Appendix 3. Long Exposure (6x) Databases, Catalogs and Images


5. Data Processing

e. Artifact Identification

Identification and flagging of spurious point source extractions of image artifacts from 6x image data follows closely the procedure used the main survey data that is described in IV.7. This procedure utilized a suite of algorithms to predict the location of latent images, diffraction spikes, filter and dichroic glints, electronic cross-talk "stripes", etc. relative to the position of parent bright stars in and around each scan, and then flagged extractions that were in proximity to the artifact locations. Association with artifacts is denoted by the contamination and confusion flag (cc_flg) in the 6x Point Source Working Database.

For 6x data processing, it was necessary to adjust many of the magnitude-dependent parameters that are given in IV.7 for the longer exposure times. Sections i-vi below give the revised parameters for 6x data and brief explanations, where applicable. Because of factor of six increase in exposure, the same source appears two magnitudes brighter in the 6x images compared to survey data, so most adjustments simply scale the artifact masks by -2 magnitudes. In other words, the artifact masks of a parent source with m=8.0 in the 6x exposures are the same as the masks of a parent source with m=6.0 in the survey exposures.

The positional parameters below are all given in units of arcseconds, the size of Atlas Image pixels.

i. Diffraction Spikes [cc_flg="D,d"]

Each band has four diffraction spikes extending in the north, west, south, and east directions (in clockwise order on the images). Since the diffraction spikes vary in length, width, and brightness with hemisphere, band, and direction, each spike in each band and hemisphere has different parameters. These spike parameter values are given in Table 1 below.

General Parameters

False or contaminated sources were not searched for in a diffraction spike if the parent source is fainter than the mthrD parameter for that spike.

Any sources found within the diffraction spike area are either part of the spike itself or are contaminated by the spike.  If a source in the spike area is brighter than mpar + mD, it is considered contaminated by the spike; if it is fainter, it is considered an artifact of the spike. 

Spike Length

The calculated length l in arcsec of each spike area, from the center of the parent source, is:

l = l0 * 10[ a * ( m0´ - mpar ) ]
where the l0 and a parameters depend on spike direction, band, and hemisphere, and the m0´ parameter depends on the source density adjustment (see subsection 3.). In addition, there is a minimum spike length of 2", and a maximum spike length of 4080", or 68', in the north and 3840", or 64', in the south. 

Source Density Adjustment

The initial value of the m0 parameter for all diffraction spikes is 8.0. However, as source densities increase, the confusion noise raises the background noise levels such that the diffraction spike lengths decrease. The source density value, src_dens, is simply the number of source detections in the scan for that band, j_n_det, h_n_det, or k_n_det from the scan information table, divided by the scan's area, 0.85 deg2 or 0.14 deg2. If the scan's source density is greater than 5883.0 per deg-2 ([jhk]_n_det > 5000 for a 6° long scan), the spike lengths are adjusted by shifting the m0 parameter using:

m0´ = 8.0 - [ log10(src_dens/5883.0) / d ]
where the d parameter depends on spike direction, band, and hemisphere.

Spike Width

The diffraction spike half-widths increase with distance from the parent source, making the spikes into wedges that fan out from the center. The equation to calculate the spike width is

w = w0 + (e * z)
where z is the distance in arcsec from the parent.

Table 1 - Diffraction Spike Parameters
Northern Hemisphere
J Band H Band Ks Band
mthrD mD l0 a d w0 e mthrD mD l0 a d w0 e mthrD mD l0 a d w0 e
N spike 10.75 6.5 91.0 0.127 0.3172 2.0 0.0167 10.25 6.0 76.0 0.132 0.2175 2.0 0.0167 10.00 6.0 62.8 0.139 0.2701 2.0 0.0167
W spike 10.00 6.5 65.4 0.121 0.3517 2.5 0.0278 9.50 6.0 55.0 0.125 0.4641 2.5 0.0300 9.00 6.0 54.2 0.141 0.3223 2.5 0.0300
S spike 10.00 6.5 79.4 0.111 0.3012 2.0 0.0167 9.50 6.0 61.6 0.129 0.4723 2.0 0.0200 9.25 6.0 59.6 0.140 0.5304 2.0 0.0233
E spike 10.75 6.5 86.4 0.114 0.2940 2.0 0.0250 10.75 6.0 65.4 0.126 0.3166 2.0 0.0296 9.75 6.0 75.0 0.112 0.2027 2.5 0.0250
Southern Hemisphere
J Band H Band Ks Band
mthrD mD l0 a d w0 e mthrD mD l0 a d w0 e mthrD mD l0 a d w0 e
N spike 10.00 6.5 92.2 0.112 0.3620 2.0 0.0200 10.25 5.5 57.0 0.124 0.6244 2.0 0.0167 10.75 6.5 63.4 0.137 0.6943 2.5 0.0167
W spike 10.75 6.5 82.2 0.105 0.3831 2.0 0.0200 10.75 5.5 88.4 0.103 0.3845 2.0 0.0200 9.50 6.5 73.2 0.101 0.3893 2.0 0.0200
S spike 10.75 6.5 85.2 0.122 0.4871 2.0 0.0200 10.75 5.5 60.4 0.127 0.4871 2.0 0.0200 10.00 6.5 56.2 0.137 0.7818 2.0 0.0200
E spike 10.75 6.5 75.0 0.116 0.4258 2.0 0.0200 9.75 5.5 61.2 0.118 0.7503 2.0 0.0200 9.00 6.5 66.2 0.113 0.4923 2.0 0.0200


