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The Second U.S. Naval Observatory CCD Astrograph Catalog (UCAC2)


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) Positions, Proper Motions and Error Calculations
d) UCAC2 Magnitudes
e) 2MASS Photometry
f) Non-stellar Data
g) Provided Utility Software
h) Arrangement of the Data Files
i) Star Identification Numbers

4) Files Contained on the CDs
a) File readme.txt
b) Directory "info"
c) Directory "sw"
d) Directory "u2"

5) Data Formats
a) Files z001 to z288 (directory "u2")
b) File u2index.da (directory "u2")
c) File u2index.txt (directory "info")

6) Description of Utility Software

7) Acknowledgment

8) References

============================================================

1) Introduction
---------------
This is the "readme" file of the Second U.S. Naval Observatory CCD
Astrograph Catalog, UCAC2. Each of the 3 CDs contains this readme
file as well as an identical copy of the "info" directory (index files
and plots) and the "sw" directory (utility software).

The UCAC2 is the second release of the ongoing UCAC project, designed
to observe the entire sky for R magnitudes of about 7.5 to 16. The
observed positional errors are about 20 mas for the stars in the 10
to 14 magnitude range, and about 70 mas at the limiting magnitude
of R ~16. For up-to-date information on the project, see our web page
at http://ad.usno.navy.mil/ucac/ . This web page will also serve as
the location that the UCAC team will post addenda to the UCAC2 catalog.

The UCAC2 is a high density, highly accurate, astrometric catalog of
48,330,571 stars covering the sky from -90 to +40 degrees in declination
and going up to +52 degrees in some areas. The northern limit is a
function of right ascension. Proper motions and photometry are
provided for all stars. Positions and proper motions are on the
ICRS (International Celestial Reference System) and given at the epoch
J2000.0.

The UCAC2 has a number of major differences with respect to UCAC1.
These differences include:
- much larger sky coverage
- reduced systematic errors of CCD observations
- positions given at a standard epoch (J2000.0)
- the addition of several new catalogs for improved proper motions
- photometry in the J, H, and K_s bands from the 2MASS project
- data in binary format for direct access
- inclusion of software to aid users in quick access of the data
Additional details of the data are found in Sections 3, 4, and 5 of
this document.

UCAC2 is the last intermediate data release before the final, all sky
catalog will be constructed. Observations will end around mid 2004;
the final catalog is expected out in 2005.

The UCAC project, observations, and first data release are described in
detail in the paper Zacharias et al. AJ 120, 2131 (2000). Versions
of that and other related papers are placed on our Web page
(http://ad.usno.navy.mil/ucac/). The paper describing UCAC2 is in
preparation (AJ, 2003). The data are freely available; however, any
publication utilizing these data should make proper reference to the
project and papers.


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 ~ half
of the northern sky were observed. In October 2001 the instrument was
moved to the Naval Observatory Flagstaff Station (NOFS) in Arizona to
continue the northern sky. Complete coverage from pole to pole is
expected by mid 2004. This second data release contains data observed
through 08 December 2002 at which time about 86% of the entire sky was
completed.

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 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 are saved.

Observations are 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, observing progresses even during nights with thin cirrus clouds.

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


2b) Reductions of UCAC Observations
-----------------------------------
Unlike the UCAC1 which utilized Hipparcos and ACT catalog stars, the
UCAC2 was processed using Tycho-2 reference stars. This doubled the
number of available reference stars per frame, however, positional
errors of individual faint stars in Tycho-2 are significantly larger
than the x,y errors of UCAC observed images.

A special effort went into dark-correcting frames. Frequent darks are
taken for all standard exposure times and camera vacuum pressures.
Twilight flats have been taken throughout the project, however, they
have not been applied for the UCAC2 processing. Observed profiles of
stellar images are fitted with a two-dimensional Gaussian model. The
obtained x,y centroid positions of the stars are then corrected for
various effects, including:
- CTE (Charge Transfer Efficiency), this manifests itself as magnitude
and coma-like terms for the x coordinate, i.e. RA;
- FDP (field distortion pattern), inhomogeneous filter,
tilt of the CCD chip w.r.t. the optical axis, optical distortion;
- pixel phase error (as a function of the pixel fraction coordinate
and the widths of the image profiles).

