Open davidwhogg opened 11 years ago
They are concentric, circular Gaussians, so the parameters are just the relative heights and the two sigmas; three parameters total.
can you point to the code that translates from the psField meta-data?
In tractor/sdss.py: get_tractor_image():
if psf == 'dg':
dgpsf = psfield.getDoubleGaussian(bandnum, normalize=True)
print 'Creating double-Gaussian PSF approximation'
(a,s1, b,s2) = dgpsf
mypsf = NCircularGaussianPSF([s1, s2], [a, b])
where
psfield = sdss.readPsField(run, camcol, field)
which is in astrometry/sdss/dr7.py and common.py :
def getDoubleGaussian(self, bandnum, normalize=False):
# http://www.sdss.org/dr7/dm/flatFiles/psField.html
# good = PSP_FIELD_OK
status = self.psp_status[bandnum]
if status != 0:
print 'Warning: PsField status[band=%s] =' % (bandnum), status
# b is the "ratio of G2 to G1 at the origin", ie, not the
# straight Gaussian amplitudes
a = 1.0
s1 = self.dgpsf_s1[bandnum]
s2 = self.dgpsf_s2[bandnum]
b = self.dgpsf_b[bandnum]
# value at center is 1./(2.*pi*sigma**2)
if normalize:
b *= (s2/s1)**2
absum = (a + b)
a /= absum
b /= absum
return (float(a), float(s1), float(b), float(s2))
reassigning this to Hogg to write it up.
DSTN: We have this request in the paper:
"For the PSF model, we use the SDSS double-Gaussian model stored in the ``psField'' file. DSTN: What do we get as parameters for that double-Gaussian model? I think it is less than 1 + 4 + 6 parameters that a K=2 D=2 MoG would have, right?"
Can you answer this either in this issue or in the paper?