Tommy Grav wrote: > I am trying to get the flux of a star in an image. I have been using numpy > and pyfits and have the code.
You will probably want to ask on numpy-discussion, instead. http://www.scipy.org/Mailing_Lists > def distance(im,xc,yc): > (rows,cols) = im.shape > dist = zeros([rows,cols]).astype(Float64) > for row in range(rows): > for col in range(cols): > dist[row,col] = sqrt(((row + 0.5) - yc)**2 + ((col + 0.5) - > xc)**2) > return dist You will probably want to use numpy.fromfunction() instead. > def apphot(im,x,y,r): > dist = distance(im,x,y) > appmask = where(dist <= r,1,0) You don't have to use where() here. (dist <= r) returns an array of bools, which are already 1 and 0 (for most purposes). > fluxim = where(appmask,im,0) > appflux = sum(sum(fluxim)) > skymask = where(dist > r, 1,0) > skyim = where(skymask,im,0) > sky = mean(skyim) > print skyim > print sky > > return 1 > > Output: >> array (20,20) , type = f, has 400 elements >> [ 45.89742126, 45.92555847, 45.8874054 , 45.88538208, 45.88244934, > 45.9353241 , > 36.75245361, 29.85816345, 27.53547668, 22.93712311, 22.93178101, > 22.93699799, 22.91038208, 27.4988739 , 29.84021606, 36.71789551, > 45.86646729, 45.86741638, 45.85328979, 45.823349 ,] > > where im is a ndarray, x and y are the position of the star and r is > the radius of the aperture. I calculate the flux inside the aperture, > but when I want to calculate the mean of the pixels outside the > aperture I run into problems as the pixels values inside the aperture > is 0 and is still considered in the mean calculation. Is there a way to > do this without using masked arrays? Sure! But ... > How would I use a masked array > to do it? ... masked arrays already do the appropriate tasks for you. In [1]: from numpy import ma In [2]: ma.average? Type: function Base Class: <type 'function'> String Form: <function average at 0x697ef0> Namespace: Interactive File: /Library/Frameworks/Python.framework/Versions/2.4/lib/python2.4/site-packages/numpy-0.9.7.2476-py2.4-macosx-10.4-ppc.egg/numpy/core/ma.py Definition: ma.average(a, axis=0, weights=None, returned=0) Docstring: average(a, axis=0, weights=None) Computes average along indicated axis. If axis is None, average over the entire array Inputs can be integer or floating types; result is of type float. If weights are given, result is sum(a*weights)/(sum(weights)*1.0) weights must have a's shape or be the 1-d with length the size of a in the given axis. If returned, return a tuple: the result and the sum of the weights or count of values. Results will have the same shape. masked values in the weights will be set to 0.0 -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco -- http://mail.python.org/mailman/listinfo/python-list
