II. User's Guide to the WISE Preliminary Data Release

4. Ancillary Products

a. Single-Exposure Images, Bit Masks, Uncertainty Maps

The WISE Single-exposure image archive is comprised of 754,854 1016x1016 pix @ 2.75"/pix FITS format image sets. Each image set consists of:

• Intensity images - Four images, one in each of the WISE band, that contain the calibrated sky images for each individual 7.7sec (W1/W2) and 8.8sec (W3/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 uncertainty maps in generating 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 World Coordinate System (WCS) standard for astronomical image data.

Single-exposure Image products have filenames as follows:

• Intensity image: frameID-wBAND-int-1b.fits


• Uncertainty image: frameID-wBAND-unc-1b.fits


• Bit-Masks: frameID-wBAND-msk-1b.fits

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, scan_id, and MMM is the frame number, frame_num, within the scan. 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.

Several of the WISE Preliminary Release Ancillary Products provide additional information about the WISE Single-exposure Image sets. The Single-exposure (L1) Frame Metadata Table contains much of the Atlas Image header content. The Single-Exposure Image Inventory Table contains derived statistical and quality assessment information pertaining to both the Single-exposure images and extracted source database. These metadata tables can be searched using any of their parameters using the IRSA Gator catalog search engine.

CAUTION: The Single-exposure image archive contains all images in the scans that are part of the WISE Preliminary Data Release scope (V.2.a), regardless of their quality. Many of these framesets suffer from the following problems:

• Poor image quality due to spacecraft jitter,


• Contamination by severe scattered light artifacts from the moon and bright planets,


• 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.a.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 > 0 - Outside of the nominal boundaries of the SAA


• MOON_SEP > 24° - Beyond the worst of the scattered moonlight (Use large values to be more conservative)


• DT_ANNEAL > 2000 sec - W3 and W4 detectors have stabilized following annealing (W1 and W2 are unaffected by anneals)

Last update: 2011 April 12