2MASS Atlas Images derive from the coaddition of the six "Read 2-Read 1" images which cover any given piece of the sky. Because of the prohibitive size of the full-resolution Atlas Image database (over 4.0 TB for just the Second Incremental Data Release), ~20:1 lossy-compressed images are currently available 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 IV.3 for a detailed description of the procedures used to construct the Atlas Images.
[Last updated: 2000 January 9, by R. Cutri, M. Skrutskie and E. Kopan]
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 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.
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.
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