[Prev][Next][Index][Thread]

Re: (IAAC) estimating magnitudes



>I would like to start including the magnitudes of some of the field
>stars in my drawings.  The problem is I don't know how to go about doing
>this.  It seems the online catalogues and megastar only go down to mag
>15 or 16.  I was wondering if there is some software out there that
>would allow magnitudes to be derived with relative accuracy from dss
>images.  If one or two of the stars in the image were of known magnitude
>they could be used to calibrate the software so that the magnitudes of
>other stars in the image could be obtained.  This would not be the most
>accurate method but would certainly beat the estimates I could make
>simply by looking at an image.
>	How do you arrive at the magnitudes of stars for which there is no
>readily available data?
>
>Thanks,
>Mark
Hi all,
The accompanying catalogue A1.0 and A2.0 from USNO are based on DSS scans
along with astrometric and photometric data for stars beyond the GSC
database (that is maybe between 15 and 20th mag). It is available for part
directly at USNO (10+ CDs for the entire version and 1 for a selected areas
version) and some software (like Guide) allow to use them. I have the 10+
CDs set but have not had the time yet to test it on my computer.
clear skies, Yann.
PS: here below is the introduction of the catalogue as the header of the files.
----------------------------------------------------------------------------
read this file first.  Near the end of this file is a list of other files
that might be helpful.  Questions and comments should be directed to
   Dave Monet
   US Naval Observatory Flagstaff Station
   PO Box 1149  (US Mail Only)
   West Highway 66  (FedEx, UPS, etc.)
   Flagstaff AZ 86002
   Voice: 520-779-5132
   FAX:   520-774-3626
   e-mail:  dgm@nofs.navy.mil
Please understand that the level of support provided will be commensurate
with the level of effort expended.  I am too busy to do your homework
for you.  E-mail works better than the phone.
============ Title ====================
                    USNO-A V1.0
              A Catalog of Astrometric Standards
                   David Monet a)
        Alan Bird a), Blaise Canzian b), Hugh Harris a), Neill Reid c),
        Albert Rhodes a), Stephen Sell a),
        Harold Ables d), Conard Dahn a), Harry Guetter a), Arne Henden b),
        Sandra Leggett e), Harold Levison f), Christian Luginbuhl a),
        Joan Martini a), Alice Monet a), Jeffrey Pier a),
        Betty Riepe a), Ronald Stone a), Frederick Vrba a),
        Richard Walker a)
a) U.S. Naval Observatory Flagstaff Station (USNOFS)
b) Universities Space Research Association (USRA) stationed at USNOFS
c) Palomar Observatory, California Institute of Technology
d) USNOFS, now retired
e) USRA, now at University of Hawaii
f) USRA, now at Planetary Science Institute, Boulder CO
============== Abstract =======================
    USNO-A is a catalog of 488,006,860 sources whose positions
    can be used for astrometric references.  These sources were
    detected by the Precision Measuring Machine (PMM) built and
    operated by the U. S. Naval Observatory Flagstaff Station
    during the scanning and processing of the Palomar Observatory
    Sky Survey I (POSS-I) O and E plates, the UK Science Research
    Council SRC-J survey plates, and the European Southern
    Observatory ESO-R survey plates.  The PMM detects and processes
    at and beyond the nominal limiting magnitude of these surveys,
    but the large number of spurious detections requires that a
    filter be used to eliminate as many as possible.  USNO-A's
    sole inclusion requirement was that there be spatially
    coincident detections (within a 2 arcsecond radius aperture)
    on the blue and red survey plate.  For field centers of -30
    degrees and above, data come from POSS-I plates, while data
    from field centers of -35 and below come from SRC-J and ESO-R
    plates.
    USNO-A presents right ascension and south polar distance in
    the system of J2000 at the epoch of the survey blue plate
    for each object, and lists an estimate of the blue and
    red magnitude.  For POSS-I sources, the photometric system is
    the photographic system defined by the O and E emulsions and
    filters, while southern sources are measured in the photometric
    system defined by the IIIa-J and IIIa-F emulsions.  It is believed
    that the typical astrometric error is about 0.25 arcseconds and
    that the typical photometric error is about 0.25 magnitudes.
    However, these error estimates are dominated by the systematic
    errors incorporated in the calibration procedure, and some
    fields may be significantly worse.  Should users be willing to
    locally recalibrate the astrometry and photometry, the errors
    arising from the PMM are believed to be in the range of 0.15
    arcsecond and 0.15 magnitude.
