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Knowledgebase: IRAC
I see large gradients and background variations in my channel 3 (5.8 micron) mosaics. What is wrong?
Posted by Seppo Laine on 29 January 2009 12:05 PM
Channel 3 suffers from a large first frame effect. Depending on the observation strategy, this may cause severe variations in the background level of the individual BCDs. The effect is worst in frame repeats (as opposed to frame dithers). To mitigate, you may want to form a delta dark by taking the medium of all the channel 3 BCDs in the AOR, normalizing this to zero, and then subtracting it from each BCD. Also, use the overlap-correction module in MOPEX before making the mosaic. Note that if you have extremely bright objects or emission regions within certain BCDs, their background may be artificially elevated by the "droop" or array pull-up effect, and needs to be manually lowered to match the spatially surrounding background level before overlap correction and mosaicking.