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

5. Data Processing

d. Extended Source Processing

Identification and characterization of extended sources in the 6x image data is based closely on the extended source processor used for the main survey data that is described in IV.5. A number of modifications have been made to optimize processing for the enhanced surface brightness and sensitivity of the deeper imaging. The most important changes are as follows:

• Adjust all SNR-related magnitude thresholds one magnitude fainter relative to those used for the main survey to account for the enhanced sensitivity of the 6x measurements.


• Replace the NASA/IPAC Extragalactic Database (NED) seedlist with the 2MASS All-Sky XSC, which is better matched in astrometry and photometry to the 6x observations, as well as being inclusive of the largest galaxies in the sky. Using the XSC also supports proper background removal and enables direct comparisons between survey and 6x photometry. Figures 1 and 2 illustrate background removal performance for a sample 6x Atlas Image.



Figure 1 - J-band 6x Atlas Image with extended sources marked in red and bright stars artifacts and masked area marked in blue. The left panel shows the raw image. The middle panel shows the "mask", and the right panel shows the background-removed image.	Figure 2 - 3-color (J+H+Ks) composite of image in Figure 1, where the left panel shows the original Atlas Images and the right panel shows background-removed Images.




• Adjust bright star masking parameters, effectively increasing the mask radii to account for the enhanced sensitivity to low surface brightness scattered light in the 6x observations. Also take the bright star seedlist for masking from the 2MASS All-Sky PSC. Bright star masking performance for 6x data is illustrated in Figures 3 and 4.



Figure 3 - J-band Atlas Image with extended sources marked in red and bright stars artifacts and masked area marked in blue. The left panel shows the raw image. The middle panel shows the "mask", and the right panel shows the background-removed image.	Figure 4 - 3-color (J+H+Ks version of image in Figure 1, where the left panel shows the original Atlas Images and the right panel shows the background-removed Images.




• Adjust the confusion noise and stellar number density thresholds to reflect the greater confusion noise at fainter flux levels.


• Eliminate e_score and g_score thresholding. In the main survey data reduction, full shape and photometric characterization was run only on sources that fell within specified e_score and g_score limits to minimize cpu runtime and to save output disk space. These savings were less critical for the modest amount of 6x data. The additional e_score and g_score is used for characterization purposes.


• Correct a "bug" that existed in survey extended source processing that on rare occasions corrupted the reported isophotal radii due to a memory register corruption.


• Add photometric analysis and diagnostic tables used in characterization and quality assurance, that compare main survey and 6x extended source results. Figure 5 is a sample diagnostic plot showing the comparison of 6x and All-Sky XSC 7" circular aperture photometry of galaxies observed on 2000 May 30 UT from Cerro Tololo. The 6x scans on this night covered the Abell 3558 cluster.

Figure 5 - 2MASS 6x Quality Assurance diagnostic plot showing the difference between 6x and All-Sky Data Release (ADR) XSC 7" circular aperture photometry of galaxies in the Abell 3558 cluster observed on 2000 May 30 UT plotted as function of All-Sky XSC magnitude. Small blue (J), green (H) and red (Ks points represent magnitude differences for individual galaxies and black point with vertical error bars are the standard deviation of difference in 0.5 mag bins.

Important Note About Large Galaxies: The 6x extended source processor is not modified or tuned to properly measure large galaxies. As a result, the 6x extended source WDB and 6x-XSC do not contain accurate measurements of extended sources that span more than one scan. Because large galaxies are well-characterized by the Large Galaxy Atlas (LGA) processing from the main survey, such processing was not carried out for the 6x data.

i. Sample 6x Extended Sources

Figures 6-9 show images of a selection of extended sources extracted during 6x data processing. In each image, the left panel shows a three-color, background-removed "postage stamp" image. The right panel shows the same image with stars removed. The Ks-band (fiducial) elliptical isophotal radius is shown in the figures in blue. The isophotal elliptical K_s magnitude (k_m_k20fe), sem-imajor axis length and axial ratio of the galaxies are given in in the figure captions. Only one image is displayed for the faint galaxy in Figure 9 because no foreground stars were subtracted.

Figure 6 - Ks=10.89 r=17.8" b/a=0.9	Figure 7 - Ks=11.06 r=21.2" b/a=0.9

Figure 8 - Ks=11.67 r=23.9" b/a=0.4	Figure 9 - Ks=15.46 r=5.0" b/a=1.0

[Last Updated: 2007 January 8; by T. Jarrett and R. Cutri]