The user is cautioned against assuming a constant value for position uncertainties in the 2MASS Sampler, even for sources brighter than a given magnitude or SNR. Uncertainties vary due to factors associated with individual point source extractions, such as signal-to-noise ratio and confusion. They also vary due to factors associated with how accurately the frame positions can be determined. The latter is primarily driven by the density and distribution within each scan of astrometric reference stars, taken from the ACT Reference Catalog. It is also influenced by the density of 2MASS extractions available to tie the frames together. Position uncertainties, which reflect a number of contributors, including those mentioned above, are given for each source in the Sampler.
In keeping with the quick-release nature of the Sampler, there has been no attempt to improve the reconstructed positions in low density ACT regions using scan overlaps to bring ACT information across scan boundaries. Position errors can therefore become much larger in these regions. The scan overlaps have been used, however, to obtain conservative estimates of the position uncertainties during these periods which are reflected in the values quoted for each source. The user should anticipate that both the positions and uncertainties (especially in low ACT density regions) quoted in the Sampler will change in the final catalog.