Photo Object Lists

1. File Naming Conventions

The principal output of the photometric pipeline (photo) is a list of detected objects with parameters in 5 bands. These outputs, expressed in instrumental units, are stored in files with the naming convention

o_0311.fit

where the prefix o is short for objc (the SDSS nomenclature for a detected object) and 0311 is the field number within a particular run and scanline. Calibrated versions of these files are stored in files with the naming convention

t_0311.fit

where the prefix t is short for tsObjc (the SDSS nomenclature for a calibrated object used by target selection algorithms). The astrometric and photometric conventions are described in data.htm: positions are for equinox J2000, and magnitudes are on the AB sytem.

For convenience of users, a subset of the outputs in this file are written out in plain ascii text in the files with the naming convention

t_0311.txt.

Several fields in the outputs contain flags. These are described briefly here, but for a fuller description, see document flags.htm and t.htm.

2. Description

The photometric pipeline processes 5 frames (one for each filter) per field simultaneously. An object detected in at least one frame is measured in all other frames, even if just to establish upper limits.

The first set of outputs describe the object as a whole:

id An id for the object within the frame; this id is the same as the object's row number in the table, and is used to tie the object lists (o_xxxx.fit) and atlas image tables (a_xxxx.fit) together.
parent For deblended objects, the id number of the object's parent, or -1 if it is a primary detection.
nchild Number of children for a deblended object. When selecting objects for analysis from the object list, one should pick objects with nchild=0.
ncolor The number of colors present in this table (5)
objc_type A flag giving the object's overall classification. Current types are:

UNK
An object of unknown type (the default).
CR
A cosmic ray.
DEFECT
Some indeterminate chip defect.
GALAXY
A galaxy.
GHOST
A ghost produced by the 2.5m optics.
KNOWNOBJ
A known object (from e.g. FIRST or ROSAT); its position and size are inputs to photo.
STAR
A Star.
TRAIL
A satellite, aeroplane, meteorite, or asteroid trail.
SKY
A pseudo-object; a location where no object was detected in any of the survey bands.

A tabulation of the encoding of these values is given in t.htm
objc_flags The union of certain of the flag bits set in each individual band (see discussion of flags below). Specifically, if the flags BLENDED, CHILD, EDGE, INTERP, MANYPETRO, NOPETRO, NOTCHECKED, or SATUR are set in any band, they are also set in objc_flags.
When analyzing these object lists, these flags should be used to exclude objects which are badly measured. The specific flags to be checked are EDGE, BRIGHT (note that objects with this flag set are actually entered twice in the object list) and SATUR.
objc_rowc, objc_colc, objc_rowcErr, objc_colcErr The canonical position of the object (and 1-sigma errors), in the r' coordinate system. If an object is detected in r', this is the r' center; otherwise it is a suitable average of the bands where the object was detected (pixels).
catID A catalog id number associated with KNOWNOBJs (not yet implemented).

The next set of outputs are reported for each band.

rowc, colc, rowcErr, colcErr The position of the object (and 1-sigma errors) in each band. The frames from the different bands are effectively aligned by computing the offset in position of many stars. (Objects with high proper motion or extended objects with extreme color distributions will exhibit additional position shift after the frames are aligned.) In the case that an object is not detected in a given band (say f'), the position is taken to be the r' position offset to the f' coordinate system (if detected in r'), and a suitable average of the other bands otherwise (pixels).
We will describe the position assigned in the r' as the r' position, even if the object was not in fact detected in r'.
sky, skyErr The sky level (and the 1-sigma), at the position of the object. (Counts/pixel²).
psfCounts, psfCountsErr The PSF-flux (and the 1-sigma error), at the position of the object measured in each band (counts).
fiberCounts, fiberCountsErr The counts inside a circular aperture of 3" diameter (and the 1-sigma error), at the r' position of the object, suitably transformed to the appropriate band.
petroRad, petroRadErr The Petrosian radius (and the 1-sigma error), measured using the $r'$ position of the object. See notes on Petrosian quantities in section 3 of this document (pixels).
petroCounts, petroCountsErr The Petrosian counts (and the 1-sigma error) within f₃r_P,r' of the r' center (counts).
petroR50, petroR50Err The Petrosian 50%-light radius (and 1-sigma error) (pixels).
petroR90, petroR90Err The Petrosian 90%-light radius (and 1-sigma error) (pixels).
Q, U, QErr, UErr The values of <col²/r² - row²/r²> and 2<col*row/r²> (and their 1-sigma errors), measured within r_P,r'. These are estimators of (a - b)/(a + b)cos(2*phi;) and (a - b)/(a + b)sin(2*phi;), and are named by analogy to the usual Stokes parameters.

