The level histograms of the 22 images is shown in Figure .
As is evident from Table and Figure , the levels in the 22 images are not the same. Two effects are seen:
To get rid of the cosmic-ray-events, the values in the tails of the two combined images (dark1 and dark2) were replaced (using imreplace) by the median value, using the following values: dark1: values outside [INDEF,20.] replaced by 5.457, dark2: values outside [7.,27.] replaced by 17.14.
The two images (especially dark1) had a number of columns with higher or lower levels (the lower ones being fat zero, the higher ones probably being remanence), which were removed by linear interpolation of the unaffected neighboring pixels (using fixpix).
The dark images had now two kinds of structure left:
some smooth variations, and a discontinuity between y = 512 and 513
(in the coordinates of the final 1024
1024 trimmed image).
The two images, dark1 and dark2, where cut in halves according to the
discontinuity, and to each of these 4 pieces a Legendre polynomial of
in x and y with cross terms was fitted. Afterwards, the corresponding
halves were joined.
It was tested that these two new dark1 and dark2 images did not have
any residuals or large scale gradients with respect to the ``unsmoothed'' ones.
The two final dark images are shown in Figure .
The two images do not exactly scale, the relative standard deviation on the quotient of the two is 2%. Given the low absolute counts, this is a small difference.
The mean dark levels the final two dark images are: dark1: 5.6 ADU/hour, dark2: 17.3 ADU/hour. The low frequency pattern, characterized by the standard deviation of the two images, is 0.12 (dark1) and 0.24 (dark2). Since they are smooth legendre polynomials, there is no pixel-to-pixel noise in them.
Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997
Bo Milvang-Jensen (email@example.com)