The photometry and position reconstruction of saturated Read 1 stars in 2MAPPS v3.0 are excellent. Photometric accuracy, measured from repeatability, is in the range 0.1-0.2 mags. The colors of stars are nominal, with only a few outliers falling more than 0.3-0.4 mags off the expected dwarf and giant tracks. Astrometric accuracy, measured by comparison with the Tycho 2 catalog, is 120-150 milli-arcsec, as good as that obtained from non-saturated high signal-to-noise stars.
All sources saturated in R1 in one or more bands (rd=3) were selected from the survey scans in the the OPS Test point source database using GATOR. Sources flagged as artifacts (cc=P,G,C,D) or within 10" of a scan edge were rejected.
Figure 1 and Figure 2 show the J-H vs. H-Ks and J-H vs. J-Ks color-color diagrams for the 2017 stars in the OPS Test DB not associated with artifacts or close to edges. The color coding in these diagrams indicates which band(s) are saturated. The scatter about the expected main sequence and giant branches is generally well-behaved. Most stars lie within 0.2-0.3 mags of the expected tracks.
Photometry of stars saturated in H-band only show the largest scatter.
The calibration
of the templates
used in the saturated star photometry was
derived using J and K photometry of bright stars published by
Johnsen et al. (1966 Comm. Lunar and Planet. Lab., 4, 99) and Lee
(1970, ApJ, 1970, 217). Those authors quote a typical uncertainty
of ~0.03 mag. The H-band photometry from those stars
was taken from sources other than Johnsen et al. and Lee, as compiled in
the CIO. Differences in the photometric systems probably elevate
the relative uncertainty in the H-band to as much as 0.1 mag.
So the larger observed scatter in the 2MASS H-band residuals are expected.
|
|
| Figure 1 | Figure 2 |
A few calibration scans contain stars that are R1-saturated in one or more bands. These stars were selected from the OPS Test Calibration Point Source working databases, and again stars marked as artifacts or within 10" of scan edges were filtered out. Between 2 and 156 repeated observations of each saturated star was available in the OPS Test DB.
The table below shows the coordinates of each saturated star in the OPS test DB, the mean and RMS Right Ascension, Declination, J, H and Ks magnitudes, and the number of repeated observations available for the star. The positions and positional RMS are given in degrees.
| < RA > | rms(RA) | < Dec > | rms(Dec) | < J > | rms(J) | < H > | rms(H) | < K > | rms(K) | N repeats | Light Curve |
| 6.123379 | 0.000047 | -2.218952 | 0.000043 | 4.031 | 0.289 | 3.386 | 0.262 | 3.231 | 0.265 | 156 | X |
| 8.280472 | 0.000128 | -39.759768 | 0.000091 | 5.049 | 0.110 | 4.198 | 0.158 | 4.031 | 0.091 | 123 | X |
| 52.979549 | 0.000039 | 36.987071 | 0.000048 | 4.754 | 0.202 | 3.716 | 0.183 | 3.370 | 0.210 | 96 | X |
| 224.159056 | 0.000055 | -45.121913 | 0.000043 | 5.112 | 0.081 | 4.167 | 0.175 | 3.947 | 0.137 | 56 | X |
| 267.125760 | 0.000047 | -45.384187 | 0.000031 | 4.732 | 0.207 | 3.522 | 0.205 | 3.082 | 0.210 | 54 | X |
| 279.996204 | 0.000074 | 49.072652 | 0.000059 | 5.118 | 0.065 | 4.307 | 0.151 | 4.038 | 0.060 | 35 | X |
| 282.825943 | - | -4.965667 | - | 5.441 | 0.003 | 4.833 | 0.019 | 4.189 | 0.008 | 2 | X |
| 310.337785 | 0.000057 | -5.750970 | 0.000056 | 5.047 | 0.091 | 4.196 | 0.195 | 3.889 | 0.147 | 33 | X |
| 352.664751 | 0.000050 | 38.662331 | 0.000055 | 4.478 | 0.268 | 4.007 | 0.170 | 3.885 | 0.161 | 64 | X |
The magnitude RMSs are typically 0.1-0.2 mags, with one star with a dispersion closer to 0.3 mags. Variability undoubtedly increases the dispersion for some of the stars since many near-IR bright stars are variables. Links are provided for the light curves of each star. The brightness spread for the apparitions of a star from the same nights are generally in the range ~0.1 mag or smaller, which should be more representative of the instantaneous repeatability.
