How to use the tractor.sdss.get_tractor_sources function in tractor

itune1 = 5
    itune2 = 5
    ntune = 0
    IRLS_scale = 25.

    bands=['r','g','u','i','z']
    bandname = 'r'
    flipBands = ['r']

    rerun = 0

    TI = []
    sources = []
    for run,field,camcol,roi in zip(runs,fields,camcols,rois):
        TI.extend([st.get_tractor_image(run, camcol, field, bandname,roi=roi,useMags=True) for bandname in bands])
        sources.append(st.get_tractor_sources(run, camcol, field,bandname,roi=roi,bands=bands))

    timg,info = TI[0]
    photocal = timg.getPhotoCal()

    wcs = timg.getWcs()
    lvl = logging.DEBUG
    logging.basicConfig(level=lvl,format='%(message)s',stream=sys.stdout)

    tims = [timg for timg,tinf in TI]
    tractor = st.SDSSTractor(tims)
    for source in sources:
        tractor.addSources(source)

    zr = np.array([-5.,+5.]) * info['skysig']

    print bands

TI = []
	for i,(run,camcol,field) in enumerate(RCFS):
		for bandname in bands:
			im,inf = st.get_tractor_image(run, camcol, field, bandname,
										  useMags=True,
										  roiradecsize=(ra,dec,S))
			#im.dxdy = (fxc - xc, fyc - yc)
			TI.append((im,inf))
	tims = [im for im,inf in TI]
	skyvals = [(info['skysig'], info['sky']) for timg,info in TI]
	zrs = [np.array([-1.,+6.]) * std + sky for std,sky in skyvals]

	# Grab sources from the LAST RCF
	roi = inf['roi']
	sources = st.get_tractor_sources(run, camcol, field, roi=roi,
									 bands=bands)

	# DevGalaxy
	s0 = sources[0]
	s0.re = 0.01
	s0.brightness.r = 22.
	
	tractor = Tractor(tims)
	tractor.addSources(sources)

	lnp0 = tractor.getLogProb()

	print 'Catalog:'
	print tractor.catalog
	print tractor.catalog.getParams()

ntune = 0
    IRLS_scale = 25.

    bands=['r','g','u','i','z']
    bandname = 'r'
    flipBands = ['r']

    rerun = 0

    TI = []
    sources = []
    for rcf in rcfs:
        print rcf
        try:
            TI.extend([st.get_tractor_image(rcf[0], rcf[1], rcf[2], bandname,useMags=True) for bandname in bands])
            sources.append(st.get_tractor_sources(rcf[0], rcf[1], rcf[2],bandname,bands=bands))
        except:
            print "Sources not in data! %s" % (rcf)

    timg,info = TI[0]
    photocal = timg.getPhotoCal()

    wcs = timg.getWcs()
    lvl = logging.DEBUG
    logging.basicConfig(level=lvl,format='%(message)s',stream=sys.stdout)

    tims = [timg for timg,tinf in TI]
    tractor = st.SDSSTractor(tims)
    for source in sources:
        tractor.addSources(source)

    zr = np.array([-5.,+5.]) * info['skysig']

if not os.path.exists(fn):
			sdss.retrieve('tsField', run, camcol, field)
		tsf = sdss.readTsField(run, camcol, field, rerun)
		astrans = tsf.getAsTrans(canonband)
		x,y = astrans.radec_to_pixel(ra, dec)
		print 'x,y', x,y
		roi = [np.clip(x-pixr, 0, W), np.clip(x+pixr, 0, W),
			   np.clip(y-pixr, 0, H), np.clip(y+pixr, 0, H)]
		if roi[0] == roi[1] or roi[2] == roi[3]:
			print 'Skipping', run, camcol, field
			continue

		TI.extend([st.get_tractor_image(run, camcol, field, band, useMags=True,
										roiradecsize=(ra,dec,pixr), sdssobj=sdss)
				   for band in bands])
		sources.extend(st.get_tractor_sources(run, camcol, field, bandname=canonband,
											  bands=bands, roi=roi))
		# (could also get 'roi' from the st.get_tractor_image info dict)


	tractor = Tractor([tim for tim,tinf in TI], sources, mp)

	for im in tractor.getImages():
		im.freezeParams('wcs', 'photocal', 'psf', 'sky')

	#tractor.freezeParam('catalog')

	for i in range(5):
		if True or (i % 5 == 0):
			print 'Thawing sky...'
			tractor.images.thawParamsRecursive('sky')

roi = [x0,x1,y0,y1]
    
    ra = 133.175
    dec = 33.4167
    itune = 10
    ntune = 5

    bands=['r','g','i','u','z']
    bandname = 'r'
    flipBands = ['r']

    rerun = 0

    TI = []
    TI.extend([st.get_tractor_image(run, camcol, field, bandname,roi=roi,useMags=True) for bandname in bands])
    sources = st.get_tractor_sources(run, camcol, field,bandname,roi=roi, bands=bands)

    timg,info = TI[0]
    photocal = timg.getPhotoCal()

    wcs = timg.getWcs()
    lvl = logging.DEBUG
    logging.basicConfig(level=lvl,format='%(message)s',stream=sys.stdout)

    tims = [timg for timg,tinf in TI]
    tractor = st.SDSSTractor(tims)
    tractor.addSources(sources)

    zr = np.array([-5.,+5.]) * info['skysig']

    print bands

wcs = im.getWcs()
				# im,info = TI[bands.index('r')]
				#wcs = im.getWcs()
				# this is a shifted WCS; S,S is the center.
				# rd = wcs.pixelToPosition(None, (S,S))
				# ra,dec = rd.ra,rd.dec
				# print 'RA,Dec', ra,dec
				cd = wcs.cdAtPixel(xc,yc)
				pixscale = np.sqrt(np.abs(np.linalg.det(cd)))
				print('pixscale', pixscale * 3600.)
				extent = pixscale * S

	#for timg,info in TI:
	#	print timg.hashkey()

	sources = st.get_tractor_sources(run, camcol, field, roi=roi,
									 bands=bands)

	print('Sources:')
	for s in sources:
		print(s)
	print()

	print('Tractor images:', TI)

	cftimg,cfsky,cfstd = get_cfht_img(ra,dec, extent)

	# Create fake tractor.Image
	#  psf = NCircularGaussianPSF([0.1], [1.])
	#  sky = timg.sky
	#  print 'SDSS Sky:', timg.sky
	#  photocal = timg.photocal

x0,y0 = roi[0],roi[2]

	# rd1 is near the position of the SDSS-catalog object
	# rd2 is four pixels above.
	rd1 = RaDecPos(225.67954, 11.265948)
	rd2 = RaDecPos(225.67991, 11.265852)

	lvl = logging.INFO
	logging.basicConfig(level=lvl, format='%(message)s', stream=sys.stdout)
	bandname = band

	sdssprefix = '%06i-%s%i-%04i' % (run, bandname, camcol, field)

	timg,info = st.get_tractor_image(run, camcol, field, bandname,
									 roi=roi)
	sources = st.get_tractor_sources(run, camcol, field, bandname,
									 roi=roi)

	wcs = timg.getWcs()
	for source in sources:
		x,y = wcs.positionToPixel(source, source.getPosition())
		print('  (%.2f, %.2f):' % (x+x0,y+y0), source)

	tractor = st.SDSSTractor([timg])
	tractor.addSources(sources)

	lnlorig = tractor.getLogLikelihood()
	zr = np.array([-5.,+20.]) * info['skysig']
	save(sdssprefix+'-orig', tractor, zr)

	# find closest source
	mini = -1