1. Image Characterization and Validation for All-Sky Release

a. Requirements

The Level 1 specifications note the following concerning the Survey's images:

1. Pixel size is 1/2 the raw frame pixel size.
2. Distortion is <0.1% edge-to-edge (i.e. < 0.5'')
3. Individual frame medians will be matched before coaddition.
4. Photometric and Astrometric calibration will be adopted from the Point Source Catalog.

b. Analysis and Validation

Points 1-3 have been previously validated or are features of the standard processing.

Point 4 is enforced as part of the processing as well, and will be valid as long as the distribution image headers contain the correct photometric and astrometric parameters from their scans. Thus, in order to validate that the scan headers do contain proper information, we considered two tests:

• Spot-check validation of image astrometry and photometry.
• Global check of image header values.

Both of which showed consistency with the requirements.

c. Exceptions and Anomalies

c.1 Image artifacts

Images can include a number of artifacts including:

1. Reflection/persistence ghost images of bright stars
2. Meteor and airplane trails
3. Insect trails (from bugs walking across the dewar window)
4. Glints from the edges of chips
5. Structure from extreme airglow spatial/temporal variations

Ghost images are predicted geometrically and can be marked on the final images.

A gallery of other artifacts with explanation will be maintained as part of the explanatory information.

c.2 FITS headers

The distribution images contain some potentially extraneous and, in the case of compressed images, incorrect header parameters. The time to make direct modifications or to develop and validate filters to reinterpret headers is prohibitive. Their presence is largely benign. The distribution web pages for the images includes a warning concerning the appropriate use of the images for photometry. We recommend distributing images as is. We will include explanatory information about reliable usage of the headers.