SUSI2 Fringing in I#814
Images obtained with the I#814 filter show a strong fringing pattern caused by night sky line interference in the thin CCDs. Since flat fields are obtained with white light, they cannot correct for these fringes. Moreover, the fringes are an additive pattern and should therefore be subtracted, not divided. To remove the fringes, the fringe pattern must be extracted from the scientific images themselves.
Fringes appear quite well in Fourier space. Therefore, checking the power spectrum of the raw and cleaned images is a good way to monitor the cleaning and to adjust the normalization factor used below. Note however, this does not work very well for crowded fields, as the objects contaminate the power spectrum.
The following procedure is will take care of fringing in a simple manner and give good results:
|Dirty image in I||Clean image in I|
|The fringing pattern|
You can retrieve the fits file corresponding to a "classical" fringing pattern by clicking on the above image. It was built with 34 I#814 images. The exposure time for all the images was 600s. If you don't want to build your own fringing pattern (we recommend that you do, but if you do not have sufficient data) you can use this one. However, note that you should:
Here, you find a new fringing pattern. It was obtained combining 5 images (I#814) with exposure time of 1200 sec each, taken on November 10, 2004. It was corrected for the bias but was not flatfielded.The average has been subtracted, so the pattern is varying around zero. We compared the image to the old one, and although the intenisty of the fringes change, we could not find any changes in the shape of the pattern.
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Last modified: Thu Nov 18 2004 (Silvia Ragaini)