ii. Persistence [cc_flg="P,p"]

Persistence artifacts in J, H, or Ks bands are searched for if the parent source is brighter than 12.5 mag, 11.0 mag, or 10.5 mag, respectively. The persistence search radii and predicted persistence position errors for parents brighter than 5.0, 4.5, or 4.0 mag in J, H, or Ks, respectively, are larger than those for fainter parents.

Algorithms

The predicted position of a persistence source found at the location of the ith frame offset following the parent source is approximately:

xiP = xpar + ( i * xoffs)
yiP = ypar + ( i * yoffs)
where (xpar,ypar) is the position of the parent, xoffs is the scan's average frame-to-frame cross-scan offset, and yoffs is the scan's average frame-to-frame in-scan offset.  The signs of the offsets depend on the scan direction.   These offsets are read from a POSFRM output file for each scan; typical values are, in arcsec, for north-going scans:
xoffs ~ 0.5"
yoffs ~ 83"

and for south-going scans:

xoffs ~ -0.5"
yoffs ~ -83"

The predicted magnitude of the ith persistence source is approximately given by:

miP = mpar + miP
where the miP values are found in Table 2 below. Note that miP is not well-determined for values of i > 7 because there are few of these sources and their parents are very bright, so the persistence sources are not point-like and their magnitudes can have large errors.

The persistence probability is determined for each source within 6" of the predicted persistence positions, or within 14" if the parent is brighter than 5.0, 4.5, or 4.0 mag in the J, H, or Ks bands, respectively. The probability that a certain source is a persistence artifact of the parent source is determined from a 2 quantity, computed as:

2 = [(xs - xiP)2 / x2 ] + [(ys - yiP)2 / y2 ] + [(ms - miP)2 / m2 ]
where xs, ys, and ms are the possible persistence source's position and magnitude, and xiP, yiP , and miP are the predicted persistence position and magnitude.  The quantities x2, y2, and m2 are the sums of the squares of the possible persistence source's uncertainties in x, y, and magnitude (found in the source data), the parent source's uncertainties in x, y, and magnitude (also found in the source data), and the uncertainties in the empirically predicted positions and magnitudes, which are 4.0" in x and y, or 10.0" for very bright parents, and 1.0 mag.

Persistence artifact searching continues for values of i up to the point at which miP, within its uncertainty, is fainter than the approximate source detection limits of the survey. Since the predicted magnitude uncertainty is approximately 1.0 mag, the search is ended when miP > 22 mag in J, 21 mag in H, or 20 mag in Ks in either hemisphere.

Table 2 - Persistence Parameters
Northern Hemisphere
i 1 2 3 4 5 6 7 8 9 10
J band miP 5.8 7.6 8.4 8.8 9.1 9.4 9.8 10.5 12.0 13.0
H band miP 6.3 7.8 8.4 8.8 9.1 9.2 9.4 11.0 12.0 13.0
Ks band miP 6.3 7.8 8.4 8.8 9.1 9.4 9.6 10.5 12.0 13.0
Southern Hemisphere
i 1 2 3 4 5 6 7 8 9 10
J band miP 6.2 8.1 8.9 9.5 10.0 10.4 10.8 11.5 12.0 13.0
H band miP 5.8 7.1 7.6 8.3 8.6 8.9 9.3 10.2 12.0 13.0
Ks band miP 6.3 8.0 8.8 9.3 9.4 9.5 9.8 11.0 12.0 13.0


iii. Filter and Dichroic Glints [cc_flg="G,g"]

All filter and dichroic glints are located close to the parent source, so they will be found only in the same scan as the parent. There are 2, 1, and 2 glints in the J, H, and Ks bands in the north and 4, 2, and 2 glints in the south, respectively.

