II. The WISE All-Sky Data Products
4. Ancillary Products
b. Single-Exposure Images, Bit Masks, Uncertainty Maps
The WISE Single-exposure image archive is comprised of 1,491,686
1016x1016 pix @ 2.75"/pix FITS format image sets. Each image set
- Intensity images - Four images, one in each
of the WISE band, that contain the calibrated sky images for each
individual 7.7sec (W1 and W2) and 8.8sec (W3 and W4) exposure.
Instrumental calibrations have been applied to these images to remove
instrumental signatures, and the headers contain photometric zero points
and full WCS information.
- Uncertainty maps - Four images that give the 1-σ uncertainty estimate for each pixel in the corresponding intensity images. These maps provide the instrumental noise component for photometric uncertainty estimates in the catalog and are also used to generate the Atlas Image uncertainty maps.
- Bit-masks - Four images, one in
each WISE band, that provide information about the status of each pixel in the Single-exposure intensity images.
WISE Single-exposure images are in the
Flexible Image Transport System (FITS) format and follow the
Coordinate System (WCS) standard for astronomical image data.
Single-exposure Image products have filenames as follows:
- Intensity image:
- Uncertainty image:
where BAND = 1, 2, 3, or 4, and frameID is
a unique identification string pertaining to the frameset.
FrameID has the form SSSSxMMM, where SSSSSx is the
identifier of the scan in which the framset was taken,
MMM is the frame number,
BAND and frameID are variables that match the values of
keywords with the same name in FITS headers. The WISE Single-exposure Image
FITS header content is described in II.4.a.i.
The Single-exposure (L1) Frame Metadata Table
provides additional information about the WISE Single-exposure image sets
including much of the image header content and
derived statistical and quality assessment information pertaining to both
the Single-exposure images and extracted source database.
This metadata table can be searched
using the IRSA Gator catalog
CAUTION: The Single-exposure image
archive contains all images in the scans that are part of
the WISE All-Sky Data Release scope (V.2.a),
regardless of their quality. Many of these framesets suffer
from the following problems:
- Degraded image quality due to spacecraft jitter,
- Contamination by severe scattered light artifacts from the moon and
- Contamination by bright satellite streaks both on an off the FOV,
- Contamination by large numbers of cosmic ray strikes because they were
taken while WISE was passing through the South Atlantic Anomaly (SAA),
- Residual long-term W3 and W4 latent images from bright sources that were not corrected by
dynamic frame calibration step,
- Unusable W3 and W4 data because they were taken <2000 sec following
the annealing of the Si:As detectors, and
- Contamination by electronic noise signatures.
Images showing example frames with many of these problems
can be seen in II.4.b.ii, the
Single-exposure Image Anomaly gallery.
Most of the low-quality framesets and frames were excluded when
selecting frames to constructed the coadded Atlas Images and
Source Catalog (V.2.c), but
all are present in the Single-exposure image archive. Use this
archive with caution.
You can identify Single-exposure image sets that are of good quality
using several of the keywords in the FITS headers and/or values in the
metadata tables as follows:
- qual_fact > 0 - Good overall quality score (includes image quality and pixel noise levels; available in Frame Metadata and Image Inventory Tables only)
- SAA_SEP (saa_sep) > 0 - Outside of the nominal boundaries of the
South Atlantic Anomaly
- MOON_SEP (moon_sep) > 24° - Beyond the worst of the scattered moonlight (Use larger values to be more conservative)
- DT_ANNEAL (dtanneal) > 2000 sec - W3 and W4 detectors have stabilized following annealing (W1 and W2 are not affected by anneals)
Last update: 2012 July 6