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

6. 6x Catalog Generation

c. Source Selection

The source selection process draws the subset of extractions from the 6x point and extended source WDBs that meet the reliability goals of the 6x-PSC and 6x-XSC while maintaining the completeness objectives of the deeper observations. The selection criteria used for the 6x-PSC and 6x-XSC are similar to those used for the All-Sky PSC and XSC, and users should refer to V.3 for detailed descriptions.

A graphical illustration of 6x-PSC and 6x-XSC source selection process results is given in Figure 1 of A3.6.a.

i. 6x-PSC Selection Criteria

Sources in the 6x Point Source WDB must satisfy the criteria described in Table 1 to be included in the 6x-PSC. These differ from the criteria used to construct the All-Sky PSC in three ways:

The brightness thresholds used for the frame-detection requirements are one magnitude fainter
The point source profile (²) requirement is imposed only for sources brighter than SNR=10
One detection of sources in scan overlap regions was always included in the 6x-PSC, regardless of whether or not it was detected more than once

Table 1 - 6x-PSC Source Selection Criteria
Category Criteria Purpose

Scan-Edge Boundaries Source lies >10" from edge of 6x scan Exclude detections on or near scan edges for which brightness and/or position cannot be accurately measured

SNR and Band-detection Source must have a valid measurement in at least one band (rd_flg="1","2" or "3"),
AND

[Have SNR>7 in at least one detected band
OR
be detected and have valid photometry in all three bands AND has SNR>5 in at least one band.]
The SNR thresholds can be satisfied by either the SNR derived from the photometric measurement uncertainty ([jhk]_msig), or the scan SNR ([jhk]_snr)
To filter out low SNR noise excursion detections

Artifacts Source must not be identified as a detection of an image artifact produced by a bright star in any band according to the cc_flg. Spurious source rejection

Frame-detection Sources brighter than J<15.9, H<14.8 or K_s<14.4 mag must be measurable on at least two out of the six (and sometimes seven) frames sampling their positions
AND
[must be detected at SNR>3 on at least 40% of the available frames,
OR
the source must have non-saturated detections in all three bands AND it must have been measurable on two frames AND detected with SNR>3 on both of those frames.]
6x point source frame detection statistics are tabulated in the ndet column.
Rejection of transients such as hot pixels, cosmic rays and residual meteor trails

Point Source Profile Non-saturated Read_2 sources with SNR<10 that are measured using profile-fit photometry (rd_flg='2') must have a ² goodness-of-fit value [jhk]_psfchi < 2 in at least one detected band.
Bright sources that saturate the Read_2 exposures and non-saturated Read_2 detections with SNR>10 are not required to satisfy this criterion. The latter exception avoids rejecting bright multiple stars and galaxies.
Rejection of transients such as hot pixels, cosmic rays and residual meteor trails

Multiply-Detected Sources For sources in Tile overlap regions that are detected more than once, the detection farthest from its scan edge is selected. If a source in an overlap region is detected only once, it is always included in the 6x-PSC. (see A3.6.d) To eliminate redundant detections.

**Table 1 - 6x-PSC Source Selection Criteria**
Category	Criteria	Purpose
Scan-Edge Boundaries	Source lies >10" from edge of 6x scan	Exclude detections on or near scan edges for which brightness and/or position cannot be accurately measured
SNR and Band-detection	Source must have a valid measurement in at least one band (rd_flg="1","2" or "3"), AND [Have SNR>7 in at least one detected band OR be detected and have valid photometry in all three bands AND has SNR>5 in at least one band.] The SNR thresholds can be satisfied by either the SNR derived from the photometric measurement uncertainty ([jhk]_msig), or the scan SNR ([jhk]_snr)	To filter out low SNR noise excursion detections
Artifacts	Source must not be identified as a detection of an image artifact produced by a bright star in any band according to the cc_flg.	Spurious source rejection
Frame-detection	Sources brighter than J<15.9, H<14.8 or K_s<14.4 mag must be measurable on at least two out of the six (and sometimes seven) frames sampling their positions AND [must be detected at SNR>3 on at least 40% of the available frames, OR the source must have non-saturated detections in all three bands AND it must have been measurable on two frames AND detected with SNR>3 on both of those frames.] 6x point source frame detection statistics are tabulated in the ndet column.	Rejection of transients such as hot pixels, cosmic rays and residual meteor trails
Point Source Profile	Non-saturated Read_2 sources with SNR<10 that are measured using profile-fit photometry (rd_flg='2') must have a ² goodness-of-fit value [jhk]_psfchi < 2 in at least one detected band. Bright sources that saturate the Read_2 exposures and non-saturated Read_2 detections with SNR>10 are not required to satisfy this criterion. The latter exception avoids rejecting bright multiple stars and galaxies.	Rejection of transients such as hot pixels, cosmic rays and residual meteor trails
Multiply-Detected Sources	For sources in Tile overlap regions that are detected more than once, the detection farthest from its scan edge is selected. If a source in an overlap region is detected only once, it is always included in the 6x-PSC. (see A3.6.d)	To eliminate redundant detections.

