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The following images compare a 1800s dark frame taken with the guide CCDs turned on ( after zero-subtraction , trimming and overscan subtraction ) , with an 1800s dark frame taklen with the guide CCDs turned off ( after zero-subtraction , trimming and overscan subtraction ) , and with a zero ( or bias ) frame ( trimmed and overscann subtracted ) . For the purposes of this example , I 'll be adding a new algorithm called ' Noise ' which will simply add ( or subtract ) a small random amount from each pixel in the image . For example , on an important , high-traffic keyword , you think you can only afford position six because you want to make $ 50 per order ( after search click media costs are subtracted ) .