Flat-field correction

No distortion, neither geometrical neither due to magnetic fields, thanks to proximity focusing. The non sensitivity of the tube to magnetic fields enables software enhancement of the images such as flat field corrections. 

Standard data reduction, i.e. bias and flat field correction, has been performed with Iraf. The Iraf task APEXTRACT/APALL was used to extract the spectra, with interactively selected background sampling, in order to avoid contamination for the star spectrum. The wavelength calibration has been done using daily He, Ne, HgCd arcs, and, in order to improve the calibration, wavelengths values for the transitions used were taken from http //physics.nist.gov/. 

Fig. 3.2 I mage analysis procedure used for the images presented in this review, (a) The fluorescence images acquired at the adsorption (/ajs) and desorption (/jgs) potentials are divided by the image which represents the distribution of the incident intensity [1 ) creating flat field corrected images which are used for further analysis. The change in fluorescence is determined by...

Ability to remove other structural (fixed-noise) patterns by flat-field correction... 

An example of the effect of a flat-fielding correction is shown in Fig. 2.8. The interval between calibrations... 

Fig. 2.8. Digital mammogram (a) before and (b) after flat-field correction (from Digital Mammography, eds. ED Pisano,...

The flat-fielding correction is generally based on an assumption that the detector responds linearly to radiation exposure. If some or all of the dels have nonlinear response, then a two-constant (slope and intercept) correction will only operate properly under exactiy the exposure conditions used to obtain the calibration constants. Under other signal levels, different nonlinearities of the del responses will result in image nonuniformities (Fig. 2.7d). 

For flat-fielding correction, an offset value and a gain is measured for each del in the detector. Therefore, the number of such constants is equal to twice the number of dels in the detector, about 7.2 million values. It is typical to remeasure offiset values between images however, the gain matrix generally need only be measured occasionally. 

Another source of noise in spectra measured using a CCD is caused by the pixel-to-pixel variation in the quantum efficiency of neighboring pixels. This variation can be corrected by illuminating the detector array with a uniform light source and measuring the signal from each pixel. Once the relative response of each pixel has been determined, a correction known as a flat field correction can be applied. 

Orion Constellation This is a mosaic picture of 18 (6x3) frames. Additional four frames are added around the Barnard Loop region to increase S/N. Vignetting of the telescope is corrected by flat fielding, using proprietary software written in IDL. Distortion correction and initial colour match is done in Registar, with final colour correction and mosaic processed using Photoshop. (Reproduced by permission of Wei-Hao Wang, Institute for Astronomy at University of Hawaii)... 

PLAN or PL stands for planar and means the lens is corrected for curvature of field, and thus generates a flat field of image ... 

Fig. 3.10. Examples of an artifacts related to the flat field procedure, (a) not corrected defective column on a homogeneity test image, (b) artifact caused by the movement of an object in the X-ray beam after the flat field calibration on a...

Side drilled holes are widely used as reference reflectors, especially when angle beam probes are used (e.g. for weld testing). However, the distance law of side drilled holes is different to that of a flat bottomed hole. In the literature [2] a conversion formula is given which allows to convert the diameter of a side drilled hole into the diameter of a flat bottomed hole and vice versa, valid in the far field only, and for diameters greater than 1.5 times the wave length. In practical application this formula can be used down to approximately one nearfield length, without making big mistakes. Fig. 2 shows curves recorded from real flat bottomed holes, and the uncorrected and corrected DGS curves. 

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