Algorithms

A source is flagged as a glint if it is found within rgl arcsec of the glint's expected position (xgl, ygl) and has a magnitude within ± mgl mag of mparmG, where mpar is the magnitude of the parent and rglmgl, and mG depend on the glint number, band, and hemisphere. The expected position is:

xgl = xpar+fgl
ygl = ypar+hgl
where (xpar, ypar) is the position of the parent, and the fgl and hgl parameters also depend on the glint number, band, and hemisphere.

Table 3 - Glint Parameters
Northern Hemisphere
J Band H Band Ks Band
glint #1 glint #2 glint #1 glint #1 glint #2
f 1.5 3.0 1.5 1.5 -4.0
h -14.0 -28.5 -14.0 -14.0 16.5
rG 2.0 3.0 2.0 2.5 2.5
mG 6.7 9.0 7.9 7.8 7.2
mG 0.9 2.0 1.6 1.8 1.3

 
Southern Hemisphere
J Band H Band Ks Band
glint #1 glint #2 glint #3 glint #4 glint #1 glint #2 glint #1 glint #2
f -2.5 1.0 -6.5 -10.0 -2.5 1.0 -2.5 1.0
h 11.0 -13.5 22.0 33.5 11.5 -13.5 11.0 -13.5
rG 3.0 1.5 3.5 5.0 3.0 1.5 3.0 2.0
mG 6.5 9.0 9.0 12.0 8.0 8.2 7.3 7.5
mG 1.0 2.0 2.5 3.0 2.0 2.0 2.4 1.5


iv. Stripes [cc_flg="s"]

As with persistence artifacts, the horizontal stripe artifacts from electronic feedback are found only in the same scan as the parent source. Only sources brighter than the approximate source detection limits in each band minus 11 mag can create stripes that are visible on the images, so parents fainter than 10 mag in J, 9 mag in H, and 8 mag in Ks skip this step in the artifact processing.  The three stripes per parent are found at the parent's y position and at y ± 256", and extend horizontally across the entire image.  They are 8" wide (full width, north to south), and often have small ghosts at the locations (x ± 0, 256"), (y ±  0, 256").   Anything found within the stripe area is flagged as contaminated by the stripe.

v. Bright Star Confusion Artifacts [cc_flg="C,c"]

Algorithms

Searches for artifact confusion are made in Read1 and/or Read2-Read1 only when the parent is saturated in that read type. However, there often are no good Read2-Read1 data for sources saturated in Read2-Read1, and thus no explicit indication that the parent was saturated in Read2-Read1. To deal with this situation, the programs also use magnitude thresholds msat to determine if the parent should have been saturated in Read2-Read1. These thresholds are found in Table 4 below. If the parent is brighter than the threshold in that band, the program will search for artifact confusion in that band.

The equation to calculate the artifact confusion radius for a read type (Read1 or Read2-Read1) is

riAC = r0AC * 10[ b * ( mi0AC - mpar ) ]
where the r0AC and b parameters depend on hemisphere and band, and the mi0AC parameter depends on hemisphere, band, and read type i. In addition, there is a minimum radius of 1", and a maximum radius of 4000", or 66.67', in the north and 5000", or 83.33', in the south. 

Table 4 - Artifact Confusion Parameter Table
Northern Hemisphere
J Band H Band Ks Band
msat 9.0 8.7 8.1
r0AC 12.90 10.20 8.50
b 0.202 0.203 0.171
Read1 m0AC 2.5 2.5 2.5
Read2-Read1 m0AC 8.0 8.0 8.0
Southern Hemisphere
J Band H Band Ks Band
msat 9.0 8.6 8.1
r0AC 15.06 9.46 7.74
b 0.207 0.201 0.194
Read1 m0AC 2.5 2.5 2.5
Read2-Read1 m0AC 8.0 8.0 8.0


vi. Photometric Confusion [cc_flg="c"]

Algorithms

The photometric confusion radii depends on both the magnitudes of the parent and the fainter neighboring source and thus is calculated separately for each possible confusion pair.  The equation to calculate the radius for a pair of sources is

rPC = r0PC * 10[ c * ( mfs - mpar ) ]
where mfs is the magnitude of the fainter source, and the r0PC and c parameters depend only on band. There is a minimum radius of 1", and a maximum radius of 1200", or 20.0'. Additionally, all sources with a blend flag greater than zero are marked as having photometric confusion in that band, since they have been passively deblended.

Table 5 - Photoemtric Confusion Parameters
J Band H Band Ks Band
r0PC 4.678 4.380 3.980
c 0.1165 0.1148 0.1243

[Last Updated: 2008 February 15; by T. Evans and R. Cutri]


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