accuracy issues with numpy arrays?

eli bressert at gmail.com
Tue Apr 29 23:03:47 CEST 2008


Hi,

I'm writing a quick script to import a fits (astronomy) image that has
very low values for each pixel. Mostly on the order of 10^-9. I have
written a python script that attempts to take low values and put them
in integer format. I basically do this by taking the mean of the 1000
lowest pixel values, excluding zeros, and dividing the rest of the
image by that mean. Unfortunately, when I try this in practice, *all*
of the values in the image are being treated as zeros. But, if I doFor
example, I take the pixel that I know has the highest value and do

x = scipy.ndimage.maximum(image)
print x
1.7400700016878545e-05

The script is below. Thanks for the help.

Eli



import pyfits as p
import scipy as s
import scipy.ndimage as nd
import numpy as n


def flux2int(name):
	d  = p.getdata(name)
	x,y	= n.shape(d)
	l = x*y
	arr1	= n.array(d.reshape(x*y,1))
	temp = n.unique(arr1[0]) # This is where the bug starts. All values
are treated as zeros. Hence only one value remains, zero.
	arr1	= arr1.sort()
	arr1	= n.array(arr1)
	arr1	= n.array(arr1[s.where(arr1 >= temp)])

	val = n.mean(arr1[0:1000])

	d		= d*(1.0/val)
	d		= d.round()
	p.writeto(name[0,]+'fixed.fits',d,h)



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