Winter - basis for astrometric CCD reduction software.

Greg Hennessy - computer administration support (USNO-DC)

Danilo Castillo - observer,  Cerro Tololo, Chile

Mauricio Martinez - observer,  Cerro Tololo, Chile

Sergio Pizarro - observer,  Cerro Tololo, Chile

Oscar Saa - telescope manager, Cerro Tololo, Chile

Trudy Tilleman - observer, U.S. Naval Observatory Flagstaff Station,
     assist with data reductions.

Stephanie Potter - observer, U.S. Naval Observatory Flagstaff Station

Dominic Marcello - observer, NOFS, joined the project in Jan. 2003

Gary Wieder - telescope upgrades, relocation, maintenance, and

John Pohlman - telescope upgrades,  maintenance, and troubleshooting

John Bowles - telescope upgrades and maintenance (USNO-DC)

Dave Smith - telescope upgrades and maintenance (USNO-DC)

Tie Siemers - telescope upgrades and maintenance (USNO-DC)

Mike Divittorio - telescope maintenance and troubleshooting (NOFS)

Steve Sell - telescope maintenance and troubleshooting (NOFS)

Albert Rhodes - telescope maintenance and troubleshooting (NOFS)

Blaize Canzian - network and computer system support (NOFS)

Sumit Dutta - summer student working on separating out minor planet
      observations and streak artifacts

Aleida Young - summer student working on identification of high
      proper motion stars

Danley Hsu - summer student working on UCAC3 release, and providing
      the UCAC3 access code included with this DVD

Other Acknowledgments
This project would not have been possible without support from the USNO
administration.  Special thanks are given to Thomas Corbin for under-
standing the need for such a project, articulating it to those with
resources, and administering the project in its early days.  Thanks
are given to the three USNO Astrometry Department Heads during the
UCAC project.  Without their continued support, it would not have come
to fruition.  They are F. Stephen Gauss, Ted Rafferty, and Ralph Gaume.
Thanks are given to the USNO Scientific Director, Kenneth Johnston, for
his approval and support of the project and for making USNO resources

Our successful stay at the Cerro Tololo Interamerican Observatory would 
not have been possible without a memorandum of understanding, worked out
by P. Kenneth Seidelmann, USNO, and the then director of CTIO, Malcolm 
Smith.  The entire staff of CTIO is thanked for their generous support.
The same holds for all the people at the Naval Observatory Flagstaff
Station (NOFS) during the northern hemisphere observing of UCAC,
in particular we want to thank the then director Jeff Pier.

Also, we wish to thank Gary Sims from Spectral Instruments for building
our 4k camera and providing outstanding support over the years.
Finally, we wish to acknowledge the following for their important
contributions;  Bill Gray for custom updates to his GUIDE software
regarding minor planet ephemerids, and Rae Stiening (Univ.of Mass.) 
for providing preliminary and pre-release 2MASS data to our project.

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This list includes items relevant to UCAC even if they are not cited 
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Rafferty, T.J., Germain, M.E., Holdenried, E.R., Pohlman, J.,
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Zacharias, N., 2002
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   2003, "First Results from AGK2 Plate Remeasurements", 
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Norbert Zacharias for the UCAC team, USNO, Washington DC, July 2009