I. Introduction

6. Cautionary Notes

d. Atlas Images

2MASS Atlas Images derive from the coaddition of the six "Read 2-Read 1" images which cover any given piece of the sky. Please see Section IV.3 for a detailed description of the procedures used to construct the Atlas Images.

i. Quicklook Images

A ~20:1 lossy-compressed version of the Atlas Images, known as "Quicklook" Images, are also available on-line. These images are suitable for finding charts and visual inspection of the near-infrared sky. Because of the loss of information during compression, the Quicklook Images should not be used to make quantitative measurements of source or region brightness. Users should always defer to the Catalogs for photometry of point and extended sources, and use only the non-compressed Atlas Images for direct measurements from the images.

ii. 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.

iii. 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 can be 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. Because the OH airglow (especially at H-band) often contains structure on scales at or below the 2MASS frame 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.

iv. 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: 2001 January 31, by R. Cutri, M. Skrutskie and E. Kopan]

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