The photometric uncertainties listed in the 2MASS Sampler Point Source Catalog entries are the measurement errors for each star only. They do not include the contribution of the uncertainty in the photometric calibration (cf.III.3.h) or any other estimates of systematic errors. A minimum photometric uncertainty of 0.015 mag is quoted in the Sampler Point Source Catalog entries to provide a measure of net error associated with high signal-to-noise sources that is determined from the photometric dispersion (Figure 1) observed in multiple measurements of stars in the calibration fields during the night.
|
|
| Figure 1 |
Cross-scan Photometric Bias
Profile-fitting photometry in 2MAPPS assume that a single PSF adequately describes point sources across the focal plane of the 2MASS cameras. Any mismatch between the actual source profile and the PSF being fit can result in an increased dispersion and small bias in the resulting photometry, relative to fits made with perfectly-matched PSFs. As with most optical systems, there is a small variation in the 2MASS system PSF across the focal plane. This distortion results in a small photometric bias (1-2%) between the east and west edges of the focal plane. Although it is small, the bias can be measured by comparing the systematic differences between photometry of sources in the scan overlap regions. For the 2MASS Sampler night, the cross-scan biases range from 1-2% in the J and H band, and ~1% in the Ks band.