In order to flag sources which are created by diffractions spikes ("spurious" sources), as well as astrophysical sources that have their photometry contaminated by diffraction spikes, we employ empirically-derived functional relations to predict spike length and width as a function of parent star brightness (Figure 1). The size (length and width) of a diffraction spike is determined by the brightness of the star producing the spike (referred to as the "parent" star). i.1 Single-frame Flagging i.1.a Spike Lengths and Widths Flagging of sources created or contaminated by diffractions spikes is similar in the single-frame and atlas images however, there are important differences outlined in the section below. Diffractions spikes also appear in atlas (coadded) images, though they are somewhat mitigated at high ecliptic latitudes where their positions angles on the sky vary due to the nature of the spacecraft's orbit. In a single frame image, they extend from a bright source at angles of approximately 45, 135, 225, and 315 degrees, where 0 degrees is aligned with the positive y-axis (see Figure 1). Diffraction Spikesĭiffraction spikes are linear features caused by diffracted light from the telescope's secondary mirror support structure. Other changes include the addition of background-dependent spike and halo sizes and adjustments for high ecliptic latitude effects on the artifacts. The bright source list is also used to supplement the list of latents parents and to supplement the WISE photometry for more accurate flagging when the extracted WISE photometry was underestimating the flux of extremely saturated stars. For each frame, ArtID searches the bright source list for sources in the surrounding area which may produce artifacts in the frame, and flags them as necessary. In order to facilitate the flagging of artifacts with off-frame parents, ArtID makes use of a bright source list, which includes stars which are bright enough to create artifacts. This is, in fact, always true of "glints" in single-frame images, which are caused when a bright parent lies just outside the frame. may still extend into the frame in question. I.e., when a bright parent falls just outside of the single-frame or atlas image, its diffraction spikes, halo, ghosts, etc. One of the major additions is the flagging of artifacts created by parents which lie off-frame. The version of ArtID used for the All-Sky Data Release incorporates numerous improvements over previous versions (including the version used for the Preliminary Data Release). We strongly recommend that you familiarize yourself with the cautionary notes in this section before using the cc_flags field in your searches. The only exception to this convention applies to the flagging of glints, In multiframe flagging (i.e., atlas images) all artifacts except glints are flagged (see below for an explanation).
Upper-case flags in the 'cc_flags' field represent spurious sources, which exist solely due to the artifact, and lower-case flags represent real astrophysical objects whose photometry is contaminated by the artifact. In single-frame images (i.e., scan frame or Level 1b images), diffraction spikes (flag='d'), halos (flag='h'), ghosts(flag='o'), latents (flag='p'), and glints (flag='g') are flagged. What follows is a description of each artifact, the characterization and flagging procedure in single frame (scan frame) and multiframe (atlas) images, and procedure for refinement of parameters.
These artifacts include diffraction spikes, scattered-light halos, optical ghosts, persistence (latents), and glints. Artifact Identification and Flagging (ARTID)īright stars produce a number of artifacts in the both the single exposure and atlas images. WISE All-Sky Release Explanatory Supplement: Data Processing
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