    To avoid the necessity of consulting many catalogs, objects
    brighter than 11th magnitude that appear in the Guide Star
    Catalog that were not detected by the PMM were inserted.
    USNO-A covers the entire sky, and goes as deep as O=21, E=20,
    J=22, and F=21 for objects with appropriate colors.  The limiting
    magnitude is brighter for objects with extreme colors, and follows
    from the requirement for a detection on both the blue and red
    survey plate.  Although it covers the entire sky, there are holes
    in the catalog in the vicinity of bright stars, regions of nebulosity,
    crowded fields, etc.
============== Statement of Intellectual Property Rights ====================
   This catalog contains data from a diverse collection of photographs,
reductions, and catalogs.  A large number of different organizations claim
copyright and/or intellectual property rights on the various components.
Although the details differ, all permissions for usage of data are
contingent on unrestricted access.  Distribution and/or other direct costs
can be recovered, but re-packaging, re-formatting, or similar activities,
especially for commercial purposes, are not permitted except as authorized
by the U. S. Naval Observatory in consultation with the other institutions
listed below and as appropriate.
   1) Palomar Observatory, National Geographic Society, and California
      Institute of Technology own Palomar Observatory Sky Surveys I and II.
   2) European Southern Observatory owns the ESO-R survey.
   3) The UK Particle Physics and Astronomy Research Council (formerly
      Science and Engineering Research Council and before that Science
      Research Council) owns the SRC-J survey.
   4) Space Telescope Science Institute (and AURA and NASA) own the
      Guide Star Catalog.
   5) US Naval Observatory owns the digitization of the plates and the
      object parameters and catalogs made from them.
   6) The Anglo-Australian Telescope Board retains the copyright to plates
      taken with the U.K. Schmidt Telescope after 15 June 1988.
In particular, we reserve the rights to compile and distribute zone catalogs,
summary catalogs, or other significant pieces of USNO-A beyond that which is
needed to support personal or institutional scientific or educational projects.
Included in this is the preparation and distribution of images generated
from the USNO-A catalog beyond those needed for finding charts and similar
purposes.  In summary, you are welcome to use the catalog, but significant
redistribution, extraction, and image rights are reserved, and permission
needs to be obtained before using this catalog for such purposes.
Please don't make us wake up the lawyers.  Please treat these data and
catalogs in the spirit in which they were created.  They are for non-profit
educational and scientific pursuits, and not for third parties to remarket
for a profit.
=======================Request for Citations=================================
It is an unfortunate aspect of modern funding that impersonal and statistical
measures are used to assess the productivity and usefulness of programs.
If you benefited from using USNO-A, we ask that you give the catalog a
citation.  By doing so, we may be able to justify the expense of continued
production of catalogs.  Whenever possible, it would be appropriate to note
which survey (POSS-I, ESO, and/or SRC) supplied the relevant data, since
these surveys are measured, in part, by the extent to which they serve the
community.
=======================How To Proceed====================================
There is no paper copy of this catalog.  All documentation that exists
has been put somewhere on the CD-ROM set.  A reasonable strategy for
learning about and using this catalog is the following.
   read.me - Contains a brief description and the necessary statement
             of intellectual property rights.
   read.use - Contains a description of the format of the various files
              and what they contain.  A companion file, demo.tar, contains
              the source code for a simple program that uses this catalog.
   catalog.tar - contains the ASCII text files that describe each of the
                 plates taken as part of the various surveys.  Of particular
                 interest is the epoch of each plate since proper motions
                 have not been computed and applied to the position of
                 each source.
   read.ast - Contains a description of the astrometric reduction procedure.
   read.pht - Contains a description of the photometric reduction procedure.
   read.pmm - Contains a description of the PMM hardware and software.
   sg1.tar - Contains the source code for all software needed to run the
             PMM and do the real time processing.  Included in this file
             are the bias and flat field frames as well as the coefficients
             used in the geometric calibration procedure.
   binary.tar - Contains the source code for about half of the routines
                used to reduce the raw PMM data and produce this catalog.
   newbin.tar - Contains the source code for the rest of the routines
                used to reduce the raw PMM data and produce this catalog.
