Readme for Public Export version of APM catalogue
Draft 0.2
Richard McMahon 2000, Jan 16
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The APM-POSS1 Sky Catalogue 1.0 (McMahon+ 2001)
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The APM-POSS1 1.0 Northern Sky Catalogue
(McMahon+ 2000)
McMahon R.G., Irwin, M.J., Maddox, S.J.
Institute of Astronomy, Cambridge, CB3 OHA, UK
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Keywords: Optical; Positional data; Photometry; Stars; Galaxies
Description:
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 61 arc/mm(16.4
micron/arcsec). Further details about the survey material
can be found in Minkowski and Abell 1963 and Lund and Dixon 1973.
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 ---
$\sim$20.0 in $E$ and $\sim$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) 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) 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 10$^6$ 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 $\sim$10 photometric sequences (Evans 1989; Humphreys
et al. 1991). Similarly, a single slope of 1.10, was assumed in
converting between the linearized APM magnitudes (Bunclark \& Irwin
1983) and the alpha-Lyrae based Johnston magnitude system. It was
noted at the time that there were significant deviations(1mag)
from a simple linear relation at magnitudes brighter than V=15.
This is not surprising bearing in mind that the POSS-I glass plates
measured by the APM are copies that may have different degrees of
saturation and have had their contrast stretched to enhance faint
features. The assumption of a constant flux limit seemed reasonable,
since the plates were all taken in similar dark sky observing
conditions with exposure times that were adjusted to ensure uniform
sensitivity. A similar assumption is made in all modern photographic
cameras where it is assumed that all photographic film has the
specified speed. The blue band ($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(+-1sigma 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. (1991a,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.
Byte-by-byte Description of FITs long format version of Catalogue
-------------------------------------------------------------------
22 fields
76 bytes per object
TTYPE1 = 'APMNo ' / label for field 1
TFORM1 = '1J ' / data format of field: 4-byte INTEGER
TUNIT1 = 'Integer ' / physical unit of field
TTYPE2 = 'RightAscen' / label for field 2
TFORM2 = '1E ' / data format of field: 4-byte REAL
TUNIT2 = 'Degrees ' / physical unit of field
TTYPE3 = 'Declination' / label for field 3
TFORM3 = '1E ' / data format of field: 4-byte REAL
TUNIT3 = 'Degree ' / physical unit of field
TTYPE4 = 'Xcoord ' / label for field 4
TFORM4 = '1E ' / data format of field: 4-byte REAL
TUNIT4 = 'Pixel ' / physical unit of field
TTYPE5 = 'Ycoord ' / label for field 5
TFORM5 = '1E ' / data format of field: 4-byte REAL
TUNIT5 = 'Pixel ' / physical unit of field
TTYPE6 = 'ImClass_1' / label for field 6
TFORM6 = '1I ' / data format of field: 2-byte INTEGER
TUNIT6 = 'Integer ' / physical unit of field
TTYPE7 = 'Stellaric_1' / label for field 7
TFORM7 = '1E ' / data format of field: 4-byte REAL
TUNIT7 = 'sigma ' / physical unit of field
TTYPE8 = 'MajorAxis_1' / label for field 8
TFORM8 = '1E ' / data format of field: 4-byte REAL
TUNIT8 = 'Arcsec ' / physical unit of field
TTYPE9 = 'Ellipt_1' / label for field 9
TFORM9 = '1E ' / data format of field: 4-byte REAL
TUNIT9 = '(1-a/b) ' / physical unit of field
TTYPE10 = 'PA_1 ' / label for field 10
TFORM10 = '1E ' / data format of field: 4-byte REAL
TUNIT10 = 'Degree ' / physical unit of field
TTYPE11 = 'Magnitude_1' / label for field 11
TFORM11 = '1E ' / data format of field: 4-byte REAL
TUNIT11 = 'Mag ' / physical unit of field
TTYPE12 = 'MagFlag_1' / label for field 12
TFORM12 = '1I ' / data format of field: 2-byte INTEGER
TUNIT12 = 'Flag ' / physical unit of field
TTYPE13 = 'ImClass_2' / label for field 13
TFORM13 = '1I ' / data format of field: 2-byte INTEGER
TUNIT13 = 'Integer ' / physical unit of field
TTYPE14 = 'Stellaric_2' / label for field 14
TFORM14 = '1E ' / data format of field: 4-byte REAL
TUNIT14 = 'sigma ' / physical unit of field
TTYPE15 = 'MajorAxis_2' / label for field 15
TFORM15 = '1E ' / data format of field: 4-byte REAL
TUNIT15 = 'Arcsec ' / physical unit of field
TTYPE16 = 'Ellipt_2' / label for field 16
TFORM16 = '1E ' / data format of field: 4-byte REAL
TUNIT16 = '(1-a/b) ' / physical unit of field
TTYPE17 = 'PA_2 ' / label for field 17
TFORM17 = '1E ' / data format of field: 4-byte REAL
TUNIT17 = 'Degree ' / physical unit of field
TTYPE18 = 'Magnitude_2' / label for field 18
TFORM18 = '1E ' / data format of field: 4-byte REAL
TUNIT18 = 'Mag ' / physical unit of field
TTYPE19 = 'MagFlag_2' / label for field 19
TFORM19 = '1I ' / data format of field: 2-byte INTEGER
TUNIT19 = 'Flag ' / physical unit of field
TTYPE20 = 'MatchFlag_1' / label for field 20
TFORM20 = '1I ' / data format of field: 2-byte INTEGER
TUNIT20 = 'Flag ' / physical unit of field
TTYPE21 = 'Col_1 ' / label for field 21
