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jeffctaylor / astro2013

A centralized place for my code this spring and summer
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Suggestion for get_native_pixelscale #5

Closed PBarmby closed 11 years ago

PBarmby commented 11 years ago

Here is a piece of code I wrote a while back. It actually does 2 things, one of which is getting the pixel scale (you might need the 'get the position angle' part later too). The CD1_2 and CD2_2 keywords are a different way of storing the coordinate system information -- generally an image will have either CDELT1,CDELT2,CROTA1,CROTA2 or CD1_1, CD 1_2, CD2_1, CD2_2 but not both sets.

def get_wcs(im): ''' given an image, returns PA in degrees and pixel size (assumed to be square) in degrees''' fimg = pyfits.open(im,'readonly') if len(fimg) > 1: hdr = fimg['SCI'].header else: hdr = fimg[0].header if hdr.has_key('CROTA2') and hdr.has_key('CDELT2'): # use CROTA1 and CDELT keywords if there cr2 = 0.0 - hdr['CROTA2']
pxsc = math.fabs(hdr['CDELT2']) elif hdr.has_key('CD1_2') and hdr.has_key('CD2_2'): # or use CD matrix: lazy case assuming no skew cd12 = hdr['CD1_2'] cd22 = hdr['CD2_2'] cr2 = math.atan2(cd12,cd22)_180.0/math.pi
pxsc = math.sqrt(cd12_2+cd22*2) else: cr2 = 0.0 pxsc = 0.0 fimg.close() return(cr2,pxsc)

jeffctaylor commented 11 years ago

Since we are asking the user to make sure that CDELT1 and CDELT2 are included as keywords in the input images, this is no longer relevant.