================Table of Contents=====================
       File                  CD-ROM
----------------------------------------------
   zone0000.acc/.cat            1
   zone0075.acc/.cat            1
   zone0150.acc/.cat            6
   zone0225.acc/.cat            5
   zone0300.acc/.cat            3
   zone0375.acc/.cat            2
   zone0450.acc/.cat            1
   zone0525.acc/.cat            4
   zone0600.acc/.cat            6
   zone0675.acc/.cat            5
   zone0750.acc/.cat            7
   zone0825.acc/.cat           10
   zone0900.acc/.cat            8
   zone0975.acc/.cat            7
   zone1050.acc/.cat            8
   zone1125.acc/.cat            9
   zone1200.acc/.cat            9
   zone1275.acc/.cat            4
   zone1350.acc/.cat           10
   zone1425.acc/.cat            3
   zone1500.acc/.cat            2
   zone1575.acc/.cat            6
   zone1650.acc/.cat            2
   zone1725.acc/.cat            3
   .lut files for all zones     7
   pmmgsc.len                   7
   read.me                      1
   read.ast                     1
   read.pht                     1
   read.pmm                     1
   read.use                     1
   demo.tar                     1
   catalog.tar                  2
   newbin.tar                   6
   binary.tar                   8
   sg1.tar                      8
================Other Notices=====================
The source codes are the intellectual property of the U. S. Naval
Observatory and are provided so that the expert user can answer detailed
questions about how the catalog was constructed.  Casual users should
avoid them because they contain no instructions as to where various useful
things are hidden.  Release of the source code is made to support such
investigations only, and is not intended for commercial or other
non-professional applications.  The source code is a protected property,
and illegal usage is prohibited.  If in doubt, please contact Dave Monet
for clarifications and protections.
The PMM program has been supported through internal funding by USNO, and
by funding provided by the U. S. Air Force through the Space Surveillance
Network Improvement Program.
This work is based partly on photographic plates obtained at the Palomar
Observatory 48-inch Oschin Telescope for the First and Second Palomar
Observatory Sky Surveys which was funded by the Eastman Kodak
Company, the National Geographic Society, the Samuel Oschin
Foundation, the Alfred Sloan Foundation, the National Science
Foundation grants AST84-08225, AST87-19465, AST90-23115 and
AST93-18984,  and the National Aeronautics and Space Administration
grants NGL 05002140 and NAGW 1710.
This catalog is based, in part, upon original material from the UK Schmidt
Telescope, copyright in which is owned by the UK Particle Physics and Astronomy
Research Council.  No charge beyond recovery of costs has been made by
USNO or PPARC for the provision of these data.  These data are provided
to its recipient for its purpose without restriction, except on the condition
that the data should not be replicated, in whole or in part, and passed
on for profit.  The plates for the SRC-J survey were taken with the
UK Schmidt Telescope (UKST).
The copyright inherent in the plate material obtained with the UK Schmidt
Telescope taken after 15 June, 1988 rests with the Anglo-Australian
Telescope Board.
The European Southern Observatory holds the copyright to the ESO-R
Survey plate material, and USNO scanned these plates under an agreement
with ESO.  This agreement states, in part, that the data derived from these
scans shall be available to the public without restriction and without
charge beyond recovery of the cost of distribution.
USNO would like to thank NOAO/KPNO for the permission to borrow the glass
copies of the SRC and ESO surveys for scanning, and to Bill Schoening and
Richard Green for their assistance.
****************Really Important Stuff**************************************
1) This file is the first level of documentation for the USNO-A2.0 catalog.  It
   discusses the changes between USNO-A2.0 and USNO-A1.0, and familiarity with
   USNO-A1.0 is presumed.  Should this not me the case, please start by reading
   the A1.0 documentation (README.V10 and associated files) before continuing
   with this file.  Questions and comments should be directed to
   Dave Monet
   US Naval Observatory Flagstaff Station
   PO Box 1149  (US Mail Only)
   West Highway 66  (FedEx, UPS, etc.)
   Flagstaff AZ 86001 USA
   Voice: 520-779-5132
   FAX:   520-774-3626
   e-mail:  dgm@nofs.navy.mil
   Please understand that the level of support provided will be commensurate
   with the level of effort expended.  I am too busy to do your homework
   for you.  E-mail works better than the phone.
2) If you have been using USNO-A1.0, all you really need to do is swap
   the new versions of the .ACC and .CAT files for the old ones.  If
   you insist on understanding what has changed, you can read the rest
   of the documentation, but the new version is intended to be as
   compatible as possible with the old one.
