what is the PSF model

[davidwhogg]

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?

[dstndstn]

They are concentric, circular Gaussians, so the parameters are just the relative heights and the two sigmas; three parameters total.

[davidwhogg]

can you point to the code that translates from the psField meta-data?

[dstndstn]

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))

[davidwhogg]

reassigning this to Hogg to write it up.