In the actual astrometric "plate" reduction step itself, a minimal
model (only linear terms) was used to keep error propagation small.
Effects from aberration and refraction were removed prior to a least-
squares fit of the x,y data to the reference stars' mean positions at
the CCD frames epoch.

As with UCAC1, all "problem cases" were excluded from the catalog.
In particular, many multiple stars are excluded in UCAC2 as their images
are often elongated. Precise centroiding on such objects is difficult
with our current software. It was decided to use the same centroiding
routines as with UCAC1 and remove these "problem" data. This software
is being modified; re-processing of the pixel data is planned for the
final catalog release.

Saturated images of stars were not used, thus no bright stars are
included. Depending on atmospheric conditions, the cut-off at the
bright end is around R magnitude 8. Weighted mean positions were
produced from the individual positions. Only stars with 2 or more
images are included in this catalog. Stars with a formal standard
error (derived from the scatter of the individual images of a star)
exceeding 200 mas in either coordinate were excluded. Those stars are
either very faint, blended images, or have other miscellaneous problems.


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 is in the process of re-measuring
the AGK2 photographic plates, taken in Bonn and Hamburg (Germany) around
1930. About 600,000 new catalog positions from this re-measurement
are included in the area north of +20 degrees declination.

For the faint stars (~12.5 to ~16), data from the Lick Northern
Proper Motions (NPM, Jones et al. 2000) and 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
and initially reduced by D. Monet. For his reductions, Monet used only
the yellow series, so the data are referred to as the "Yellow Sky"
(currently unpublished). The data were subsequently massaged
at USNO Washington by S. Urban to minimize systematic errors. The use
of the Yellow Sky for the faint stars is a departure from UCAC1, which
utilized the USNO A2 catalog. Not surprising, statistics show the
Yellow Sky -- observed using astrographs -- is about twice as accurate
as the A2 -- observed with Schmidt telescopes.

There are 18,604 previously known high proper motion stars in the UCAC
position file. These were identified by Gould and Salim utilizing the
New Luyten's Two-Tenths Catalog and graciously forwarded to the UCAC
team. However, only those high proper motion stars with an early epoch,
astrometric position available were included. There were 8282 stars
found in our standard catalogs (Yellow Sky, AC 2000.2, Tycho-2,
Hipparcos, other transit circle and photographic catalogs), while 7666
stars were supplemented utilizing positions from the USNO A2, thus a
total of 15,948 NLTT stars are in the UCAC2. Note that these are the
only stars for which the USNO A2 is utilized, as we are trying to
minimize the reliance on Schmidt plate data in the UCAC2.

The computation of proper motions is performed similarly to the procedure
used for the UCAC1 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 UCAC2 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 exclusively Yellow Sky data, typical errors are
estimated to be 4 to 6 mas/yr.

All stars in the UCAC2 have proper motions. Stars are not included if:
- they were not observed in the astrograph CCD program;
- they were observed, but the UCAC positional reduction failed for
some reason;
- the epoch span between first catalog position and UCAC does not
exceed 10 years;
- no proper motion could be computed (no one-to-one match found in
other position catalogs).
- proper motions exceed 180 mas/yr but stars were not identified in
the revised NLTT or Hipparcos Catalogue;
- errors in proper motion based on scatter of the individual
positions exceed 20 mas/yr.

All totaled, 48,330,571 stars of the 58,843,138 stars observed with
the astrograph up to December 8, 2002 are included in UCAC2.


3) Properties of the Catalog and Important Notes for the User
-------------------------------------------------------------


3a) Sky Coverage
----------------
Included in UCAC2 are all areas of the sky from the South Celestial
Pole to between +40 and +52 degrees declination. The northern limit
is a function of right ascension. The plots skycov_u2.ps (black and
white) and skycov_u2c.gif (color) in the "info" directory show all
survey fields which are included in this release (see Section 4b for
further details on these plots). These are all fields taken up to the
night of 07/08 December 2002. The plot skycov_nofs.ps and
skycov_nofs.pdf show only the northern fields completed at US Naval
Observatory Flagstaff Station up to the same date. The data for these
plots are much fewer but show the same northern limit of the UCAC2.
Their southern limit shows the border line between observing performed
at CTIO and NOFS.


