Flat image is the response of the system to uniform illumination obtained for the purpose of correcting for anomalies in the optical path. In effect, this concerns the flat field correction - korekcja tła.

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Note added at 10 hrs (2015-06-02 00:25:57 GMT)
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Flat Fielding

What is flat fielding and why do it?

A flat field is the response of the telescope-camera system to a source of uniform illumination. Flat field frames are used to correct for anomalies in the optical path, such as specs of dust on the optical surfaces, sensitivity variances between pixels in the CCD, and illumination variations within the optical system (vignetting).
A flat field frame is basically an image of a uniformly illuminated field. Since this flat contains the variations in illumination due to variations within the optial system, dividing the science image by the normalised flat filed frame will correct the image for these variations.
This procedure is referred to as flat fielding.
When to do flat fielding?

Flat fielding to remove inhomogeneities in the filed illumination is always performed in astronomy research; however, within the scope of AstroLab is not always necessary. Flat fielding is particularly useful when precise photometry is required, such as for light curve determination projects. Ideally one would like to use one flat field per night.
https://community.dur.ac.uk/physics.astrolab/flat_fielding.h...

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Note added at 10 hrs (2015-06-02 00:28:44 GMT)
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Flat-field correction is a technique used to improve quality in digital imaging. The goal is to remove artifacts from 2-D images that are caused by variations in the pixel-to-pixel sensitivity of the detector and/or by distortions in the optical path. It is a standard calibration procedure in everything from pocket digital cameras to giant telescopes.

Flat fielding refers to the process of compensating for different gains and dark currents in a detector. Once a detector has been appropriately flat-fielded, a uniform signal will create a uniform output (hence flat-field). This then means any further signal is due to the phenomenon being detected and not a systematic error.
http://en.wikipedia.org/wiki/Flat-field_correction
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Automatyczna korekcja FFC (flat-field correction)
http://resource.boschsecurity.com/documents/VOT_320_Data_she...
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ook Up Table), łączenie pikseli w poziomie i/lub pionie (binning), korekcja tła (ang. Flat Field Correction),
http://automatykaonline.pl/Artykuly/Systemy-wizyjne-i-RFID/D...