Appendix 3. Long Exposure (6x) Scan Databases, Catalogs and Images
5. Data Processing
a. Modifications Relative to Final Survey Processing
Reduction of the 2MASS Long Exposure (6x) scan data was carried out
using a minimally modified version of the 2MASS Automated Production Pipeline
(2MAPPS v3.0) that was used for final processing of all of the primary
survey data (see IV.1).
The basic processing steps shown in
Figure 1 of IV.1 remained unchanged from
the final survey processing, and the pipeline output products from
6x processing were very similar in form to those from the survey data
The modifications to 2MAPPS for the 6x processing fell into three categories
and priorities. Highest priority was given to the fundamental
changes that were necessary so that the 6x data would successfully process
without software failures, such as corrections dealing with the
longer exposure times and resulting higher sky background count levels
and structure. Second priority was given to changes that
fixed software bugs that were known to exist 2MAPPS v3.0, or
that were discovered since the release of the All-Sky Catalogs.
Finally, modifications to the software and/or the quality assurance
process that further improved data quality and brought output data
formats closer to those of the release products were made as time
and resources permitted.
Brief summaries of the major
modifications made to each of the data reduction subsystems
are given below.
A link to the pertinent section of the main survey supplement
is given for each subsystem.
Further descriptions of the changes made to the astrometric and
photometric calibration procedures, extended source processing
and artifact identification for the 6x data are
given in Sections A3.5.b,
and A3.5.e, respectively.
As with the main survey, 6x data processing was done in two steps.
Preliminary processing was carried out shortly after the
6x observations were made using a 2MAPPS version that was
modified only to accommodate the longer exposure times of the
6x data. The results of the preliminary reduction were used
to assess the basic 6x data quality for observatory support
and to prepare for the final processing.
Final 6x data processing was carried out as part of the
2MASS Extended Mission, and incorporated best practices learned
from the survey itself, as well as analysis of the preliminary
6x processing results. Because final 6x processing followed
the release of the final survey data products, it was possible
to incorporate Quality Assurance steps that compared the
6x products to the well-characterized All-Sky Catalogs and Images.
i. Synopsis of Reduction Pipeline Changes By Subsystem
Instrumental Calibration (IV.2)
- Generate new bias/dark exposure frames using longer R2 exposure data.
6x processing used the same responsivity (flat-field) images and saturation
threshold levels as were used for the main survey.
Atlas Image Generation (IV.3)
- Blank frames containing very bright stars to avoid large scale
background matching errors all along the scan. This blanking was applied
to only a small number of scans containing extremely
bright stars, such as that covering R Doradus in the 6x LMC scans.
The result of this blanking is a slightly lower depth-of-coverage in the
region of the Atlas Image immediately surrounding the star.
Point Source Photometry (IV.4.b)
- Adjust variance maps of existing point spread function templates to
produce profile-fit photometric uncertainties consistent with
repeatability analysis. This modification removed the need for
the corrections to the
uncertainties that were used during processing of the main
survey and calibration data. Point source photometric uncertainties are called
[jhk]_msig in the 6x point source database, and [jhk]_cmsig
in the survey and calibration databases to reflect this difference.
Position Reconstruction (IV.6)
- Use the 2MASS All-Sky Point Source Catalog as the primary astrometric
reference for 6x position reconstruction. This modification
is discussed in A3.5b.
- Correct the software bug
(I.6.b.xiii) that passed
on READ1 source detections for bandmerging
before frame distortion corrections could be applied during main
survey and calibration scan data processing (see
A3.5.b). Bright source astrometric
residuals should be slightly smaller for 6x point sources than
for the PSC.
Account for frame position errors in absolute
position uncertainties by computing residuals with respect
to the 2MASS PSC positions in 0.5° long segments within
6x scans, and adding in quadrature to the relative
position errors generated during the bandmerging
process. For survey data processing, this was accomplished using
position offsets computed in the overlap region between adjacent
scans. Overlap scan information was not always available for
the 6x data, and the higher density of PSC reference stars provides
a more robust estimate of the absolute errors with respect to
the reference frame.
Optical Associations (IV.4.f)
- Correct software bugs (I.6.b.xvi
that resulted in missed associations between
2MASS point sources and USNO-A and Tycho 2 sources,
and incorrectly setting the associated
catalog identifier (a) to "0"
when the separation between 2MASS and optical
source was larger than 5".
Extended Source Processing (IV.5)
- Modifications to the extended source processing for
the 6x data, and examples of its performance are discussed in
- Modify magnitude-dependent and SNR-dependent thresholds to enable
measurements of fainter objects.
- Adjust confusion noise and stellar number density thresholds to
accommodate deeper exposures.
- Tune background fitting to compensate for higher background levels
and increased contrast in apparent structure.
- Increase the effective masking radius size around bright stars
to compensate for brighter instrumental magnitudes.
- Replace bright galaxy and star seedlists with 2MASS All-Sky XSC
and bright star list drawn from the PSC.
Artifact Identification (IV.7)
- Adjust magnitude thresholds and geometric artifact identification
parameters to account for longer exposure times and resulting larger
instrumental source brightnesses. A summary of the revised parameters
is given in A3.5.e.
Photometric Calibration (IV.8)
- Modify Read_2 instrumental zero point magnitudes to compensate
for longer exposure times.
- Develop new secondary standard star network in the special LMC and
SMC calibration fields used to support the 6x observations of those
regions. The 6x calibration strategy is discussed further in
Quality Assurance (IV.10)
- Implement photometric and astrometric comparisons between the
6x source lists and the 2MASS All-Sky Release PSC and XSC
to validate photometric accuracy, position reconstruction
performance, completeness and reliability of the 6x processing.
[Last Updated: 2007 January 8; by R. Cutri]
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