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 m_thrD 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 m_par + _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´ - m_par ) ]}

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 ]

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)

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 K_s 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 K_s, respectively, are larger than those for fainter parents.

Algorithms

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

x_iP = x_par + ( i * xoffs)
y_iP = y_par + ( i * yoffs)

xoffs ~ 0.5"
yoffs ~ 83"

and for south-going scans:

xoffs ~ -0.5"
yoffs ~ -83"

The predicted magnitude of the i^th persistence source is approximately given by:

m_iP = m_par + _miP

where the _miP values are found in

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 K_s 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 = [(x_s - x_iP)² / _x² ] + [(y_s - y_iP)² / _y² ] + [(m_s - m_iP)² / _m² ]

_x², _y²,

Persistence artifact searching continues for values of i up to the point at which m_iP, 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 m_iP > 22 mag in J, 21 mag in H, or 20 mag in K_s 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 K_s 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 r_gl arcsec of the glint's expected position (x_gl, y_gl) and has a magnitude within ± _mgl mag of m_par + _mG, where m_par is the magnitude of the parent and r_gl, _mgl, and _mG depend on the glint number, band, and hemisphere. The expected position is:

x_gl = x_par+f_gl
y_gl = y_par+h_gl

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 K_s 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 m_sat 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

r_iAC = r_0AC * 10^{[ b * ( m_i0AC - m_par ) ]}

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

r_PC = r_0PC * 10^{[ c * ( m_fs - m_par ) ]}

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