From schuehle@linmpi.mpg.de Thu Nov 20 15:03:13 2003 Date: Thu, 13 Mar 2003 18:20:58 +0100 From: "[iso-8859-1] \"Schühle Dr., Udo\"" Reply-To: sumer-soft@esa.nascom.nasa.gov To: SUMER_SOFT Subject: SUMER-SOFT: flatfield shift in SUMER images This message is about the (sub-pixel) shift of the SUMER image data and the corresponding flatfield correction data. It provides information about flatfield data that can be correlated better with your image data if you allow a shift of the flatfield data. __________________________________________________________________________ It had been found very early in 1996, by analysing the flatfield correction of SUMER detector A data, that the flatfield pattern of the detectors are not constant with time. This can be found by correlating different flatfield data. The correlation can be maximised by a shift of the flatfield pattern, which is mostly less than one pixel (in X and Y direction), but can amount to several pixels between different flatfields. There may be several reasons for this shift of the "fixed" pattern: One reason lies in the scrubbing of channel plates and the resulting gain loss of the lowest of the three channel plates. Since the channels are inclined with respect to the anode, the gain loss causes a shift of the charge cloud which is located on the anode by the position encoding. The gain loss has always been partly compensated when the high voltage has been raised after a gain calibration has been done. But the high voltage change may have a distortion effect on the electrical field between the channel plate and the anode. It may also have an effect on the position encoding if it affects the dielectric constant in the delay lines. Both may lead to a shift of the image pattern. Normally, a new flatfield image had been acquired always directly after each gain calibration. As a first approximation, it is recommended to use the flat field data which are closest in time to your data set. If you apply the flatfield correction using the flatfield data that are closest in time to your observations, the shift will be smallest. However, since the gain loss occurs more or less constantly during usage of the detector and the compensation can only be done stepwise, there is a shift between your data and the flatfield pattern. In addition, a uniform scrubbing of the detector cannot be achieved, and therefore a differential (, or local) scrubbing, which is due to the non-uniform illumination of the detector during use, results in a shift pattern that is not uniform: depending on which part of the detector area has been used more, the shift is higher in these areas. Thus, the next better approximation is to consider a constant shift within your data frames, although in general it will be different in each location of the image. !!!There is, however, a very strong fixed pattern in the flatfield data that never changes!!! It is the nonlinearity of the detector ADC in the position encoding electronics, which causes the difference of responsivity of odd and even rows. This causes that the count rate in one row is about 9.5% higher than average, while in the adjacent row it is 9.5% lower than average, thus making roughly a 19% difference between the rows. This odd/even pattern is always present along the slit direction, and it has been found to be very stable throughout the time of all flatfield images we have. Note that this pattern only exists in the rows of the image, not in the columns where it has been avoided by a technique called "dithering". This effect is also effectively averaged out if you use an even number of binning along the slit direction. If you want to improve the flatfield correction by shifting the flatfield pattern to better correlate with your data, the odd/even pattern has to be removed separately. For this purpose I have extracted the odd/even pattern from the flatfield raw data and produced new flatfield arrays, which have the odd/even pattern removed. This was done in the following way: For the A and B detector separately, the average odd/even pattern was determined from the row-sums of all flatfield exposure raw images available. From the row-sums the odd/even pattern was extracted by subtraction of the two-pixel average. Since the pattern is a non-linearity of the ADC, it must be the same all along the slit. Thus, the average along the row-sum was taken to determine a single value for the upper and lower deviation, respectively, from the average. These two values were taken to construct an artificial image array of the odd/even pattern of 1024 by 360 pixels. This array will be used to remove the odd/even pattern from images by multiplication (in the same way as the usual flatfield function). It has also been applied to all the flat field raw images, in order to remove from them the odd/even pattern and to produce the new flatfield correction arrays without odd/even pattern. In order to apply the flatfield correction to your A-detector or B-detector data, you can FIRST use the files ODD_EVEN_ARRAY_A or ODD_EVEN_ARRAY_B, respectively, to your data and THEN the flatfield array, named 'ff_*_NOE.RST', which you can shift, or not, as you wish. This will first remove the odd/even pattern from your data and then correct the rest of the flatfield pattern. Both corrections can be applied by using the SUM_FLATFIELD.PRO function, as usual. This will take care of your sub-frame image size, if you have not chosen full size images for your data. A routine to apply only the odd/even correction to a data set of IDL-files is supplied under CONTRIB:[SCHUEHLE.TOOLS]FF_ODDEVEN.PRO, and it can be called under IDL (SSWIDL) by the command >ff_oddeven, 'filespec', 'det', where 'filespec' is defining the file names of your data set and 'det' is either A or B. You can then apply the usual flatfield correction (and geometric distortion correction by calling FF_GC.PRO), now making sure that 'flatfile' is specified as one file of the type 'ff_*_NOE.RST'. The files needed for these corrections are available for all users in the directory CALIBRATION:[FLIGHT.FF], where all the flatfield correction data are located. If you apply these two corrections without shifting the flatfield, the result should be the same as using the original flatfield array. Please make extensive use of this and inform me about any problems. A description of the way to determine and apply the shift of the flatfield will be the subject of a forthcoming mail. Udo Schuehle Lindau, 13. March 2003 ___________________________________________ Udo Schühle Max-Planck-Institut für Aeronomie Max Planck Strasse 2 37191 Katlenburg-Lindau Germany Tel:+49 5556 979 458 Fax:+49 5556 979 6458 e-mail:schuehle@linmpi.mpg.de