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hsubtract stsdas.playpen



hsubtract -- Baade-Lucy background subtraction algorithm.


hsubtract input background output


This task implements the Baade-Lucy background subtraction algorithm described in "Proceedings of the 1st ESO/ST-ECF Data Analysis Workshop", Garching, April 17-19 1989, edited by P.Grosbol, F.Murtagh and R. Warmels, pp 169-172. The following is their discussion of the algorithm..

"The method depends on the availability of a reference background area, either in the same or in a separate image, from which a statistically significant histogram of the background, B, can be constructed. If a histogram, H, with identical bins is also computed for the image, I, which is to be corrected, the binwise ratio Fk = Bk / Hk for each bin k is proportional to the probability Pk that the flux of the image pixels in bin k is due to background only. The proportionality constant,, alpha, simply is the inverse ratio of the number of pixels Nb, in the reference background area, to the number of pixels Nbi in I which are due to the background only.

Because of the wide variety of source contents of astronomical images, generally no algorithm can be given to calculate Nbi. However, a recipe that will provide a reasonable guess for a fair range of applications is to determine a scaling factor after multiplication with which the background histogram best match the image histogram in the range 1 < k < kmax. The choice of the upper cut-off bin kmax (above which sources start to make significant contributions) may have to be made by trial and error and in any case requires competent judgment of the nature of the sources. Once a scaling factor has been found, it may be used as the constant of proportionality, alpha, introduced above, and probabilities can be computed as Pk = alpha * Bk / Hk for all k.

If Pk >= 1 (an inequality may result from statistical fluctuations), none of the pixels in bin k contain any information other than background. In order to correct these pixels for background, they simply can be set to zero..

If Pk < 1, only a fraction of the pixels in bin k contain a pure background signal. Accordingly, only alpha * Bk out of a total of Hk pixels in the bin may be set to zero. It is of course just the nature of the problem that it is impossible to know which pixels these are. One may, however, argue that for all pixels in bin k the probability of their flux being due to background only is larger when the the flux in the ambient pixels is lower. Conversely, a relatively large flux in nearby pixels gives rise to the assumption that the pixel considered contains also some valid non-background information. The approach adopted here is, therefore, to rank all pixels in bin k according to the flux in their respective eight nearest neighboring pixels and then to zero the alpha * Bk lowest ranking pixels. For the remaining pixels the background contribution must be smaller than Sk, the flux level of bin k. We choose to subtract the most probable value, namely the expectation value for all bins 1,...,k-1 with flux levels below the one from bin k."

In the hsubtract implementation, the scaling factor alpha is computed by equating the cumulative histograms of image and background at bin kmax.

The method will achieve optimum performance only on images that have already been corrected for any large scale (low-frequency) structures in the background distribution.

The task can process an image template or list as input. In this case, the output is either a matching list of images or a directory.


input [file name template]
Input 2-dimensional image section, template, or list of images.
background [file name template]
Input 2-dimensional background image section, template, or list of images. The number of images passed to this parameter must be the same as the number passed to input.
output [file name template]
Output file name, list or directory. Output images are always of type real, regardless of the input image type.
min = 0.[real]
Pixels below this value are not included in the histogram computation, but are set to zero.
max = 10.[real]
Pixels above this value are not included in the histogram computation, and are not processed at all.
binsize = 1.[real]
Histogram bin size.
kmax = 10.[real]
Upper threshold level. Object and background histograms will be matched in bins 1 < k < kmax. Pixels below kmax will be set to zero, pixels above it will have background subtracted following the algorithm rules.
(verbose = no) [boolean]
Print file names and processing information?


1. Subtract background from images a0.hhh thru a9.hhh. Background is in between columns 100 and 200, and lines 50 and 150 of the same images. Resulting images will be stored in directory out/. Compute histograms between -100 and 300 pixel units. Bin size is 10. The kmax parameter will be set at 45.:

pl> hsubtract a?.hhh a?.hhh[100:200,50:150] out/ min=-100 max=200 \
>>> binsize=10  kmax=45.



This task was written by I.Busko


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