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 socalled labdark is subtracted from each frame, including skydark frames. So, in simple mathematical terms, 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,
(objsky) / 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.
 
