- Instrumental Calibration:
- Detailed Documents: Darks, Flats
- Construct and use "canonical" dark and flat-field images from
averages of large blocks of survey data, appropriate to
telescope and hardware changes. Objective is to
smooth variations due to nightly statistical fluctuations (darks)
and seasonal twilight sky illumination variations (flats)
- Residuals between individual and canonical flats < 1-2%, so
modifications minor
- Flat-field cross-scan correction maps derived from point source
mapping tests taken throughout the survey applied to canonical
flats. Objective is to remove 1-2% wavelength dependent
residuals that induce small, but observable color variations
- Status - Green. Production
calibration images generated and tested.
Figure 1a - Mean flux residuals as a
function of cross-scan position for southern 2MASS camera. 2-d residuals
measured by mapping relative brightness of high SNR point sources in
a 6x10 grid around the focal plane. These plots show the residuals
collapsed in the in-scan direction. Each line represents a measurement
taken during a different time of the survey.
Figure 1b - Observed mean J-H color
of high SNR sources as a function of cross-scan
position (distance from focal plane edge). The red curve shows
sources measured from CTIO and the blue curve sources measured
at Mt. Hopkins.
Figure 1c - Predicted color-dependence
on cross-scan position from the measured flux residual maps (as in Fig. 1a).
Color coding is as in Fig. 1b. Compare the top panel with Fig. 1b.
- Meteor Streak Removal from Atlas Images:
- Detailed Document
- Meteor streaks identified by linear configuration of
spurious single-frame detections. Stack 3 bands to improve
signal.
- Streak blanked only in affected JHKsframes. Coaddition
to produce normal Atlas Image with trail removed.
- Objective is to minimize spurious detections of trails.
Meteor trail "sources" are major source of unreliability
in extended source detection at high galactic latitude.
- Detection/blanking reliability >90%, Completeness ~75%.
Some faint trails persist becuase they do not trigger enough
"detections."
- Status - Green. Code implemented
and tested.
Figure 2a - Atlas Image pair showing a meteor
trail with detections marked with green crosses on the left, and the same
image on the right when the meteror trail is blanked in a single frame in
all three bands prior to coaddition.
- Astrometry:
- Detailed Document
- Frame distortion maps measured for all arrays and throughout
survey by comparison with high density external astrometric catalog.
Corrections for distortion now applied to positions of
extracted sources.
- Astrometric solutions improved by using higher density Tycho-2
reference catalog. Not available during preliminary processing.
- Astrometric solutions improved in lower reference-star-density
regions by propagating solutions across scans using information
from multiply observed sources in scan overlap regions.
- Demonstrated RMS residuals with respect to UCAC astrometric
catalog of ~120 mas on each axis, with reduced incidence
of outliers (UCAC has estimated accuracy of ~20 mas).
- Status - Yellow. Distortion
maps measured and implemented. Tycho 2 implemented. Code
modifications implemented and tested. Preprocessing
for cross-scan solution propagation is ~80% finished, and will
be complete by July 1. No schedule risk because can start
operational processing with nights for which preproduction
data are ready.
Figure 3a - Distribution of DEC offsets (in arcsec)
between 2MASS and UCAC positions for 2MASS sources with SNR>30 in all bands.
Red curve shows offsets from preliminary 2MASS processing (v2)
and the blue curve shows the results of 2MAPPS v3 test processing.
Figure 3b - Detail of DEC offset curves
shown in Figure 3a to show the wings of the distributions. The number of outliers are greatly reduced
and the distribution is more symmetric with v3.0 processing.
Figure 3c - Distribution of RA offsets (in arcsec)
between 2MASS and UCAC positions for 2MASS sources with SNR>30 in all bands.
Red curve shows offsets from preliminary 2MASS processing (v2)
and the blue curve shows the results of 2MAPPS v3 test processing.
Figure 3d - Detail of RA offset curves
shown in Figure 3C to show the wings of the distributions. The number of
outliers are greatly reduced and the distribution is more symmetric with
v3.0 processing.
- Very Bright Star Photometry and Positions:
- Detailed Document
- Stars saturated in R1 exposures are measured by fitting
template to non-saturated part of 1-d radial profile.
Template derived empirically by measuring profiles of large numbers
of bright stars of known magnitude.
- Although no requirement for saturated star photometric accuracy,
goal is ~0.3 mags. Achieving ~0.2 mag RMS. Good brightness and
position estimates necessary
for proper artifact identification which is key driver to meeting
general reliability requirements for survey.
- Single reliable detection of very bright stars with positions
accurate to <0.3" RMS.