3) This file is subject to being updated.  We are in the process of moving
   the USNO Flagstaff Station Web site from
       http://www.usno.navy.mil/nofs
   to
       http://www.nofs.navy.mil
   Please be patient during the transition.  This version of the file
   was all that I could prepare in time for the CD-ROM distribution.
   As changes, mistakes, and additions are processed, the new version
   of this file will be available from our Web site.
*********************The Rest Of The Stuff********************************
                    USNO-A V2.0
              A Catalog of Astrometric Standards
                   David Monet a)
	Alan Bird a), Blaise Canzian a), Conard Dahn a), Harry Guetter a),
	Hugh Harris a), Arne Henden b), Stephen Levine a),
	Chris Luginbuhl a), Alice K. B. Monet a), Albert Rhodes a),
	Betty Riepe a), Steve Sell a), Ron Stone a), Fred Vrba a),
	Richard Walker a)
a) U.S. Naval Observatory Flagstaff Station (USNOFS)
b) Universities Space Research Association (USRA) stationed at USNOFS.
============== Abstract =======================
   USNO-A2.0 is a catalog of 526,280,881 stars, and is based on a
   re-reduction of the Precision Measuring Machine (PMM) scans that
   were the basis for the USNO-A1.0 catalog.  The major difference
   between A2.0 and A1.0 is that A1.0 used the Guide Star Catalog
   (Lasker et al. 1986) as its reference frame whereas A2.0 uses the
   ICRF as realized by the USNO ACT catalog (Urban et al. 1997).
   A2.0 presents right ascension and declination (J2000, epoch of the
   mean of the blue and red plate) and the blue and red magnitude
   for each star.  Usage of the ACT catalog as well as usage of new
   astrometric and photometric reduction algorithms should provide
   improved astrometry (mostly in the reduction of systematic errors)
   and improved photometry (because the brightest stars on each plate
   had B and V magnitudes measured by the Tycho experiment on the Hipparcos
   satellite).  The basic format of the catalog and its compilation is the
   same as for A1.0, and most users should be able to migrate to this
   newer version with minimal effort.
   This file contains a discussion of the differences between A1.0 and
   A2.0, and those points not discussed remain unchanged.  For convenience,
   the documents circulated with the A1.0 catalog are included in this
   distribution.
================= Discussion =========================
1.  REFERENCE FRAME
   USNO-A2.0 has adopted the ICRS as its reference frame, and uses
   the ACT catalog (Urban et al. 1997) for its astrometric reference
   catalog.  The Hipparcos satellite established the ICRS at optical
   wavelengths, but stars in the Hipparcos catalog are saturated on
   deep Schmidt survey plates as are the brighter Tycho catalog stars.
   Fortunately, the fainter Tycho stars have measurable images, so each
   survey plate can be directly tied to the ICRS without an intermediate
   astrometric reference frame.  The proper motions contained in the
   ACT catalog are more accurate than those in the Tycho catalog, so
   the ACT was adopted as the reference catalog.  USNO-A1.0 use the Guide
   Star Catalog v1.1 as its astrometric reference catalog, and the
   availability of the ACT was the driving force behind the compilation
   of USNO-A2.0.
2.  STAR NAMES
   USNO-A2.0 continues the policy established for USNO-A1.0 of not
   assigning an arbitrary name to each object.  Without explicit star
   names, the IAU recommendation is to use the coordinates for the name.
   Since USNO-A2.0 contains a complete astrometric rereduction, the
   coordinates of objects are not the same, so the names for USNO-A1.0
   stars are NOT PRESERVED in USNO-A2.0.  If you need a name for a star,
   you can use either the coordinates or the zone and offset so long
   as you are careful to cite USNO-A2.0 as the source.
   (If anybody has a clever solution to the problem of star names that
   does not waste lots of space or CPU cycles, please let me know.)
3.  PHOTOMETRIC CALIBRATION
   The Tycho catalog provides B and V magnitudes for its stars.  USNO-A2.0
   uses these and Henden's photometric conversion tables between (B,V)
   and (O+E+J+F) to set the bright end of the photometric calibration for
   each plate.  This is an improvement over USNO-A1.0.
