Here Be Dragons:
Characterization of ACS/WFC Scattered Light Anomalies


We present a study characterizing scattered light anomalies that occur near the edges of the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) images. The study is based on all full-frame WFC raw images ever produced by ACS. Using the 2MASS catalog, we identified stars that cause two particular scattered light artifacts known as ”dragon’s breath” and edge glow. These artifacts are caused by stars located in narrow bands outside the ACS/WFC field of view. We have completed this study for the ACS F606W and F814W filters. The results for both filters are similar when expressed in total fluence, or flux multiplied by exposure time. We provide a map of risky areas around the ACS chips and an upper limit of magnitudes to be concerned about. We will use these results to develop interactive tools that will aid the astronomical community in the proposal process for ACS/WFC.


ACS/WFC images can suffer from a number of optical and scattered light anomalies. Most of the optical anomalies that effect ACS have been well characterized. Hardware, software, and optical anomalies are discussed in ISR 2008-01. This is not the case for the scattered light anomalies known as ”dragon’s breath” and edge glow. Dragon’s breath is caused by reflections being scattered back to the detector. There is a knife-edged mask in front of the CCD that scatters light back to the detector when its back side is illuminated by reflections from the CCD surface. These phenomena were discovered in early testing of ACS and were mitigated by sharpening the knife edges and coating them black. However, when point sources fall on the edge of the mask, scattering still occurs (Hartig et. al.).

Figure 1a: Dragon’s Breath