Atlas/Quicklook Images: Cautionary Notes
2MASS Atlas Images derive from the combination of the six "Read 2-Read 1" frames which cover any given piece of the sky. Please see Section IV.3 of the Explanatory Supplement to the 2MASS Second Incremental Data Release for a description of the procedure used to construct the Atlas Images.
i. Quicklook Images
A ~20:1 lossy-compressed version of the Atlas Images, known as "Quicklook"
Images, are currently available on-line. These images are suitable for
finding charts and visual inspection of the near-infrared sky.
Because brightness information is lost during compression, especially
at low signal levels,
the Quicklook Images are not recommended for photometric
measurements of sources or regions.
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.
The non-compressed Atlas Images covering the full sky may be
available sometime in 2004.
For more information on the degradation of photometry due to compression
on the Quicklook Images, please see
Aperture Photometry from 2MASS Compressed/Uncompressed Images.
ii. Low Coverage Areas
At certain pixel locations sky coverage is reduced from the nominal six
apparitions by either noisy or bad pixels, cosmic rays, or meteor
trails. If 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 combined 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. Focal Plane Distortion
Correction for optical distortion in the focal plane of the
2MASS cameras has been applied to the extracted source positions for optimal
astrometric reconstruction. However, distortion corrections
are not applied to the 2MASS Atlas/Quicklook Images.
The effects of focal-plane distortion appear primarily in the
east-west (cross-scan) direction in the Atlas Images.
North-south distortion is minimal, because the combination of
multiple frames offset in the in-scan direction averages down
the distortion residuals.
Offsets between Catalog source positions and
positions measured off the images are zero near Image center
line and increase towards the Image right ascension edges. The maximum
distortion at the frame edges ranges from approximately 0.08´´ to
0.2´´;
the greatest distortion is seen in the northern observatory H-band data. (See
Section VI.6c of the Explanatory
Supplement).
v. Transients
An effort is made to remove transient "sources" such as intermittent
noisy (or "hot") pixels, isolated cosmic ray hits, and
meteor trails during the generation of the Atlas Images. For small,
point-like transients, such as hot pixels and cosmic rays, 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 single frame event (i.e.
solo) and will remain in the Atlas Image.
Meteor trails are removed from single frames by identifying linear patterns
in the solos identified in the frames and blanking the corresponding portion
of the frame in all three bands prior to coaddition. This process allowed
removal of most of the meteor trails from the images, but fainter trails
and some anomalous cases were not identified by this technique, so users
will occasionally see remnant trails on the frames either as solid streaks
or broken streak segments.
This process also serendipitously removed some artifacts
due to bright stars, satellites, and other anomalies, but a number of these
were incompletely blanked. Examples of incomplete blanking may be found
in the gallery of Anomalies
in Atlas/Quicklook Images.
vi. Image Artifacts
The 2MASS Atlas and Quicklook Images can contain a number of
artifacts produced by the optical and electronic effects of
bright stars, and by transient phenomena such as meteor trails,
aircraft, and insects crawling on the camera dewar windows.
Compact artifacts that can be mistaken for point sources are
marked on the Image overlays provided with the
IRSA 2MASS Quicklook
Image Server. Extended and diffuse artifacts are usually
apparent to the user when viewing full Atlas/Quicklook Images.
As an aid to discriminating between real diffuse structure
and artifacts, please see the gallery of
Anomalies in Atlas/Quicklook Images.
The 2MASS Atlas and Quicklook Images are constructed with no attempt
to correct or identify saturated data. The "Read 2-Read 1" frames from
which the Atlas Images are constructed saturate at approximately J<9.0,
H<8.5, and Ks<8.0. Any pixel in an Atlas Image brighter than a few
thousand DN is potentially contaminated by saturation. At levels
J<5.5, H<5.0, and Ks<4.5, the "Read 1" exposure also begins to
saturate, and this will be apparent in the images as a depression in the
center of the saturated image. The precise saturation limit varies,
depending on the atmospheric seeing conditions and the sky background.
Saturation is handled correctly during
source photometry in pipeline processing. Therefore, users are once again
advised to defer to the Catalogs for photometry
of point and extended sources.
[Last updated: 2002 September 30, by R. Cutri, M. Skrutskie and E. Kopan]
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