ii. 6x-XSC Selection Criteria

Sources in the 6x Extended Source WDB must satisfy the criteria described in Table 2 to be included in the 6x-XSC. There are two differences between the source selection for the 6x-XSC and All-Sky XSC:

The e_score and g_score limits are relaxed slightly for the 6x-Xsc
One detection of extended sources in scan overlap regions was always included in the 6x-XSC, regardless of whether it was detected multiple times

Table 2 - 6x-XSC Source Selection Criteria
Category Criteria Purpose

Scan-Edge Boundaries Source lies >15" from edge of 6x scan Exclude detections on or near scan edges for which brightness and/or position cannot be accurately measured

SNR Source must have SNR>7 in at least one band.
This threshold can be satisfied by the SNR derived from uncertainties on either the 7" circular aperture photometry, ([jhk]_msig_7), or the K_s=20 mag arcsec^-2 elliptical isophotal photometry ([jhk]_msig_k20fe).
To filter out sources with low confidence extended classifications

Artifacts Source must not be identified as a spurious detection of an image artifact produced by a bright star or as a piece of a large, bright galaxy according to the cc_flg Spurious source rejection

Extended Classification Source must have:
0 < e_score < 1.5 OR 0 < g_score < 1.5
See Section IV.5 for a description of the "Oblique Decision Tree" classifier which is used to determine the e_score and g_score values for candidate extended sources.
To reject "false" extended sources such as multiple stars and low SNR point sources

Multiply-Detected Sources For sources in Tile overlap regions that are detected more than once, the detection farthest from its scan edge is selected. If a source in an overlap region is detected only once, it is always included in the 6x-XSC. (see A3.6.d) To eliminate redundant detections.

**Table 2 - 6x-XSC Source Selection Criteria**
Category	Criteria	Purpose
Scan-Edge Boundaries	Source lies >15" from edge of 6x scan	Exclude detections on or near scan edges for which brightness and/or position cannot be accurately measured
SNR	Source must have SNR>7 in at least one band. This threshold can be satisfied by the SNR derived from uncertainties on either the 7" circular aperture photometry, ([jhk]_msig_7), or the K_s=20 mag arcsec^-2 elliptical isophotal photometry ([jhk]_msig_k20fe).	To filter out sources with low confidence extended classifications
Artifacts	Source must not be identified as a spurious detection of an image artifact produced by a bright star or as a piece of a large, bright galaxy according to the cc_flg	Spurious source rejection
Extended Classification	Source must have: 0 < e_score < 1.5 OR 0 < g_score < 1.5 See Section IV.5 for a description of the "Oblique Decision Tree" classifier which is used to determine the e_score and g_score values for candidate extended sources.	To reject "false" extended sources such as multiple stars and low SNR point sources
Multiply-Detected Sources	For sources in Tile overlap regions that are detected more than once, the detection farthest from its scan edge is selected. If a source in an overlap region is detected only once, it is always included in the 6x-XSC. (see A3.6.d)	To eliminate redundant detections.

[Last Updated: 2006 November 10; by R. Cutri]

Previous page. Next page.
Return to Explanatory Supplement TOC Page.