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(±1σ 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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
out.sam 140 1000 Sample Output
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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
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Bytes Format Units Label Explanations
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1- 14 A14 --- APM-ID [EO0-9-] Designation of the object (plate and
sequential numbers separated by a dash -)
16- 25 F10.6 deg RAdeg Right Ascension (J2000)
27- 35 F9.6 deg DEdeg Declination (J2000)
37- 44 F8.1 pix Xpix X Position in pixel from center of plate
46- 53 F8.1 pix Ypix Y Position in pixel from center of plate
55- 59 F5.2 mag rmag Magnitude of red image
61- 61 A1 --- l_rmag [L] 'L' when rmag is the plate limit (3)
63- 64 I2 --- rClass [-1,2] Red Image class code (1)
66- 71 F6.2 --- 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
89- 93 F5.2 mag bmag Magnitude of blue image
95- 95 A1 --- l_bmag [L] 'L' when bmag is the plate limit (3)
97- 98 I2 --- bClass [-1,2]? Blue Image class code (1)
100-105 F6.2 --- 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 --- b-r Colour
130-130 A1 --- l_b-r [L] 'L' when b-r is affected by plate limits
132-132 A1 --- Mflag [*] '*' if edge effect in matching.
134-140 F7.2 yr Epoch Epoch of observation (red plate)
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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.
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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 1989A&AS...78..249E
Humphreys, R.M., Landau, R.L., Ghigo, F.D., Zumach, W., LaBonte, A.E. 1991,
1991AJ....102..395H 1991AJ....102..395H
Irwin, M. 1985MNRAS.214..575I 1985MNRAS.214..575I
Irwin, M. 1994, IAU Commission 9 Newsletter, 5, 25.
Lund, J., & Dixon, R. 1973PASP...85..230L 1973PASP...85..230L
Maddox, S.J., Sutherland, W.J., Efstathiou, G. & Loveday, J., 1990a,
1990MNRAS.243..692M 1990MNRAS.243..692M
Maddox, S.J., Efstathiou, G. & Sutherland, W.J. 1990b, 1990MNRAS.246..433M 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 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.
History:
* 13-Aug-2024: format migration (CDS)
(End) Richard G. McMahon [Cambridge] Francois Ochsenbein [CDS] 05-Dec-2000