   Unfortunately, GSPC-II and other large catalogs of faint photometric
   standards are not available, so the faint end of the photometric
   calibration came from the USNO CCD parallax fields in the North,
   and from the Yale Southern Proper Motion CCD calibration fields
   (van Altena et al. 1998) for fields near the South Galactic pole.
   Hence, the faint photometric calibration of USNO-A2.0 may not be
   any better than for USNO-A1.0.  Sorry.  When better sources of faint
   photometric calibration data become available, new versions of USNO-A
   will be compiled.
   A new algorithm for doing computing the photometric calibration.
     a) Since there are 300 or more ACT(==Tycho) stars on each plate,
        the computed J+F+O+E magnitude for each star can be computed
        from B+V.  Given the relatively poor nature of this conversion,
        subtleties of the various photometric systems were ignored.
        Please remember that all Tycho stars are toasted on deep Schmidt
        plates, and we were lucky that PMM could compute decent positions
        and brightnesses for any of them.  Four solutions were done
        (O+E+J+F) which fit an offset for each plate and a common
        slope for all plates.  For example, there were 825 free parameters
        in the solution for the 824 POSS-I O plates, 824 offsets and 1 slope.
        This solution isn't quite as good as fitting individual slopes
        for "good" plates, but is much more stable than fitting individual
        slopes for "bad" plates.
     b) There are 215 POSS fields and 42 SERC/ESO fields with faint
        faint photometric standards.  Again, the ensemble of plates was
        divided into 4 solutions (O+E+J+F), and the fit allowed an
        offset for each plate but a common value for the linear and the
        quadratic term.  For example, there were 217 free parameters in
        the POSS-I O plate solution, 215 offsets, 1 slope, and 1 quadratic
        term.  Again, this offers stability at the expense of accuracy
        on the "good" plates.
     c) A number of iterative solutions for using the calibrated plates
        to calibrate the rest were tried, and most failed.  Finally,
        a stable solution was found.  For each of the 4 sets of plates,
        the faint zero points were fit as a function of the bright
        zero points.  Using this relationship, the faint zero points for
        all plates were computed.  (We chose to use the fit instead of the
        individual solutions for those plates which had the faint
        photometric standards.)  Note that this relationship provided
        the fifth (and final) parameter for the photometric calibration
        (i. e., bright offset, bright slope, faint offset, faint slope,
        faint quadratic).
        Once the coefficients were known for all plates, the overlap
        zones on adjacent plates were used to smooth the solution over
        the whole sky.  In an iterative scheme, the faint mean error
        for each plate was computed from all stars in common with other
        plates, and then the faint offset was adjusted after all the
        mean errors were computed.  This algorithm converged in 3 or 4
        iterations, and makes the plate-to-plate photometry as uniform
        as possible given the paucity of faint standards.
     d) No vignetting function was used.
4.  ASTROMETRIC CALIBRATION
   A startling result of the comparison between PMM and ACT is that
   decent astrometry can be done on stars as bright as about 11th magnitude.
   Visually, these images have spikes and ghosts, and are not the sort of
   images commonly associated with the word "astrometry".  Since there
   are 300 or more ACT stars on a single Schmidt plate, each plate
   can be tied directly to the reference catalog without an intermediate
   coordinate system.  This solution includes corrections for systematic
   errors in the focal plane and for magnitude equation, and these
   are discussed below.  It should be emphasized that the raw measures
   are the same for USNO-A2.0 and USNO-A1.0, and the difference is in
   how these are combined to produce the coordinates found in the catalog.
   a) Schmidt telescopes have field-dependent astrometric errors, and
      these must be sensed and removed.  Because there are hundreds of
      reference stars on each plate, the algorithm used was as follows.
      Data from the exposure log are used to do the transformation from
      mean to apparent to observed to tangent plane coordinates using
      the relevant routines from Pat Wallace's SLALIB package.  The
      first set of solutions finds the best cubic solution between the
      PMM measures (corrected for the known Schmidt telescope pin cushion
      distortion) and the predicted positions.  Once an ensemble of these
      solutions have been done, the residuals are accumulated in 5mm by
      by 5mm boxes of position on the plate.  By combining the residuals
      from hundreds of plates, the systematic pattern can be determined
      with good precision.  The second step is to repeat the cubic fit
      between predicted and observed positions after correcting the
      observed positions using the pattern determined in the first step.
