The Level 1 Specifications place three requirements on saturated star photometry (specifically "Read 1" photometry).
- 5% photometric precision for Ks=8.0 mag (meaning just above the "Read 2 - Read 1" 1.3s exposure saturation threshold (designated by rd_flg=1)).
- 10% photometric precision for at Ks=4.0 mag (meaning just below the "Read 1" 51ms saturation threshold (thus also rd_flg=1)).
- No more than 2% bias for Ks>4.0 mag
There are no Level 1 requirements associated with sources that saturated even the 51 ms "Read 1." (designated by rd_flg=3). These sources do have large uncertainties significantly in excess of the Level 1 Specifications for fainter sources.
a. Bright Star Photometric Precision
Requirements 1 and 2 are addressed in the photometric precision section above, which discusses uncertainties derived from repeated observation across virtually the entire flux range observed in 2MASS. The relevant figure is reproduced in Figure 1 below and shows that the uncertainty in the magnitude range 4.0<Ks<8.0 is substantially better than the 10/5% requirements in items 1 and 2.
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| Figure 1 |
b. Bright Star Photometric Bias
After substantial discussion, the team has not found a means to rigorously statistically validate the third item -- demonstrating less than 2% bias across the Read1 (Ks = 4.0-8.0) regime -- from internal survey data.
Bias can be addressed at the "faint" end of the saturation regime due to some overlap between the 1.3s "Read2-Read1" photometry (rd_flg=2) and the 51ms "Read1" photometry (rd_flg=1) since, in addition to saturated stars, the 51ms "Read1" pipeline photometers stars fainter than the "Read2-Read1" saturation threshold. Processing QA monitored the overlap between these two assessments of a star's magnitude. Figure 2 shows a typical result. In Figure 2 the blue dots represent legitimate 1.3s "Read2-Read1" (rd_flg=2) detections which transition to saturated 1.3s detections at the red points. Near the transition the the two magnitudes agree well. In the regime of red dots the plots deviate because the "Read2-Read1" pipeline is operating on a saturated image. At faint magnitudes the plots deviate because the singly-correlated 51ms "Read1" integration becomes a poor estimator of flux when there is little detectable flux.
Although this result does not bear directly on the bias in saturated observations it does show that the 51 ms integration frames were consistently tied to the 1.3s frames at the crossover point between two methods of photometry and that the "Read2-Read1" 1.3s integration photometry was linear right up to the saturation threshold. The 51ms "Read1" saturation threshold was characterized and implemented in a similar fashion and the linearity characteristics at this threshold should be similar.
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| Figure 2 |
Direct comparison with externally determined calibration networks provides both a test of the accuracy of Read1 photometry as a whole and an estimate of the bias between the bright end and faint end of the Read1 saturation range. Since these other systems suffer from the same validation challenges as 2MASS, differences may characterize biases in the other systems rather than in 2MASS. Nevertheless, the ensemble should provide upper limits to bias in 2MASS.
Below are comparisons of Read1 photometry with established calibrator magnitudes in five different popular calibration systems. The green points in the diagram correspond to 2MASS rd_flg=1 sources (while the red points illustrate the performance of the algorithm that extracted stars saturated even in the 51ms exposures -- rd_flg=3 -- for which there are no specifications). Table 1 below (not yet available) quantifies the mean zeropoint offset for each photometric system. The table also quantifies the difference in zeropoint offset (bias) between the faint end and bright end of the Read1 regime for each system. If fewer than 5 stars appear in either of the magnitude bins for this calculation a result does not appear in the table.
From these tabulated results the Science Team concludes that the photometric bias across the Read1 range is certainly <4% and possibly with the specification of 2%.
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| Figure 3 | Figure 4 | Figure 5 |
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| Figure 6 | Figure 7 |
| System | J offset | H offset | Ks offset | J bias (5-6 mag) vs. (8-9 mag) |
H bias (5-6 mag) vs. (8-9 mag) | Ks bias (4-5 mag) vs. (8-9 mag) |
| AAO | -0.083 +/- 0.012 | -0.022 +/- 0.007 | -0.039 +/- 0.006 | ---- | ---- | ---- |
| CIT | 0.001 +/- 0.007 | 0.018 +/- 0.005 | -0.017 +/- 0.003 | ---- | ---- | ---- |
| ESO | -0.056 +/- 0.003 | 0.011 +/- 0.005 | -0.041 +/- 0.003 | 0.018 +/- 0.009 | 0.013 +/- 0.009 | -0.016 +/- 0.007 |
| MSSSO | -0.016 +/- 0.004 | 0.007 +/- 0.006 | -0.032 +/- 0.004 | ---- | ---- | ---- |
| SAAO | -0.040 +/- 0.003 | 0.026 +/- 0.004 | -0.016 +/- 0.002 | 0.023 +/- 0.006 | 0.016 +/- 0.010 | -0.007 +/- 0.006 |
To be complete in this discussion one should note that there is a known and characterized source of bias in the Read1 photometry immediately adjacent to the Read1 saturation magnitude. This bias arises because Read1 photometry is calculated from only those apparitions (of the 6 possible for a given source) that were unsaturated. For a source at the saturation threshold effects such as intrapixel quantum efficiency variation and instantaneous seeing fluctuations will cause some of the six apparitions of the source to be saturated while others will not. The unsaturated apparitions are naturally biased to fainter fluxes yielding a biased estimate of the star's flux. Users can identify affected sources by consulting the column in the database that lists the number of useful detections of a source in a given band and the number of opportunities for detection (ndet). Read1 sources with two or fewer surviving unsaturated apparitions can be also identified because they receive ph_qual='F' -- a flag which means that insufficient data were available to calculate a meaningful uncertainty.
c. Saturated Read1 Performance (mag < 4.5)
At conception 2MASS had no intention to extract photometry for stars which saturated even the 51 millisecond Read1 exposure. Thus the Level 1 Specifications do not even address this flux regime. Analysis at IPAC demonstrated that reliable fluxes could be extracted for sources even as bright as -4 magnitude (e.g. Alpha Ori and Alpha Tau) using the scattered light wings in the images. These extractions are called "Read3" extractions and receive a rd_flg=3 if they appear as default magnitudes in the database. Figures 3-7 above include Read3 results for a large number of bright sources and demonstrate that their photometry is consistent with previously cataloged magnitudes. Figure 1 above, which assesses uncertainty from repeated observations as a function of magnitude, includes rd_flg=3 sources and shows that the price of this novel method of flux estimation is excess uncertainty. Since this subset of Read3 sources overlaps the brightest visible stars on the sky, the Science Team deemed it reasonable to include these very reliable sources at the "catalog" level despite their substantial uncertainty relative to the Level 1 requirement for fainter stars. Read3 photometry is biased by seeing. This seeing correction has been estimated, but has not been applied to the data because it did not demonstrably improve stellar colors when applied although it improved the match between 2MASS Read3 photometry and catalog photometry. The biases, expected to be <20% in the worst cases, lie within the quoted large uncertainties for these sources. The calibration of these sources is a hybrid between existing bright calibrators from other photometric systems and overlap with unsaturated Read1 photometry. As such, it has a somewhat different heritage than the rd_flg=2 and rd_flg=1 photometry which ultimately tie entirely to the network of faint near-infrared Survey standards.