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rachanab
 02/10/2017 01:36PM (Read 879 times)  
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Hi,

I am trying to convolve my image with a psf and I am struggling with the following two things:

1) It takes a really long time to time to get the output file (~20mins).
2) My input fits file has 0 values outside the field of view, but once this image is convolved with the psf using fconvolve, I can see that it has introduced noise outside the FOV also. The problem I am facing because of this is that when I run Sextractor on it, its detecting this noise also and giving me thousands of detections when there are actually only 200 sources in the image. Does fconvolve introduce noise in your convolved image like this?

Thanks,
Rach

 
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fitz
 02/10/2017 04:04PM  
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Registered: 09/30/2005
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1) It takes a really long time to time to get the output file (~20mins).


Without knowing more about the image or hardware being used I can't really comment on the time. FCONVOLVE does use the FFT of both images and requires that the psf image be the same size as the input so perhaps another task such as GAUSS or CONVOLVE would be more efficient and accomplish the same thing.

2) My input fits file has 0 values outside the field of view, but once this image is convolved with the psf using fconvolve, I can see that it has introduced noise outside the FOV also. .....


The GAUSS/CONVOLVE tasks do have parameters for boundary extension but this is different than 0 values outside a FoV. You could try setting the 0 values to something like the image median to minimize the convolution artifacts, but a better approach might be to first create a mask of the input image (either the FoV or the outside region), do the convolution and then apply the mask to reset the values back to 0 before running SExtractor. You can use the command "cl\$this->_split2($m[0]) refer mask" to find suitable masking tasks or see the help page for IMEXPR for a way to create or apply masks.

 
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