- Bridges gap with TMSS.
- Status - Green. Templates
produced. Code implemented and tested.
Figure 4a - Residuals between template fits
and published photometry from saturated R1 stars measured at CTIO.
Figure 4b - Residuals between template fits
and published photometry from saturated R1 stars measured at Mt. Hopkins.
Figure 4c - JHKs color-color diagram
for stars in 12 RTB nights with R1 saturation in one or more bands.
The green and red lines denote the main sequence and giant tracks
from Bessel and Brett. The color-coding indicates which bands are
R1-saturared.
- Improved Point Response Functions (PSFs) for Profile-fit Photometry
- Detailed Document
- New production PSFs generated using data spanning entire survey,
reflecting all significant hardware changes and seeing conditions.
- Objective was to remove magnitude-biased chi-squared statistics
obtained for profile-fit photometry in preliminary processing.
Higher SNR sources have systemmatically low chi-squares.
- Efforts to smooth chi-squared statistics increases
magnitude-dependent brightness bias between profile-fit
and aperture photometry.
- Conclusion that PSF-modeling is reaching limit what can be
reasonably expected of raw data that is 10x undersampled,
given resources and schedule of survey.
- Status - Yellow. Science Team
has concluded that quality of photometry should take precedence over
statistics of photometry. Decision has been made to revert
to original PSF-generation algorithm. Production PSFs are being
generated now and will be ready to support operational test period
after validation. Will include extensive characterization
of photometric statistics in final Explanatory Supplement.
Figure 5a - Profile-fitting chi-squared
statistics plotted versus magnitude for sources observed on 9803019s, from
preliminary 2MASS processing. Black points are individual stars and large red
points are the trimmed average chi-squared for all stars in 0.2 mag bins.
Figure 5b - As in Figure 5a, but
from 2MAPPS v3.0 test processing.
Figure 5c - Difference between profile-fit
and aperture photometry plotted versus magnitude for sources observed on
9803019s, from preliminary 2MASS processing. Black points are individual
stars and large red points are the trimmed average magnitude differences
for all stars in 0.2 mag bins.
Figure 5d - As in Figure 5c, but
from 2MAPPS v3.0 test processing.
- Point Source Deblending:
- Detailed Document
- Allow multiple PSF-components to be fit to single detections
if chi-squared statistic above specified threshold. Objective
is to provide better brightness estimates in complex regions.
- Adopt conservative approach that strives for highly reliable
deblends (e.g.Relatively high chi-sqaured threshold,
and strict deblend acceptance criteria). This minimizes impact
on runtime.
- Simulations show accurate deblending of ~80% of SNR>40 doubles with
2-3" separation with magnitude differences of 1 mag. Deblending
reliability decreases with larger magnitude differences and
smaller separations.
- Status - Green. Deblending code
implemented and thresholds defined. Need to characterize
deblended population on large set of data in operational testing.
Figure 6a - Fraction of double sources deblended
as a function of magnitude in 2.8" separation delta-magnitude = 1.0 simulation.
Figure 6b - Chi-squared distributions before
and after deblending in simulated data.
Figure 6c - Distributions of deblended
component separations in 2.8" (top) and 2.0" (bottom) simulations.
Figure 6c - Comparison of expected
and measured brightnesses as a function of blend brightness for
2.8" separation simulated data.
- Point Source Photometric Normalizations:
- Detailed Documents: Aperture Curve-of-Growth, R1/R2 Normalization
- Aperture curve-of-growth and R1/R2-R1 normalizations derived
as a function of seeing (FWHM) using all survey data.
- Objectives are to use normalization factors with much better
statistical accuracy than empirical values determined for
each scan, to improve runtime since empirical normalizations
no longer determined, and to fix bug in which empirical normalization
measurements were sometimes contaminated by very bright star in
scan.
- Uncertainty in corrections for given FWHM < 0.004 mags.
- Status - Green. Global normalization
values as function of seeing derived. Code modifications
implemented and tested. Parameters will be validated
made when final production PSFs are implemented.
Figure 7a - Aperture Curve-of-Growth corrections as a function
of seeing shape (proportional to FWHM) for different periods of northern
operations. J-band is represented in blue, H-band in green and
Ks-band in red.
Figure 7b - R1/R2-R1 normalization
corrections as a function of seeing shape for southern data. These are
corrections to the nominal exposure time differences (3.49 mags).