nprof Three measurements of the radial profile are computed. This parameter tells how many points are included in the profile (larger objects have more steps). The profiles are measured using fixed radii as given in the following table:

Outer radius (pixels)	Outer radius (arcsec)	Enclosed area (pixels)
0.56	0.23	1
1.69	0.68	9
2.58	1.03	21
4.41	1.76	61
7.51	3.00	177
11.58	4.63	421
18.58	7.43	1085
28.55	11.42	2561
45.50	18.20	6505
70.51	28.20	15619
110.5	44.21	38381
172.5	69.00	93475
269.5	107.8	228207
420.5	168.2	555525
657.5	263.0	1358149

profMean The mean surface brightness within the innermost nprof annuli, with fixed radii given above; these fluxes may be used to recover the annular counts exactly (counts/pixel).
profMed The `median' surface brightness within the innermost nprof annuli, with fixed radii given above (counts/pixel).
profErr An estimate of the uncertainty in the profiles (counts/pixel); note that this is not the photon noise (which can be recovered from profMean), but an estimate of the true uncertainty allowing for contamination by stars, HII regions, etc.
iso_rowc, iso_colc, iso_a, iso_b, iso_phi, iso_rowcErr, iso_colcErr, iso_aErr, iso_bErr, iso_phiErr, iso_rowcGrad, iso_colcGrad, iso_aGrad, iso_bGrad, iso_phi The center, major and minor axes, and position angle of a certain isophote (pixels). For the files on the CDROM, the chosen isophote is 25 mag/arcsec² in all bands. The parameters are determined from the 2-dimensional extracted profile. The Grad quantities are correction terms allowing us to correct for variations in the isophotal radius in case the photometric calibration changes.
r_deV, counts_deV, ab_deV, phi_deV, r_deVErr, counts_deVErr, ab_deVErr, phi_deVErr Parameters of the de Vaucouleurs profile convolved with the PSF that best fits the radial profile (as determined by the cell array), and errors. The r and counts parameters are the effective radius and the total flux (in counts) of the model.
r_exp, counts_exp, ab_exp, phi_exp, r_expErr, counts_expErr, ab_expErr, phi_expErr Parameters of the exponential profile convolved with the seeing profile that best fits the radial profile (as determined by the cell array), and errors. The r and counts parameters are the effective radius and the total flux (in counts) of the model.
counts_model Total flux in counts of the model that best fits the objects.
star_L, exp_L, deV_L Likelihoods for the fits of a pure PSF, an exponential disk convolved with the PSF, and a de Vaucouleurs profile convolved with the PSF to the object. More specifically, the values quoted are the probabilities of finding a value of chi² at least as large as that found for the model fits.
fracPSF The fraction of the total light in the profile that can be assigned to a point source. Not currently calculated.
texture A measure of the roughness of the object, based on the residuals after inverting the image and subtracting. Not currently calculated.
type The type assigned to the object in this colour; the possibilities are described for the objc_type field.
flags Detailed flags describing the outcome of the data processing on the object. The encoding of the flags is defined in t.htm. A fuller description of the flags and how they are set in detail is given in flags.htm.

3. Petrosian Quantities

Let I(r) be (a spline fit to or other smooth representation of) the measured azimuthally averaged surface brightness of an object in r'.

The profMean profile will be used. The profile used for calculating Petrosian quantities is cutoff at the first radial bin for which the S/N is below 1.0. To avoid the necessity of interpolating to determine fluxes, the actual radial profile will be recorded out to the adopted Petrosian radius.

Define the Petrosian ratio R_P(r) as the ratio of the local surface brightness at radius r to the mean within r:

Petrosian radius

Mark all the radii r_i, i = 1,..., N where R_P falls to a specified value f₁, and for which I(r) > f₂.

If there is at least one such radius (N > 0), the largest of the r_i will be taken as the Petrosian radius r_P; If no such radius exists, there are two possibilities: that the lowest surface brightness in the object is above a specified value f₃ (in which case we adopt r_P = r_max}, the largest "good" radius in the profile), or that the highest surface brightness is below f₃ (in which case we take r_P = f₅).

The Petrosian flux F_P is defined as the total flux as measured within a certain number of Petrosian radii:

The Petrosian half-light r₅₀ is defined by the implicit equation:

Petrosian half-light radius

The Petrosian 90% radius is defined by the implicit equation:

The adjustable parameters used for the data sets on the CD-ROM are:

f₁	Petrosian Ratio	0.25
f₂	Minimum Surface Brightness at r_P	26.7
f₃	Fallback surface brightness	25.7
f₄	Multiple of r_P for Petrosian flux	3
f₅	Fallback Radius	3"