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Detailed Description of I/267 :
The APM-North Catalogue (McMahon+, 2000)

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Note : this is the description file of the original catalog. This HTML page corresponds well to the original description, but some minor changes in the format may have been introduced in the FITS output files in order to allow for arithmetic operations on quantities such as coordinates and times. h:m:s and d:m:s units are normally converted into degrees, and YYYY-MM-DD into julian days.
Similarly, some slight changes may have been introduced in the units; the value given in the HTML files supersedes the value indicated in the ReadMe file for this catalogue.
I/267               The APM-North Catalogue                 (McMahon+, 2000)
================================================================================
The APM-North Catalogue
     McMahon R.G., Irwin, M.J., Maddox, S.J.
    <Institute of Astronomy, Cambridge, CB3 OHA, UK (2000)>
================================================================================
ADC_Keywords: Surveys ; Photographic catalog ; Positional data ;
              Photometry, photographic
Keywords: Optical; Positional data; Photometry; Stars; Galaxies


Description:

   ******************************************************************
    This version is a preliminary adaptation of the APM, covering
    the Northern sky at high galactic latitudes only.
   ******************************************************************

    The catalogue APMCAT-POSS1-1.0 is derived from the first epoch
    (1949-1958) Palomar Observatory-National Geographic Sky Survey (POSS).
    The catalog is based on digitised scans with the laser based Cambridge
    Automated Plate Measurement(APM) machine of both the blue O plates and
    red E plates. The plates are scanned with a pixel sampling 8microns
    which corresponds 0.49 arcsecs at the nominal plate scale of 61arcsec/mm
    (16.4 micron/arcsec). Further details about the survey material
    can be found in Minkowski and Abell 1963 and Lund and Dixon 1973.

Astrometry of the APM Catalogue:

    The main properties of the APM catalogue that differentiate it from
    other public catalogues of digitisation programs of the POSS1 are:

  o scanned at high spatial resolution 0.49arcsec pixels (cf DSS which
    used n.n arcsec pixels

  o image are classified into stars and galaxies(unlike USNO)

    The APM catalog currently includes all plates with J2000 plate centres
    between +90 declination and 0 declination inclusive and |b|>25. The
    catalogued area covers over 10000 square degrees and contains over
    100million objects. At the plate limit we have attempted to detect all
    images that are detectable on the plates with the inevitable price
    that some spurious image are detected.

    The APM POSS1 catalogue contains measurements from both the blue(O)
    and red(E) plates. The POSS is based on plates taken in two wavebands
    during the period 1949 to 1955. The blue plates were taken using
    Eastman 103a-O emulsion, and the red plate were taken with Eastman
    103a-E emulsion and hence are commonly known as O and E plates
    respectively.

    Between 1991 and 1995, the SERC Automated Plate Measuring Machine
    (APM) at the Institute for Astronomy, Cambridge (Kibblewhite et al.
    1984) was used to digitize these plates at a resolution of 8.0 microns
    (0.54 arcsec), the highest spatial resolution yet applied to these
    images (McMahon and Irwin 1992). An object catalog has been
    constructed from these data which includes all objects down to the
    plate limits --- ~20.0 in E and ~21.5 in O --- and contains
    approximately 2000 stars and 2000 galaxies deg^-2 at high Galactic
    latitudes. The catalog contains positions, magnitudes, morphological
    classification parameters, major and minor axes, and isophotal areas
    for each source; a merged catalog which matches objects between plates
    also contains a color (or an upper limit thereto) for each entry. An
    automated classification algorithm interprets the morphological
    parameters to classify each object. Here, we present the basic
    procedures which establish the astrometric and photometric calibration
    of the APM catalog, and discuss the limits of the image classification
    system.

    The APM machine measures the x and y positions of all objects
    detected. The conversion relationship between these measured APM
    positions and celestial coordinates is derived by matching stars in
    the Tycho catalog (Hog et al. 1997, see Cat. I/250) with stars
    detected on each plate using a `standard' six plate-constant model
    that allows for shift, rotation, scale, and shear. The algorithm uses
    2-sigma clipping to give a robust fit; the typical rms on the fitted
    positions of the Tycho stars are 0.4"-0.8"

    Irwin (1994) has studied the two-dimensional systematic errors in an
    earlier version of the APM catalog positions by investigating the
    intraplate residuals between the measured positions for bright stars
    in the Positions and Proper Motions Catalog (Roser and Bastian 1991,
    Cats I/146 and I/193)) and the astrometric fit. He found
    significant, systematic residuals ranging up to 0.5". A residual map
    generated from this analysis is applied to positions in the standard
    APM catalog.

