I. Introduction

6. Cautionary Notes

d. Atlas Images

2MASS Atlas Images derive from the coaddition of the six Read2-Read1 images which cover any given piece of the sky. Because of the prohibitive size of the full-resolution Atlas Image database (over 0.5 TB for just the First Incremental Data Release), 20:1 lossy-compressed images are kept on-line. The compression degrades the photometry of the sources particularly at lower SNR and this version of the image product should not be used independently to extract source photometry. Users should always defer to the Catalogs for source photometry.

Please see Section III.3.b for a detailed description of the procedures used to construct the Atlas Images.

i. Low Coverage Areas

At certain pixel locations sky coverage is reduced from the nominal six apparitions by either noisy or bad pixels, or cosmic rays. If only one or zero apparitions are available for a given pixel location, a zero flux value is inserted for this pixel. These pixels are  apparent in the images because the natural sky background in the frames is much larger than zero.

ii. Backgrounds

The Atlas Images preserve the observed background sky levels measured relative to camera dark frames with the shutter closed. This background is normally largest in the Ks band, although it is often even larger in the H-band due to atmospheric OH airglow emission. The only background compensation that is made during Atlas Image construction is to adjust the frame backgrounds by a constant to produce seamless coadded images. However, because the OH airglow (especially at H-band) often contains structure on scales at or below the Atlas Image size, the resulting Atlas Images show large background variations.  Also, if the background is changing shape from frame to frame due to a bright star or a time-variant background, discontinuities may be seen at the frame edges.

iii. Transients

An effort is made to remove transient "sources" such as isolated cosmic ray hits and hot pixels during the generation of the Atlas Images. This is accomplished by identifying point source detections above a specified SNR threshold seen on only a single frame, and masking them in the offending frame before combining the six frames. When the transient effect is confused with a source, it will not be identified as a solo and will remain in the Atlas Image. This "solo-blanking" has the side effect of removing part or all of many meteor trails. However, users will often see remnant trails on the frames either as solid streaks or broken streak segments.

[Last updated: 1999 May 7, by R. Cutri, M. Skrutskie and E. Kopan]

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