      Examination of the systematic pattern produced by the second
      step indicated that there was a small residual pattern that arose
      from the interdependence of the fixed pattern and the cubic
      polynomial fit.  A third iteration was done, and the resulting
      systematic pattern was consistent with random noise.
      The iterative process of determining the systematic pattern of
      astrometric distortions was done separately for each telescope
      in each color, and intermediate solutions based on zones of
      declination were examined for the effects of gravitational
      deflection.  None were found, so the final patterns were determined
      through the co-addition of all plates taken by a particular telescope
      in a particular color.  Hence, USNO-A2.0 uses 4 specific patterns
      instead of the single mean pattern used for USNO-A1.0.
   b) Inspection of the astrometric residuals from high declination
      fields (where the overlop between plates is large) showed that
      there was a significant radial pattern.  This, and the analysis
      of the residuals from the UJ reductions for the USNO-B catalog,
      suggested that magnitude equation was present.  This is hardly
      a surprise because the images of Tycho stars show spikes, ghosts,
      and other problems whereas the faint stars show relatively clean
      images.  The effect is small to non-existent within a radius of ~2.2
      degrees of the center, and then rises to 1.0 arcsecond at ~3.0
      degrees and continues to rise into the corners.  The effect is
      more or less the same for the POSS-I O, POSS-I E, and SERC-J plates,
      but a different behavior was seen for the ESO-R plates.  The
      source of this different behavior is not understood, and may
      indicate a software problem associated with the different size of the
      ESO plates (300x300 mm vs 14x14 in).
      The analysis of the UJ plates (like POSS-II J except with a 3 minute
      exposure) shows a similar behavior when the Tycho stars are subdivided
      into bins of <9, 9, 10, 11, and 12 magnitude.  Since the nominal
      difference between UJ and POSS-I is something like 4 magnitudes,
      the effect was assumed to be zero for stars fainter than 15 and
      rises linearly until it becomes the same for all stars brighter
      than 11.  This is an empirical correction, and more work needs to be
      done to verify its behavior.
5.  NUMERICAL REFOCUS
   The most common mode for the PMM to mis-measure a plate is that it
   does not determine the distance between the camera and the plate
   accurately.  The PMM starts by using the granularity of the emulsion
   as a signal for setting the focus (i.e., minimum background smoothness),
   and then does 15 exposures separated by 0.5 millimeters to compute
   the actual pixels per millimeter.  In many cases, this algorithm is
   not sufficient, and the raw scans have relatively large astrometric
   errors, and show a sawtooth pattern in the residuals.
   Since PMM saves many more data than are contained in this catalog,
   it is possible to refocus the plate after the scan.  To do this, the
   known positions of the ACT stars are fit as a function of the new
   Z distance between the camera and the plate.  Minimization of these
   residuals indicates what the proper focus should have been, and then
   the entire set of raw measures are corrected for this effect.  In
   general, this processed tightens the histogram of the number of plates
   as a function of the astrometric error.  The good scans are unaffected
   but the bad scans get better.  This algorithm has been applied to all
   plates used in USNO-A2.0.
6.  EPOCH OF COORDINATES
   In USNO-A1.0, the coordinates were computed from the positions measured
   on the blue plate (O or J), so they were J2000 at the epoch of the
   blue plate.  For USNO-A2.0, we believe that the uncertainties in the
   positions are no longer dominated by systematic errors, so it makes
   sense to average the blue and red positions.  Hence, USNO-A2.0 coordinates
   are J2000 at the epoch of the mean of the blue and red exposure.  For
   POSS-I plates, this difference is trivial because the plates were taken
   on the same night.  For SERC-J and ESO-R, there can be a significant
   epoch difference between the blue and red plate, and stars with small
   proper motions will be affected.  Note that stars with large proper
   motions will be selectively deleted from the SERC-J+ESO-R portion
   of the sky because they will fail the test of blue and red positions
   within a 2 arcsec radius, and that this omission depends on the
   epoch difference of the plates for the individual fields.
7.  MULTIPLE ENTRIES
   We have done our best to remove multiple entries of the same star, but
   they still remain.  The improved astrometric reduction decreased the
   number of stars in the catalog by about 0.8% (about 4 million stars),
   but this reduction is masked by the increase in the number of stars
   associated with moving the north/south transition from about -33 degrees
   to about -17.5 degrees.  In the north/south overlap zone, double
   entries are generated for stars with large proper motions since if
   they were detected in each survey separately but moved far enough
   to escape the double detection removal algorithm.  There shouldn't
   be too many of these, but they may be obvious because they are
   statistically brighter than the typical catalog entry.