- Extended Source Photometry and Characterization:
- Detailed Document
- Improved foreground star subtraction to provide better photometry
of extended sources in the Galactic Plane and to minimize
subtraction of pieces of galaxy in edge-on systems. Foreground
sources subtracted rather than masked. Star-subtracted
images will be saved along with regular Extended Source
Postage Stamps.
- Kron photometry improved by extending first-moment integration
out to 4 scale lengths, to match better the radial light profile
of the galaxies.
Objective is to reduce the 20% underestimate of total brightness
measured for preliminary processing
and Total source photometry utilizing feedback
from published studies of galaxy photometry of 2dF survey.
Objective is to remove measured bias between Kron and Total
brightness estimates (total magnitude is measured by integrating
extrapolated elliptical radial profile out to
20 mag/arcsec2 isophote).
.
- Improve Oblique Decision Tree (OBDT) classification of extended
sources with greatly expanded training set, and insert OBDT
classification sources in pipeline. Objective is to improve
runtime and efficiency (filter out obvious false extended sources)
and to reduce final product generation task load.
- Status - Green. All code
implemented and tested. Further validation to be done with
large data sets from operational testing.
Figure 8a - Example of foreground star
substraction for extended source processing in high source density regions.
Top panels show J, H, Ks and 3-color composite of original
Atlas Image containing Ks=12.5 galaxy. Bottom panel
shows field after foreground stars are subtracted.
Figure 8b - Difference between Kron
and Total magnitudes plotted versus Kron magnitude as measured using
2MAPPS v3.0 tests for galaxies in the Abell 3558 field. White points
represent galaxies with no foreground star contamination, and green
crosses are galaxies with foreground stars subtracted.
- Artifact and Confusion Flagging:
- Detailed Document
- Refine artifact flagging parameters using data accumulated for
entire survey.
- Improved artifact flagging in response to variable background
source density.
- Sources within "confusion" radius of brighter stars now
flagged as having biased photometry biases. Radius is defined
as separation where flux bias is >5%. Simulations used
to measure confusion radius as a function of magnitude difference
between parent and biased source.
- Status - Green. Artifact search
parameters derived. Simulations complete. Code modifications
implemented and tested. Final adjustment of parameters
will be made using large set of data from operations testing.
Figure 9a - Relative extracted source
density around all northern 7.0 < J < 7.25 stars. Plot is made by
extracting sources within 100" of all bright stars, shifting to
coordinate grid centered on the bright stars, and plotting all sources on
a common grid. The overdensity of sources are caused by spurious
extractions on diffraction spikes, dichroic glints, latent images, etc.
Figure 9b - Measured brightness of artifical
stars placed around brighter stars plotted as a function of separation
from brighter star. Three cases corresponding to a 5 mag difference
between "parent" and "child" stars. Close to the parent, the
brightness of the child is systemmatically overestimated.
- Photometric Calibration:
- Detailed Documents: Standard Star Mags, Extinction Coefficients, Nightly Zero Point Fitting
- Derive improved photometry of calibration stars using global
analysis of all calibration data collected during survey. Derives
photometry of calibration stars with highest degree of internal
consistency. Change in standard star magnitudes from global
analysis using ~half of survey data typically << 1%.
- Global calibration analysis also provides monthly average
atmospheric extinction coefficients
- Higher frequency fits to nightly zero points made, reflecting
atmospheric behavior observing over course of survey. J-band
transmission changes most rapidly, so fit on hourly basis.
H and Ks much more stable, so use only a linear
fit with time over the night.
- Status - Green. Global calibration
analysis complete. Final standard star magnitudes and extinction
coefficients derived. Code modifications implemented and tested.
Figure 10a - Difference between
previous and new globally-calibrated standard star magnitudes
for 2MASS calibration. J-band is on the top, H-band center, and
Ks-band bottom. Vertical scales are +0.05 mags. Horizontal
scales are RA of standards.
Figure 10b - Monthly atmospheric extinction
coefficients (mag/airmass) for northern 2MASS site determined from global
calibration processing. J-band is blue, H-band is green, and
Ks-band is red. Solid lines show the survey-monthly-average
extinction values.
Figure 10c - Same as for Fig. 10b, but for
the southern 2MASS site.
Figure 10d - Distribution of differences
between "true" standard star magnitudes and magnitudes determined
by calibration each against all other standards observed on all survey
nights. Different colors represent residuals calculated
using different zero point fits for each night.
Black curves show residuals to the constant zero point fits,
red shaded curve shows the linear fit residuals,
green shaded curve shows the quadratic fit residuals,
and the blue curve shows the piecewise fit residuals.
The smallest spread in residuals is found for piecewise fits in J,
and linear in H and Ks.