3b) Completeness
----------------
The UCAC2 is NOT complete, neither in the above mentioned sky coverage
(above +40 degrees), nor for individual magnitude ranges for a variety
of reasons:

- Stars whose images were saturated on the detector are not included.
If a star is brighter than R ~8.0, it is likely not in the UCAC2.

- Multiple stars with separations of 3 arcsec or less are excluded
by design (unique matching requirement). Most multiple stars
in the 3 to 6 arcsec separation range are likely missing due
to blended images.

- All "problem stars", e.g. with large internal errors in the image
combination step, are excluded.

- Stars with fewer than 2 observations meeting the UCAC quality
standards are excluded. Single detection observations are not used
at this time.

- Stars where a one-to-one match was not made with the proper motion
input catalogs are excluded. Many of these will be in the final
UCAC catalog.

- If a star's proper motion was computed to be larger than 180 mas/yr
in either coordinate, but it is neither a known NLTT star nor
found in the Hipparcos Catalogue, it is excluded from the UCAC2.

- If a proper motion could not be determined due to lack of early
epoch data, either it being non-existent or less than a 10 year
span from the UCAC observation, the star is excluded.

- If the proper motion error based on the scatter in the positional
data exceeded 20 mas/yr in either coordinate, it is excluded.


3c) Positions, Proper Motions, and Errors
-----------------------------------------------------
The astrometry provided in the UCAC2 is on the Hipparcos system, i.e.
the International Celestial Reference System (ICRS), which is
consistent with the (old) J2000.0 equator and equinox. Positions are
given at the standard epoch of Julian date 2000.0, thus the UCAC2 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 given in the catalog. 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. (The UCAC2
observational epochs are in the range of 1998.1 to 2002.9.) The proper
motions are given at the central epoch.


3d) UCAC2 Magnitudes
--------------------
The UCAC2 magnitude range is about 8 to 16 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 seeing conditions at the time of observation.

UCAC magnitudes are only approximate with an estimated error of
about 0.3 mag (absolute). The relative precision over < 0.5 degree
and medium magnitude range can be much better, ~0.1 mag. Note:
some data were obtained in non-photometric conditions and no "aperture
photometry" type reductions were made. The UCAC2 magnitudes are only
provided for identification purposes.


3e) 2MASS Photometry
--------------------
The procedures to derive the UCAC2 observed photometry is unchanged
from the UCAC1 procedures, giving only very approximate R magnitudes
(see Section 3d). However, for the UCAC2 release, the data from the
Two Micron All Sky Survey, 2MASS, has kindly been made available to us
by R. Stiening, shortly before the public release. 2MASS photometric
data, i.e. the J, H, and K_s magnitudes, and cross references
(star ID numbers in the 2MASS point source catalog) are provided for
matched stars as a convenience to the user. These numbers are copied
directly from 2MASS; for more information (flags, errors on photometry
etc.) the 2MASS catalog should be consulted:
http://www.ipac.caltech.edu/2mass/releases/allsky/ .
In addition to J, H, and K_s magnitudes a modified "ph_qual" and "cc"
(contamination) flag has been copied into UCAC2. A match radius of
2 arcsecs has been used between the UCAC and 2MASS positions (no proper
motions were available at that time), which still could lead to a few
wrong identifications for binaries on that separation scale. In total,
48,079,174 stars have 2MASS photometry listed, which is 99.5% of the
UCAC2 stars.


3f) Non-stellar Data
--------------------
The UCAC2 contains some galaxies, particularly at the faint end.
No flag indicating a galaxy or star is provided with this release;
however, extended objects are very unlikely to be in the UCAC2 due to
the detection and reduction quality control procedures adopted for the
catalog construction. Also, galaxies of integrated magnitudes of ~15
or fainter are likely to show cores fainter than 16th magnitude thus
will likely not be in the UCAC2.