8.  BRIGHT STARS
   Images for stars brighter than about 11th magnitude are so difficult to
   measure that their computed positions may differ with the correct
   position by more than the 2 arcsecond coincidence radius used in the
   reductions.  For really bright stars, all that appears are an ensemble
   of spurious detections associated with diffraction spikes, halos, and
   ghosts.  To make USNO-A2.0 a useful catalog, bright stars were inserted
   into it so that the catalog is a better representation of the optical
   sky.  For may applications, it is better to know that a bright star
   is nearby than it is to insist that the poorly measured objects be
   deleted from the catalog.  In compiling USNO-A2.0, a list of all
   ACT stars that were correlated with PMM detections was kept.  For
   these stars, USNO-A2.0 contains the PMM position, not the ACT position,
   and the flag bit is set to indicate the correlation.  In the compilation
   process, all uncorrelated ACT stars were inserted into the catalog
   using the ACT coordinates.  However, ACT is not complete at the bright
   end because it omits stars with low astrometric quality.  Hence,
   a final pass inserted all Tycho stars that do not appear in the ACT
   catalog at the Tycho position.  According to the documents published
   with the Tycho catalog, every effort was made to make it complete at
   the bright end, even for stars with low astrometric quality.
   Note that one should not use the coordinates of ACT and Tycho stars
   presented in USNO-A2.0 for critical applications.  ACT stars appear
   at the epoch of the plate, but because the proper motions for the
   non-ACT Tycho stars are unreliable, these stars appear at the epoch
   of the Tycho catalog.
9.  PRETTY PICTURES
   The all-sky pretty pictures generated from USNO-A2.0 used an algorithm
   to reduce the over-density of southern stars that arises from the fainter
   limiting magnitudes of the SERC-J and ESO-R plates.  This was done
   by using a random number generator and omitting the star if the
   random number was less than 0.45.  That is to say, the southern
   over-density is not quite a factor of 2 more objects per unit area
   than found from the northern surveys.  Again, all objects are in
   USNO-A2.0 and the over-density was removed to make the pretty pictures.
11.  SOURCE CODE
   As with USNO-A1.0, we have published the source code for all computations
   and for all calibration.  The compilation code is in ALPHA13.TAR in
   the directories ./newbin/procN.  The code for the numerical refocus
   is in NEWBIN.TAR ./newbin/newz0 and for the fixed pattern removal
   in ./newbin/tycho2xtaff.
   The code is published as a service to those who wish to understand
   USNO-A2.0 and not so that we can be ripped off.  Please respect the
   intellectual property rights contained in the source code, and
   do not make us wake up the lawyers.
Enjoy!  If you use USNO-A2.0 for neat stuff, drop me an e-mail.
-Dave
=====read.1st - the top level readme file==================
This CD-ROM contains USNO-SA1.0, the spatially sub-sampled version of USNO-A1.0
created to provide an astrometric reference catalog for asteroid hunters and
other folks who do not need the entire USNO-A catalog.  The format is the same
as USNO-A1.0 and the various read files from that distribution are included
on this CD.  Also, a tar file called ugapa.tar contains the source code for
the routines used to extract this catalog from USNO-A and to check it.
The rest of this file contains a summary of the extraction process, but
serious users should consult the source code to verify the exact sense of
the limits and parameters, particularly those contained in getqual.f and
each.f.
Please refer to read.me for the Copyright Notification, the Request for
Citation, and an introduction to the USNO-A1.0 catalog from which this
catalog was extracted.
Please refer to the PMM page located under the USNO web page
   http://www.usno.navy.mil
for pointers to updates and third party software that can be used to read
this catalog, etc.
Third-party catalog access software for UNIX and MS-DOS has been written by
Bruce Koehn of Lowell Observatory. Source (FORTRAN) and executable (MS-DOS
only) code can be downloaded from Lowell Observatory's Web page at
   http://www.lowell.edu
------------------------------------
Discussion of the Extraction Process
------------------------------------
The size of the sampling pixel is 7.5/285 degrees on a side.  It was chosen
because it
   (a) equally divided the 7.5 degree zone width of USNO-A,
   (b) produced a catalog of approximately 55 million entries, which is
       just about as much as a CD-ROM can hold,
   (c) is very close to making a pixel with an area of 2.5 square arcminutes.
