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Knowledgebase: IRAC
Almost all the background pixels in my image are negative. What is wrong?
Posted by Seppo Laine on 29 January 2009 12:03 PM

Most likely the skydark that was subtracted from your data over-subtracted the background in your images. You can add back the subtracted skydark background value using the BCD header keyword SKYDRKZB. See section 6.3 in the IRAC Instrument Handbook. You will need to scale the SKYDRKZB value by the extended source throughput to get the "as measured" surface brightness. For example in channel 4, SKDRKZB / 0.74 will be the best estimate. Adding this back to a BCD image will result in a BCD image with a background that reflects the measured background counts in the raw image. Users are reminded that IRAC is not an absolute photometer, and are advised against interpreting the resulting measured value as the absolute surface brightness of the sky. For comparison, the predicted zodiacal background for your images is in the BCD header keyword ZODY_EST.