 Spitzer Science Center

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Knowledgebase: IRAC How is the dark/sky subtraction actually performed in the pipeline. Can you give a mathematical representation of it? Posted by Seppo Laine on 29 January 2009 12:10 PM A so-called labdark is subtracted from each frame, including skydark frames. So, in simple mathematical terms, [(obj-lab) - (sky-lab)]/flatfield = (obj-sky)/flatfield = BCD where "obj" is a science object frame, "lab" is the appropriate labdark, and "sky" is an appropriate skydark. Please note that the lab image subtracted from the object is not necessarily equal to the lab image subtracted from the sky frame, they are just very similar. The subtracted labdark is based on the exposure time, Fowler sampling and operation history of the arrays and therefore is different for the object frame and the skyframe. The DC (bias) levels change as a function of time elapsed since the previous frame was taken (the "first frame effect"). This effect is largest for channel 3 and can lead to background variations from frame to frame, especially in channel 3. Note that you can download the darks from the Spitzer Heritage Archive by selecting the "cal data" box during the packaging process. For the warm mission, the arrays were set to a new bias due to the higher temperature. For the warm mission duration, there were no measured labdarks that had been created at this new temperature and bias level. Therefore, only a skydark was subtracted from each frame, including skydark frames. So, in simple mathematical terms,  (obj-sky) / flatfield = BCD where "obj" is a science object frame and "sky" is an appropriate skydark.    Please note that both the sky image and the obj image contained a “lab dark” signal  based on the exposure time, Fowler sampling and operation history of the arrays. This DC (bias) level changes as a function of time elapsed since the previous frame was taken (the "first frame effect”), so there will be a small residual left from the subtraction. Note that you can download the darks from the Spitzer Heritage Archive by selecting the "cal data" box during the packaging process.