Aperture photometry is performed for each non-saturated source detection made on the Atlas Images to provide a reference to the absolute photometric scale for the profile-fit photometry, statistics on the detectability of an object, and as a back-up source of brightness information when profile-fitting fails to converge to a valid measurement.
As with the profile fitting,
the aperture photometric measurement is made at the position of a
detection on the individual frames. On each frame, the brightness is
measured in a series of apertures ranging in radius from 3´´ to
14´´, in 1´´ steps, by summing pixels entirely
within the aperture and interpolating
pixels partially within the aperture. The sky background for each object
is computed in an annulus with an inner radius of 14.0´´ and
an outer radius of 20.0´´. Pixels in the sky annulus are entirely
included or excluded based on the distance of their centers from the source.
The sky value is estimated by first excluding saturated, masked, or
unreasonably low pixels. A 
-trimmed median
of the surviving sky pixels is then used as the sky brightness estimate.
The aperture measurements from each of the six input frames are combined
using an unweighted average which takes into account non-detections
in frames to avoid flux-overestimation.
Aperture measurements are usually possible on all six
(and sometimes seven) frames that sample the position of each detection.
However, if one of more of the frames contains a masked or saturated pixel
within 4" of the source centroid, or if the source centroid
falls within <4'' of a frame edge, that frame is excluded from
the the measurement. The aperture measurements from
each of the remaining available frames are combined
using an unweighted average.
The statistics of the aperture measurements are compiled in
the ndet
parameter included in the PSC record. Ndet is a six-digit flag,
with two digits per band
(NJMJNHMHNKMK)
that tabulates for each band the number of frames on which a source
was detected with >3 For example,
an ndet value of "665634" indicates that a source had
six >3
ii. Curve-of-Growth Correction
The "standard aperture" used for 2MASS aperture photometry had
a 4'' radius (or ~2 camera pixels).
However, the point-spread-function of the 2MASS optical system
is broad enough that light from the wings of star images
is not completely captured within this aperture.
Depending on the atmospheric seeing conditions, between 2% and 15%
of the total flux from a point source will be missed.
To correct for the loss of light in the standard aperture, a
curve-of-growth correction is applied to the
measurements, and it is this corrected photometry that is
listed in the standard aperture magnitudes
(j_m_stdap,
h_m_stdap,
k_m_stdap)
in the PSC source records.
The curve-of-growth correction is a constant factor
that when added to the magnitudes measured in the 4'' radius aperture
makes them equivalent to an "infinite" size aperture.
Correcting small-aperture measurements rather than
using much larger apertures avoids the degradation in signal to noise ratio
due to increased sky photon noise and possible confusion with nearby sources
that would result from simply using larger aperture measurements.
However, the correction means that the aperture photometry quoted
for 2MASS sources assumes that the sources are unresolved, and
have a profile consistent with the point-spread-function of the
optical system plus seeing. Resolved or multiple sources
may not be accurately characterized by the standard aperture measurements.
The curve-of-growth corrections applied to standard aperture photometry
during final 2MASS data processing were drawn from look-up tables
indexed by atmospheric seeing. Photometry for all sources in a scan
observed within specified ranges of measured seeing conditions were
corrected using the same factor, per band.
The correction-table values
were derived empirically using the multi-aperture
photometry of large ensembles of non-saturated sources
(rd_flg="2")
observed in relatively low source density scans
from each observatory under a wide variety of seeing conditions.
For each observatory, time period and seeing value, the differences
between magnitudes in successive aperture were compiled, and
the point at which the differences converge to zero was determined.
The correction factor, in magnitudes, is the median difference
between the 4´´ aperture magnitude and the magnitude in the
aperture at which the magnitude differentials become indistinguishable from
zero. The mean correction values for each band and
each seeing range were internally consistent; the median and the mean
values agree to <0.01 magnitudes.
J-band curves-of-growth evaluated during periods of good (2.5´´
FWHM) and bad (3.4´´ FWHM) seeing in scans from the Second
Incremental Data Release are shown in Figure
1 and Figure 2, respectively. The
x-axes show aperture radius in pixels (2´´/pixel), and the
y-axes show the magnitude differences measured in successive aperture pairs
vs.
the outer radius of the pair. Indicated on the figures are the radius
of the "standard aperture" (open circle), and the radius at which the
curve-of-growth was determined to converge (open star). The good seeing
case shown in Figure 1, converges
at a smaller radius, 2.5 pix (5´´) than the poor seeing case,
4.0 pix (8´´).
The curve-of-growth correction determined in these examples were
-0.016±0.005 mags and -0.121±0.015 mags, respectively.
Note that the net corrections in each case are the sum of the differentials
for all pairs between 4'' and the convergence radii.
in aperture the photometry,
Nb, and
the number of frames which were available for measurement, Mb.
Thus, the frame detection thresholds are also referred to
as the "N-out-of-M" criteria.
detections
on all six of the available frames in the J-band, five detections out of
six possible frames in the H-band, and three detection on four possible
frames in the Ks-band. For brighter sources, the
ndet parameter can be used as a reliability indicator
and is used in source selection for
the final PSC.
