How are the photometric corrections applied?
The dark count frame should always be subtracted first because the dark count is not affected by the effective CCD pixel sensitivity nor the number of photo-electrons collected at the pixel. If the automatic routine is used in the multiple file processor, then the dark frame intensity is weighted by the ratio of the “BottomThermalCount” in the image to be processed to the “BiasLevel” in the dark count image. The “BiasLevel” in the dark count image should represent the median intensity in the image, if the “Create thermal frame” function on the multiple image processing form was used. The purpose of the prior subtraction of the bias and later scaling of the dark count frame is to allow dark count frames obtained for different integration times to be used. Otherwise, dark count frames must be produced for each integration time used, which is rarely practical.
The “flat-fielding” operation consists of dividing the intensities in the image to be processed (minus the offset level of the image) by the flat-field image. The resulting image is automatically scaled in intensity so that the most is made of the dynamic range of a 16-bit integer. An important point to remember is that the flat-fielding produces an image whose pixel intensities more accurately reflect the light intensity of the imaged object, however the connection of the pixel intensities to the collected pixel photo-electrons is lost due to the somewhat arbitrary scaling used in the flat-fielding procedure.
Note that if the multiple image processing tool is employed, then three options are available to determine the bias to subtract from the images. The bias can be estimated from a histogram of a rectangular region of the image, determined from the quoted value in the image header or a fixed value can be selected. The first method is not recommended because the bias estimate will also include contribution from sky glow. If you are worried about a fixed pattern in the image offset frame (that is a frame obtained in the dark under conditions where the thermal count is negligible) then a series of offset frames can be generated and subtracted from the images prior to dark-frame and flat-field correction. During flat-field and dark-frame correction the bias to subtract should then be set to 0. Note that no fixed pattern has been noticed in the Pyxis KAF401e prototype camera offset frame – the spatial (meaning from pixel to pixel in the same image) and temporal (meaning from frame to frame looking at the same pixel) noise properties of the offset frames are identical.