Standard construction

There are two ways to produce flat fields for use with IR data. The standard method is to take 'LAMP ON' and 'LAMP OFF' exposures of the calibration spot on the telescope dome. The 'LAMP ON' exposure contains light from both the calibration lamp, which should provide the even illumination needed for the flat field, together with potentially uneven illumination from the telescope and instrument environments. To retain only the even calibration illumination, the ``LAMP OFF'' image must be subtracted from the ``LAMP ON'' image. This subtraction also removes the bias pattern from the flat field image.

The subtracted image should then be normalised for later use as a flat field.

In MIDAS, you may directly use COMPUTE/IMAGE to perform the subtraction. The image mean may then be calculated using STATISTICS/IMAGE or similar. The ``LAMP on - LAMP OFF'' image can then be divided by this value using COMPUTE/IMAGE for normalisation. Alternatively, the procedure MKFLAT/IRAC2 combines these actions. To improve the signal-to-noise ratio of the flat-field, a number of flat field images may be used together. These should be combined using COMBINE/CCD (see later).