APM Photometry:

    The APM measures photographic density rather than flux; moreover, the
    central regions of all objects more than a factor of 10 brighter than
    the sky produce a nonlinear response and/or are saturated. The
    algorithms used to overcome these inherent difficulties are discussed
    in detail by Irwin (1985). Briefly, a local background is determined
    for each of 106 locations on each plate by producing a histogram of
    the pixel values in 64 x 64-pixel regions and finding the mode of each
    distribution using a maximum likelihood estimator; a two-dimensional
    smoothing is applied to these million background estimates to derive a
    background model for the plate. The image detection algorithm then
    finds connected regions of pixels above a threshold level (typically
    2-sigma above the estimated background level for the given plate
    position). This background-following technique has the advantage that
    faint objects lying in the halos of bright objects can be detected.
    However, large objects such as bright stars and galaxies with angular
    extents >30' have their raw fluxes underestimated. An additional
    problem for large images is that the limited memory available to the
    software means that bright objects sometimes overflow the pixel
    buffers and are lost. This occurs for images with sizes greater than
    roughly 1--2mm (i.e. 1-2'), corresponding to stellar magnitudes
    brighter than around V=9.

    Another inherent problem arises in attempting to derive magnitude
    estimates for extended objects from saturated images. Saturation
    effects can be corrected for in stars by assuming that stellar images
    have an intrinsic density profile independent of magnitude, and that
    this profile can be derived from the unsaturated parts of stellar
    profiles. A high signal-to-noise intrinsic profile is constructed by
    taking the core from faint stars and the wings from brighter stars
    (see Bunclark & Irwin (1983) for further details). This profile can
    then be integrated and used to derive a calibration curve to convert
    saturated stellar magnitudes to a linear system. In the APM catalog,
    this calibration is applied to all images. This has the unfortunate
    consequence that galaxies, which have shallower surface brightness
    profiles and lower central surface brightnesses than stars of the same
    total magnitude, will have their magnitudes over-corrected. This is a
    fundamental problem for galaxy photometry determined from photographic
    sky survey plates (see Metcalfe, Fong, and Shanks (1995) for a
    discussion).

    The basic APM catalog is to have a red-band (E) plate limit of
    m(r)=20.0. This limit was established during the early stages (1991)
    of the creation of the APM catalog via comparison with ~10 photometric
    sequences (Evans 1989; Humphreys et al. 1991). Similarly, a single
    slope of 1.10, was assumed in converting between the linearized APM
    magnitudes (Bunclark & Irwin 1983) and the alpha-Lyrae based Johnston
    magnitude system. It was noted at the time that there were significant
    deviations(1mag) from a simple linear relation at magnitudes brighter
    than V=15. This is not surprising bearing in mind that the POSS-I
    glass plates measured by the APM are copies that may have different
    degrees of saturation and have had their contrast stretched to enhance
    faint features. The assumption of a constant flux limit seemed
    reasonable, since the plates were all taken in similar dark sky
    observing conditions with exposure times that were adjusted to ensure
    uniform sensitivity. A similar assumption is made in all modern
    photographic cameras where it is assumed that all photographic film
    has the specified speed. The blue band (O) limit was defined with
    respect to the red limit; for the 428 fields available in March 1999,
    this has a range of m(o)=20.6--21.3(±1{sigma} range).

    Eventually, a full photometric recalibration of the APM using the
    Guide Star Photometric Catalog (GSPC -- Postman et al. 1998a) CCD
    sequences is planned. Preliminary comparisons with CCD photometry for
    5% of the POSS-I plates show that the APM magnitudes for stellar
    objects have a global rms uncertainty of 0.5 magnitudes over the range
    16 to 20. As discussed above, the uncertainties in the magnitudes of
    galaxies are more complex, since galaxies have a range of surface
    brightness distributions, and hence may have complex, partially
    saturated surface brightness profiles on the POSS-I plates. This is
    compounded by the range in calibration slopes observed. At faint
    magnitudes (18--20) where the image profiles are unsaturated, the APM
    magnitudes may be more reliable, but it is left to the reader to
    verify this where precise magnitudes are required. For many programs,
    a uniform set of magnitudes or uniform selection criteria are more
    critical.

    The APM scans result in a parameterization of each detected image
    which includes an x,y position, a peak intensity, a total isophotal
    intensity, second moments of the intensity distribution, and areal
    profiles (defined as the number of pixels above preset levels which
    increase by powers of two above the threshold level). In addition, a
    parameter, psf, is calculated which reports by how many sigma the
    object differs from the stellar point-spread function of its plate.
    These parameters are then used to classify all images into one of four
    categories: stellar (consistent with the magnitude- and
    position-dependent point spread function, cl=-1), non-stellar (a
    measurably extended source, cl=1), merged objects (sources with two
    local maxima within a single set of connected above-threshold pixels,
    cl=2), and noise (objects with nonphysical morphologies, cl=0). For
    further details of the principles involved, see Maddox et al.
    (1990a,b). Very bright images can often be misclassified, since the
    limited set of parameters does not provide an adequate description and
    the background-following algorithm attempts to track over them in
    order to detect the faint images in the source halos. The
    merged/non-stellar boundary is not as reliable as the
    stellar/non-stellar boundary, so merged stars are often found in the
    non-stellar list (with a smaller number of galaxies in the merged
    list). Some objects classified as noise are real; objects found on
    both plates are the obvious examples.


