Some of the other factors that will impact the 2MASS sensitivity and accuracy include sky and telescope background levels (see Gizis 1997 2MASS Background Levels Over Time) , atmospheric transmission and telescope and camera stability, etc. 2MASS Quality Assurance will assign quality levels to each survey scan by comparing statistical measures of these factors to empirically determined thresholds relating these factors to achieved point and extended source sensitivity and reliability. Scan data not meeting minimum thresholds will be assigned low or failed quality levels that will result in reobservation.
As part of the effort to understand the affect of atmospheric
seeing (and other factors that degrade image quality) on survey performance,
I have compiled image size and shape statistics for all processed 2MASS
scans acquired between the dates of 21 May and 19 December 1997.
Data from 125 days and a total of 7470 scans are included in this analysis.
The majority of scans processed to date are calibration data, but at least
10% are survey scans. Image size and shape are standard quality assurance
output diagnostics from the 2MAPPS system for all scan data.
This relationship was measured from PROPHOT point-spread function (psf) images derived from scan data. Figure 1 shows the measured FWHM as a function of seeing shape for a series of actual J (blue), H (green) and Ks (red) psf's used in the 2MAPPS system. Note that the relationship is independent of wavelength, as should be the case. The minimum practical limit for seeing shape is ~0.90-0.95 (FWHM=2.4"-2.5") because of the raw camera pixel size.
Figure 1 - Measured PROPHOT psf
FWHM(") as a function of average seeing shape.
Figure 2 - Average J, H and Ks seeing
shapes for all processed scans plotted versus running survey day number.
Figure 3 - Seeing shape measurement
uncertainty plotted as a function of seeing shape, for J, H and Ks
Figure 4 - Histograms of seeing
shape distributions.
J | J | H | H | Ks | Ks | |
shape | FWHM(") | shape | FWHM(") | shape | FWHM(") | |
Mode | 1.03 | 2.8 | 1.02 | 2.7 | 1.04 | 2.8 |
Median | 1.07 | 2.9 | 1.05 | 2.8 | 1.06 | 2.9 |
90% | 1.23 | 3.4 | 1.19 | 3.3 | 1.20 | 3.3 |
Figure 5 - Cumulative distributions
of measured seeing shapes.
The average image symmetry is characterized during processing of each 2MASS calibration and survey scan by measuring the ratio of the cross-scan to in-scan second image moments of composite star images. These composite images are generated by combining individual star images from an entire scan. Values of the second image moment ratio (R2) larger then 1.0 indicate images elongated in the cross-scan (RA) direction, and values less than 1.0 indicate elongation in the in-scan (DEC) direction. Figure 6 shows a time history of R2 measured for all scans as a function of running survey date. Severe elongation occurred in June 1997 because the initial calibration of the auto-focus parameters were made in April and May when temperatures were significantly lower than in June. Better calibration was carried out at the beginning of July and since that time the telescope auto-focus performance has been excellent.
Figure 6 - Measured average image
second moment ratios for all processed scans plotted versus running survey
day number.
Figure 7 - Histograms of log second
moment ratios for all scans.
Figure 8 - Relationship between
average second moment ratio (R2) and average seeing shape for all scans.
Figure 9 - Cumulative second image
moment ratio distribution for first six months of operation.