TFORM21 = '1E ' / data format of field: 4-byte REAL
TUNIT21 = 'Colour ' / physical unit of field
TTYPE22 = 'Colflag_1' / label for field 22
TFORM22 = '1I ' / data format of field: 2-byte INTEGER
TUNIT22 = 'Flag ' / physical unit of field
see Notes after the description of the ASCII header
Byte-by-byte Description of long format version of APM POSS1 Catalogue
Bytes Format Units Label Explanations
1- 4 I4 --- FldNo POSS1 field number
6- 13 I8 --- APMObjno Original APM catalogue object number
15- 16 I2 hr RAh Right Ascension J2000 (hours)
18- 19 I2 min RAm Right Ascension J2000 (minutes)
21- 25 F5.2 sec RAs Right Ascension J2000 (seconds)
28 A1 --- DECsign Declination J2000 (sign)
29- 30 I2 deg DECd Declination J2000 (degrees)
32- 33 I2 arcmin DECm Declination J2000 (minutes)
35- 48 F4.1 arcsec DECs Declination J2000 (seconds)
50 I1 --- ImClasE Image classification on E plate
51- 57 F7.2 --- StelE Stellaricity on E plate
58- 65 F7.2 arcsec RmajE Major axis diameter of image on E plate
66- 83 F7.3 arcsec EllipE Ellipticity of image on E plate
84- 90 I6.1 degree PAE Position angle of image on E plate
91- 96 F6.2 mag MagE Magnitude of image on E plate
97 I1 flag MagEFlag Magnitude flag
98-110 A12 --- EpochE Epoch of E plate
111-118 A8 POSS1 E Plate
119 I1 --- ImClasO Image classification on O plate
120-126 F7.2 --- StelO Stellaricity on O plate
127-133 F7.2 arcsec RmajO Major axis diameter of image on O plate
134-140 F7.2 arcsec EllipO Ellipticity of image on O plate
141-145 F5.1 degree PAO Position angle of image on O plate
146-152 F7.2 mag MagO Magnitude of image on O plate
153 B flag MagOFlag Magnitude flag
154-165 A12 --- EpochE Epoch of O plate
166-171 A6 POSS1 O Plate
172 I1 --- Maflg BR Match flag
173-179 F7.2 colour OEcol B-R colour
180 B flag Col1Flag 0:OK; 1:upper limit; 2:lower limit;3:other
Notes:
Notes on APM catalogue name
This is a running number field at the moment but
maybe this should be ASCII name eg EO1393-nnnnnnn ie A14
Notes on Right Ascension and Declination
Maybe the RA, Dec should be HH MM SS.SS SDD MM SS.S
The right ascension and declination are based solely on the red plates
for objects that are detected on the red plates. In cases where object
is only detected on the blue plate the position on the blue plate is
provided.
Notes on Positional errors:
No positional errors are provided in the APM catalogue.
However empirical comparions with external radio catalogue
indicate that the rms positional errors are 0.5".
Notes on the Match Flag and plate matching:
The APM matching is carried out in the original measuring
machine pixel reference frame.
An initial match is done using a search radius of 5 arcseonds.
This match is used to analyze the residual and a distortion map
is used. A nearest match search is then repeated at a
radius of 5 arcseconds. Objects that lie within 2" radius are
those within 2 to 5 arcsecs are termed "nearly matched".
In some cases, eg large images, the centroids may be more than
5" apart and hence these objects appear in the catalogue twice
as neighbouring E only detections and O only detections.
Notes on the magnitudes
Notes on the colours
Notes on the Image Classification
Image Class Codes
-1: stellar
0: noise
1: non-stellar; probably galaxy
2: merged image
psf is statistical deviation(sigma) from normalised psf
Notes on the Position Angle(PA)
The position angle PA is in degrees the standard astronomical
sense ie East of North ie anti-clockwise looking at the original
plates. The APM processing measures the PA in a cartesian frame.
The image PA's have been converted to a celestial
orientation.
Notes on Magnitude flag
0 if OK
1 if magnitude is a limit
-1 if edge effect
Response for POSS1 O and E wavebands
# reference to origin
#
# Tabulated from Fig 1 in
# Minkowski, R.L., \& Abell, G.O., 1963, Stars and Stellar Systems Vol
# III, Basic Astronomical Data, University of Chicago
# Press IX, 481.
#
# see also
# National Geographic Society and Palomar Observatory Sky Survey
# Catalogue of Plates, 1960.
#
#
# 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.
#
# The blue(O)
# band pass is primarily defined on the UV side by the transmission
# characteristics of the glass corrector and the atmosphere,
# and on the red side by the emulsion.
#
# The red(E) bandpass
# is primarily defined on the blue side by a coloured filter
# (red Plexiglas 2400 which is similar to Wratten 29). The
# red side is determined by the emulsion response.
#
#
# Response of plate-filter-corrector combination:
#
#
# POSS-1 O:
#
#
3200.00 0.
3300.00 2.00000E-02
3400.00 0.340000
3500.00 0.580000
3600.00 0.740000
3700.00 0.850000
3800.00 0.920000
3900.00 0.980000
4000.00 1.00000
4100.00 1.01000
4200.00 0.990000
4300.00 0.950000
4400.00 0.880000
4500.00 0.800000
4600.00 0.700000
4700.00 0.600000
4800.00 0.490000
4900.00 0.380000
5000.00 0.270000
5100.00 0.150000
5200.00 5.00000E-02
5300.00 0.
#POSS-1 E
5800.00 0.
5900.00 2.00000E-02
6000.00 6.00000E-02
6100.00 0.180000
6200.00 0.360000
6300.00 0.680000
6400.00 0.960000
6500.00 1.00000
6600.00 0.600000
6700.00 0.180000
6800.00 2.00000E-02
6900.00 0.