Some asteroids might be hidden in the UCAC2. The observing schedule
actively avoided all major planets and bright asteroids (to 12th mag).
However, asteroids in the ~12 to 14 mag range can appear on both the
long and short exposure taken within 2 minutes and could give a
satisfactory position match. Fainter asteroids could enter the
UCAC2 only if overlapping fields with the object were taken within
a short period of time, which might be the case for about 30% of the
fields. Many overlapping fields are taken the following night, with a
possible asteroid having moved far enough that it will not be matched
on at least 2 frames, hence it will not be present in the catalog
(2 detections of an object are required to be in UCAC2). Additionally,
with the requirement that each object have a proper motion using an
older catalog, it is unlikely that many asteroids are present in the
UCAC2.


3g) Provided Utility Software
-----------------------------
Some utility software is included with the CD-ROMs. It is not
required to use the data; however, this software serves as an example
for a fast access to the data. No guarantee or support is provided.
See Section 6 below for more details.


3h) Arrangement of the Data Files
---------------------------------
The data are arranged in declination zones of 0.5 degree width, with
zone number 1 starting at the South Pole and increasing as one goes
north. There is a separate file for each zone; there are 288 data
files in total. Due to the amount of data, 3 CDs were required, each
with about 680 MB of mostly binary data. Records in the individual
data files are sorted by right ascension. One record contains the
data for one star.


3i) Star Identification Numbers
-------------------------------
Official UCAC2 star ID numbers should be utilized for identification
purposes and for communication with the UCAC team. However, these
numbers are not explicitly stored in the binary files but are generated
when extracting data using the provided utility software. These star
numbers increase along RA within a given declination zone of 0.5 degree
width, and increase with declination zone starting from the South Pole.
If the provided utility software is not used the UCAC2 star ID number
can be generated from the provided index files using the following
method:

ID = n0 + running star number in zone z

where z is zone number where the star in question is located
(1 to 288; zone 1: -90 to -89.5 decl., ...), n0 is the largest star
ID number of the previous zone and ID is this official UCAC2 star
ID number. To get the value of n0, one needs "nx", the index array
provided in the file u2index.da (also available in the ASCII text file,
u2index.txt). The nx array is a 2-index array containing the largest
ID number in each area of the sky. The first index is the zone number
as described above; the second is the right ascension bin of 0.1 hours
width. Hence, nx has (288x240) elements. For z=1, n0 = 0. For any
other value of z, n0 = nx (z-1,240).

If needed, this 8-digit star ID number should be given in connection
with the IAU sanctioned acronym of this project and this data release
number, like "2UCAC12345678". The star ID number runs from 00000001
to 48330571. Leading zeros are to be used to fill 8 digits in the
star name. Also, the position of a star can be used as an identifier.


4) Files contained on the CDs
-----------------------------
All 3 CDs contain the same directory structure. There is a
"readme.txt" file (this file), and 3 directories; "info", "sw", and
"u2". The "readme.txt" file, the "info" directory, and "sw" directory
are identical on all CDs. However, the contents of the data directory
"u2" are different. Here is a list of all files:


4a) File "readme.txt"
----------------------
readme.txt = This file, it is the only file outside a subdirectory.
This contains general information on the catalog, the
observing program from which the catalog is made,
information about the CDs, data formats, and references.


4b) Directory "info"
---------------
This directory contains information which may be beneficial to the user.

format.txt = Format description of the main data records.
This is a copy of Section 5 of "readme.txt" (this file).
Due to its anticipated frequent reference, it was copied
into a separate file for ease of the user.

rdp_pole.ps = Color plot of UCAC2 star distribution in RA,Dec
near the South Pole (generated with u2rdplot.f).

rdp_smc.ps = Color plot of UCAC2 star distribution in RA,Dec
for the Small Magellanic Cloud area. Note the large
number of faint stars (blue) and the absence of data
in the core of the globular cluster due to crowding.

skycov_u2.ps = Black and white, high resolution, PostScript file
of a plot showing the sky coverage of the UCAC2 data.
Areas not covered by dots are not included in this
release.

skycov_u2c.gif = similar to skycov_u2.ps, however in color and as
GIF file. Colors represent approximately one calendar
year of observing starting from the beginning of the
project. However, we decided to graphically show
when we moved the telescope from CTIO to NOFS.