The positional dispersions given in the table above range from ~0.1-0.3 arcseconds. Position repeatability over long timescales probably isn't the best measure of astrometric accuracy for bright stars since proper motion may drive up the dispersions.
There are 875 stars in the OPS Test point source database that are saturated in all three bands and are positionally associated with Tycho 2 stars. Tycho stars that are saturated in all 2MASS bands are not used in the astrometric solutions for the survey. Therefore, the offsets between Tycho and reconstructed 2MASS positions for the saturated stars provides a valid external measure of astrometric accuracy.
Figure 3 shows the Right Ascension and Declination 2MASS-Tycho offsets for the 875 R1-saturated Tycho stars. The residuals show no measureable declination bias and 38 mas positive bias in RA. The RMS of the distributions are 121 and 148 mas in RA and DEC, respectively. No 3-band saturated stars have reconstructed positions that are separated by >0.5 arcsec from the reported Tycho 2 position.
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| Figure 3 |
Because the saturated star detection and photometry in 2MAPPS v3.0 is done independently in each band, we can use the band-merging results to obtain a crude estimate of the internal completeness of the saturated R1 star detections.
There are six instances of missing saturated R1 magnitudes out of the 2017 stars R1-saturated stars in the OPS tests. Five stars are missing one measurement, and one is missing measurements in two bands. Three of these stars have upper limits in the missing band(s) and two have rd_flg=9, meaning there was a detection, but a magnitude couldn't be extracted. The stars missing band-detections are not particularly bright in the detected bands, so this is not a problem with overly bright stars.
The completeness for very bright stars can be estimated from:
where Nmissed is 6, and Ndet is the sum of stars*bands for which "good" magnitudes are available: (2016(J)+2016(H)+2013(Ks)=6045). Therefore, C = 0.9990.
This differential analysis does not account for bright stars that are missed in all three bands.
Gene Kopan has investigated the five stars with missing band-detections and reports the following:
Roc, I checked out the 5 sat r1's with missing bands, disposition below gene ------------ h date scn ra dec rd diagnosis n 02/14/1999 62 115.924309 75.875534 130 fell in bad K column s 04/08/2000 47 183.375251 -62.965122 003 ?? looks ok in *_1.fex band merge train wreck? see below- n 11/30/1997 79 68.988383 28.514526 339 fell in bad K column n 10/28/1998 144 131.767728 13.247221 339 fell in bad K column n 04/03/1998 55 143.555789 36.397583 339 fell in bad K column ------ from 980408s/s047/src/s047.short: 39458 183.374896 -62.965385 5.523 0.001 4.508 0.017 6.039 0.001 110 000 000 000 000 000 U022513623875 13.40 10.90 1.17 110 1 9 39468 183.374801 -62.965221 6.994 0.001 4.539 0.047 6.057 0.001 020 010 000 000 000 0C0 U022513623875 13.40 10.90 0.96 78 1 9 39470 183.375251 -62.965122 7.045 0.001 6.816 0.001 3.700 0.266 003 000 000 000 000 000 U022513623875 13.40 10.90 1.77 72 1 9 looks like 39458+39470 contain the 3 r1 measurements.
The most common reason for missing a band-detection appears to be cases where the star falls along the noisy column on the Ks array in the north. This column is masked out in the early northern data, and will lead to some incompleteness in Ks.
There is one instance where the bright star was detected in all three bands. BANDMERGE successfully merged the J and H non-saturated R1 detections, but did not merge those with the saturated R1 detection. So the detection is there, but the source is incorrectly listed as two objects.
Last Updated: 6 September 2001
R. Cutri - IPAC