    Bright objects (e.g. O, E < 13) cover a large number of pixels in the
    APM scans and, as a consequence, magnitude and source-size estimates
    are very sensitive to small uncertainties in the plate sky level and
    details of the background-following algorithm; as a result, large
    uncertainties in the parameter estimations can result, and very bright
    sources can even be completely missing from the catalog. In addition,
    bright galaxies with complex surface brightness distributions can be
    broken up into a swarm of discrete sources. At fainter magnitudes, the
    limitations of the plate material make reliable separation of stellar
    and non-stellar sources problematic.

File Summary:

FileName Lrecl Records Explanations
× 80 . This file × ReadMe 80 . This file × out.sam 138 100 Sample Output
See also: http://www.ast.cam.ac.uk/~apmcat/ : APM Catalog Home Page VI/25 : POSS Plate Data (Palomar Obs. 1960) I/252 : The USNO-A2.0 Catalogue (Monet+ 1998) Byte-by-byte Description of file: out.sam
Bytes Format Units Label Explanations
1- 14 A14 --- APM-ID [EO0-9-] Designation of the object (plate and sequential numbers separated by a dash -) 16- 23 F8.1 pix Xpix X Position in pixel from center of plate 25- 32 F8.1 pix Ypix Y Position in pixel from center of plate 34- 43 F10.6 deg RAdeg Right Ascension (J2000) 44- 53 F10.6 deg DEdeg Declination (J2000) 56- 60 F5.2 mag rmag Magnitude of red image 61 A1 --- l_rmag [L] 'L' when rmag is the plate limit (3) 63- 64 I2 --- rClass [-1,2] Red Image class code (1) 65- 71 F7.2 --- rStel [-100,100]? Statistical deviation from PSF (2) 73- 78 F6.1 arcsec rDiam ? Major axis diameter of image on red plate 80- 83 F4.2 --- rEll [0,1]? Ellipticity (1-a/b) on red plate 85- 87 I3 deg rPA ? Position angle of the ellipse 90- 94 F5.2 mag bmag Magnitude of blue image 95 A1 --- l_bmag [L] 'L' when bmag is the plate limit (3) 97- 98 I2 --- bClass [-1,2]? Blue Image class code (1) 99-105 F7.2 --- bStel [-100,100]? Statistical deviation from PSF (2) 107-112 F6.1 arcsec bDiam ? Major axis diameter of image on blue plate 114-117 F4.2 --- bEll [0,1]? Ellipticity (1-a/b) on blue plate 119-121 I3 deg bPA ? Position angle of the ellipse 123-128 F6.2 mag b-r Colour 129 A1 --- l_b-r [L] 'L' when b-r is affected by plate limits 130 A1 --- Mflag [*] '*' if edge effect in matching. 131-138 F8.3 yr Epoch Epoch of observation (red plate)
Note (1): the flag takes the values: -1 = stellar 0 = noise 1 = non-stellar, probably galaxy 2 = merged image Note (2): the stellaricity is the statistical deviation from a normalized point-spread function (PSF): a negative value indicates a profile sharper than a star, and a positive value smoother Note (3): the letter 'L' is set when no image could be found in that color; the limit adopted is the plate limit.
History: Prepared by Richard McMahon References: Bunclark, P.S., & Irwin, M.J. 1983, in "Proc. Statistical Methods in Astronomy", ESA SP-201, edited by E.J. Rolfe, p195 Evans, D.W. 1989A&AS...78..249E Humphreys, R.M., Landau, R.L., Ghigo, F.D., Zumach, W., LaBonte, A.E. 1991, 1991AJ....102..395H Irwin, M. 1985MNRAS.214..575I Irwin, M. 1994, IAU Commission 9 Newsletter, 5, 25. Lund, J., & Dixon, R. 1973PASP...85..230L Maddox, S.J., Sutherland, W.J., Efstathiou, G. & Loveday, J., 1990a, 1990MNRAS.243..692M Maddox, S.J., Efstathiou, G. & Sutherland, W.J. 1990b, 1990MNRAS.246..433M McMahon, R.G., & Irwin, M.J. 1992, in "Digitised Optical Sky Surveys", eds. H.T. MacGillivray & E.B. Thomson (Dordrecht: Kluwer), p417 Metcalfe, N., Fong, R., Shanks, T. 1995MNRAS.274..769M Minkowski, R.L., & Abell, G.O., 1963, Stars and Stellar Systems Vol III, Basic Astronomical Data, University of Chicago Press IX, 481. Postman, M., Bucciarelli, B. Sturch, C., Borgman, T. Casalegno, R., Doggett, J., & Costa, E. 1998a, in "New Horizons from Multi-Wavelength Sky Surveys" (IAU Symposium 179), ed. B.J. McLean, D.A. Golumbeck, J.J.E. Hayes, & H.E. Payne (Amsterdam: Kluwer), p379. ================================================================================ (End) Richard G. McMahon [Cambridge] Francois Ochsenbein [CDS] 05-Dec-2000