The colors represent frames taken over the following
periods:
Yellow = mid-February 1998 to mid-February 1999;
Blue = mid-February 1999 to mid-February 2000;
Red = mid-February 2000 to mid-February 2001;
Green = mid-February 2001 to September 2001 at which
time the telescope was disassembled for
re-location to NOFS;
Pink = Start up at NOFS October 2001 to October 2002;
Light blue = October 2002 to December 8, 2002 (latest
frames used in this release).

skycov_nofs.ps = Similar to skycov_u2.ps. Shows fields covered from
the U.S. Naval Observatory Flagstaff Station site,
i.e. the northern part of the survey. The northern
limit shows the boundary of this UCAC2 release,
the southern limit shows the border between
observations made at Cerro Tololo and Flagstaff.

skycov_nofs.pdf = same as skycov_u2.ps, but in PDF format.

zone.txt = Table of contents for the main data giving a list of zones.
Each line of the table gives; the number of stars,
accumulated number of stars, right ascension minimum and
maximum values, and declination minimum and maximum values
per zone. Note a zone is 0.5 degrees wide.

u2index.txt = ASCII table of the full index, organized in declination
zones and sections of 0.1 hour width along RA.
Again, the number of stars per bin and the accumulated
number of stars are given. This information can be
used to locate the desired section (record number) on
a zone file for a given area in the sky. See Section 5c
for further description of the data.

u2chk.log = Output of the u2chk program, containing statistics,
and histogram tables of all items. The range of values for
each item is given as well as the distribution of the
central epochs and errors.

u2tab1.txt = An ASCII table formatted sample of a selection of 14 stars.
This was generated with u2access and covers stars in the
5 to 14 mag range for RA = 23.9 to 0.1 hours, DE = -82.1 to
-81.9 degrees. Format 1 is used, i.e. all items from the
binary file are output as integers.
The UCAC2 star identification number is appended as well.

u2tab2.txt = The same as u2tab1.txt, except using format = 2, which
gives positions updated for the epoch = 2003.0, RA, DE
in degrees and decimals, along with proper motion
information and errors (decimals).

u2tab3.txt = The same as u2tab1.txt, except using format = 3, which
gives RA, DE in hms and dms format, respectively. Flags
and 2MASS photometry are included in this format
(decimals).

u2tab4.txt = The same as u2tab1.txt, except using format = 4, which
gives a short table with only RA (hour, decimal), DE
(degree, decimal), U2Rmag, error on position and star
ID number.


4c) Directory "sw" (utility software, Fortran only)
-------------
The utility software (Fortran source code) is not required to use
the UCAC2 data. The software serves as an example of efficient use
of the data with fast, random access to arbitrary areas in the sky.
It is provided for the convenience of the user only. No support is
provided by USNO. Additional UCAC2 access software in C/C++
has been kindly provided by Bill Gray (GUIDE Software) and
can be found on the web at http://www.projectpluto.com/ucac2
Executable files of the 2 main access programs are provided for
Linux (Intel Pentium processor), Windows (DOS), and HP-UNIX. For
future additions, please see our web page http://ad.usno.navy.mil/ucac/ .

See Section 6 for a complete list and short description of the programs.


4d) Directory "u2" (main catalog data)
-------------
u2index.da = binary file with an index to all zones = "nx" array,
see Sections 3i and 5b. This file is used by some of the
utility software and is expected in the same directory
as the data files. There is an identical copy on each
of the 3 CDs.

This information can also be obtained from the u2index.txt file
provided in the "info" directory. The u2index.da file is used
for faster access with the utility programs. Both index files,
the ASCII and the binary version, can be re-created by the user
by running the u2chk program (see Section 6).


Files z??? = Binary data files containing the UCAC2 catalog. See
Section 5a for details on the data formats. Note that
each file contains data for a 0.5 degree wide zone,
beginning at the South Celestial Pole. The CDs contain
zones from different declinations.

z001 to z106 = -90.0 to -37.0 declination on CD #1
z107 to z182 = -37.0 to +01.0 declination on CD #2
z183 to z288 = +01.0 to +54.0 declination on CD #3

In addition, z001 is copied on CD #2 and CD #3. It is used for a
byte-flip check in the utility programs.

A recommended option is to copy the u2index.da and all data files
from the 3 CDs into a single directory on hard disk in order to avoid
the need of switching CDs. The utility programs should work in either
case. If a needed data file is not present, a warning will be issued
from the program. The user may then change the CD and continue.
A single path name for all binary data files is assumed.