refmag = refmag[np.flatnonzero(mag)]
refmagerr = refmagerr[np.flatnonzero(mag)]
mag = mag[np.flatnonzero(mag)]
if(len(mag) == 0):
print "No good sources to plot"
sys.exit(1)
# Now apply the exposure time and nom_at_ext corrections to mag
et = float(ad.exposure_time())
if(ad.is_type('GMOS_NODANDSHUFFLE')):
print "Imaging Nod-And-Shuffle. Photometry may be dubious"
et /= 2.0
etmag = 2.5*math.log10(et)
nom_at_ext = float(ad.nominal_atmospheric_extinction())
mag += etmag
mag += nom_at_ext
# Can now calculate the zp array
zp = refmag - mag
zperr = np.sqrt(refmagerr*refmagerr + magerr*magerr)
# Trim values out of zp where the zeropoint error is > 0.1
zp_trim = np.where((zperr<0.1), zp, None)
zperr_trim = zperr[np.flatnonzero(zp_trim)]
refmag_trim = refmag[np.flatnonzero(zp_trim)]
refmagerr_trim = refmagerr[np.flatnonzero(zp_trim)]
zp_trim = zp[np.flatnonzero(zp_trim)]