The pixels were created in a reasonable manner, but are not exactly square.
Please refer to rdpixels.f for the exact details, but the algorithm finds
the nearest integer number of pixels of the requested size in a band of
declination, and then computes the limits as constant alpha and delta.
The zero point for each zone of pixels is offset so that if there were the
same number of pixels in adjacent zones, then the centers of one zone
would be under/above the edges of the next zone.
The USNO-A catalog is read, and the pixel for each entry is computed.  Each
new entry is evaluated according to the following rules, which are contained
in getqual.f and each.f
   (a) If this is the first entry for a pixel, take it.
   (b) If the quality of this entry is better (lower) than the current
       value, take this entry.
   (c) If the quality of this entry is the same (equal) to the current
       value, then choose whichever is closest to the center.
The quality is defined by the following algorithm.
   (a) If 16.0 <= m_B <= 19.0 then q = 1
   (b) Else if 14.0 <= m_B <= 20.0 then q = 3
   (c) Else q = 5
   After defining q, a penalty is added for a weird color.
   (a) If (m_B-m_R) < 0.5 or (m_B-m_R) > 2.0 then q = q+1
   These limits are approximately defined by F0 to K5 spectral type.
The distance from the center of the pixel is not quite the geometric one
because of the desire to choose a brighter star in dense regions.  The
algorithm proceeds as follows.
   (a) i = 5*SQRT((x-x0)**2 + (y-y0)**2)/SIZE
   This divides the pixel into radial zones from 0 at the center to 7 in
   the corner.  Making an integer means that all entries in a particular
   zone are assigned the same weight.  This radial zone is them modified by
   (b) r = 1000*i + 10*m_B
   For stars closer to the center of the pixel, any brightness will be
   chosen over a star in a more distant zone.  For stars in the same zone,
   a brighter star will replace a fainter star.  Remember that the quality
   test is applied first, so really bright or faint stars don't enter
   into this decision.
The goal of the algorithm was to find a spatially uniform sample of solar
type stars, and to choose brighter ones in crowded regions.  I hope that
this algorithm is a reasonable approximation.  Using this algorithm,
54,787,624 stars were selected, leaving approximately 10% of the pixels
empty.  The breakdown according to quality is as follows.
   Quality     Number of entries
   -----------------------------
      1           34,454,509
      2            5,170,963
      3            6,014,824
      4            2,264,809
      5            4,745,419
      6            2,137,100
Since the format is the same as USNO-A catalog, I have included the
demo.tar file which contains the program square.f which extracts stars
in something like a square.  You will need to hack it to point to the
catalog on your system, but it ought to be a reasonable template for
accessing the catalog.  If there are problems, let me know.
-Dave Monet	(dgm@nofs.navy.mil)
This is the top level documentation file associated with the USNO-SA2.0
distribution.  Fortunately, it can be really simple.  USNO-SA2.0 is quite
similar to USNO-SA1.0, and if you are using the earlier version, all
you really need to do is substitute the new .ACC and .CAT files.
If you are new to USNO-SA, then you should start by reading READMESA.V10
which contains the READ.1ST file from that distribution.
The rest of the READ* files in this distribution come from the USNO-A2.0
distribution (and from USNO-A1.0), and provide extended documentation.
We are migrating Web sites, so please be patient during this transition.
The old site is
   http://www.usno.navy.mil/nofs
and the new site is
   http://www.nofs.navy.mil
Our intention is to provide the latest versions of the various READ* files
and other useful stuff on the PMM's page in the Web site, and to make the
long files available by FTP or CD-ROM.
If you wish to use USNO-SA for commercial applications or intend to
redistribute it, please check with us first.  This catalog contains
intellectual property rights claimed by many organizations (see README.V10)
and these must be respected.
-Dave
	(Dave Monet is dgm@nofs.navy.mil)
-------------------------------------------------------------------------------
Yann Pothier
11 impasse Canart, 75012 PARIS, FRANCE
yann.pothier@fnac.net
http://www.astrosurf.com/cielextreme
http://astrosurf.com/skylink/publi/cielextreme
To UNSUBSCRIBE from the 'netastrocatalog' lists, use the Web form at:
http://www.tiac.net/users/lewkaren/netastrocatalog/subscribe.html

Follow-Ups: References: