Bug about dual image mode?

[WeidaHu]

Dear All,

I met a problem with sextractor dual image mode.
I tried to use sextractor with dual image mode and single image mode to do photometry to same image:

sex -c default.sex a.fits -WEIGHT_IMAGE a.weight.fits -PARAMETERS_NAME dual.param

sex -c default.sex a.fits,a.fits -WEIGHT_IMAGE a.weight.fits,a.weight.fits -PARAMETERS_NAME dual.param

Theoretically speaking, I should get same results for same sources.
However, they are different.
The SNR of the source in dual image mode is much lower than that in single image mode.
Is any one know what happened to my image? Bug?
Any help would be appreciated!

Here is the default.sex:
# Default configuration file for SExtractor 2.19.5
# EB 2016-04-12
#

#-------------------------------- Catalog ------------------------------------

CATALOG_NAME test.cat # name of the output catalog
CATALOG_TYPE ASCII_HEAD # NONE,ASCII,ASCII_HEAD, ASCII_SKYCAT,
# ASCII_VOTABLE, FITS_1.0 or FITS_LDAC
PARAMETERS_NAME default.param # name of the file containing catalog contents

#------------------------------- Extraction ----------------------------------

DETECT_TYPE CCD # CCD (linear) or PHOTO (with gamma correction)
DETECT_MINAREA 4 # min. # of pixels above threshold
DETECT_MAXAREA 0 # max. # of pixels above threshold (0=unlimited)
THRESH_TYPE RELATIVE # threshold type: RELATIVE (in sigmas)
# or ABSOLUTE (in ADUs)
DETECT_THRESH 1.2 # <sigmas> or <threshold>,<ZP> in mag.arcsec-2
ANALYSIS_THRESH 1.2 # <sigmas> or <threshold>,<ZP> in mag.arcsec-2

FILTER Y # apply filter for detection (Y or N)?
FILTER_NAME gauss_4.0_7x7.conv # name of the file containing the filter
FILTER_THRESH # Threshold[s] for retina filtering

DEBLEND_NTHRESH 32 # Number of deblending sub-thresholds
DEBLEND_MINCONT 0.005 # Minimum contrast parameter for deblending

CLEAN Y # Clean spurious detections? (Y or N)?
CLEAN_PARAM 1.0 # Cleaning efficiency

MASK_TYPE CORRECT # type of detection MASKing: can be one of
# NONE, BLANK or CORRECT

#-------------------------------- WEIGHTing ----------------------------------

WEIGHT_TYPE MAP_WEIGHT # type of WEIGHTing: NONE, BACKGROUND,
# MAP_RMS, MAP_VAR or MAP_WEIGHT
RESCALE_WEIGHTS Y # Rescale input weights/variances (Y/N)?
WEIGHT_IMAGE weight.fits # weight-map filename
WEIGHT_GAIN N # modulate gain (E/ADU) with weights? (Y/N)
WEIGHT_THRESH # weight threshold[s] for bad pixels

#-------------------------------- FLAGging -----------------------------------

FLAG_IMAGE flag.fits # filename for an input FLAG-image
FLAG_TYPE OR # flag pixel combination: OR, AND, MIN, MAX
# or MOST

#------------------------------ Photometry -----------------------------------

PHOT_APERTURES 7.5,5 # MAG_APER aperture diameter(s) in pixels
PHOT_AUTOPARAMS 2.5, 3.5 # MAG_AUTO parameters: <Kron_fact>,<min_radius>
PHOT_PETROPARAMS 2.0, 3.5 # MAG_PETRO parameters: <Petrosian_fact>,
# <min_radius>
PHOT_AUTOAPERS 0.0,0.0 # <estimation>,<measurement> minimum apertures
# for MAG_AUTO and MAG_PETRO
PHOT_FLUXFRAC 0.5 # flux fraction[s] used for FLUX_RADIUS

SATUR_LEVEL 50000.0 # level (in ADUs) at which arises saturation
SATUR_KEY SATURATE # keyword for saturation level (in ADUs)

MAG_ZEROPOINT 0.0 # magnitude zero-point
GAIN_KEY GAIN # keyword for detector gain in e-/ADU
PIXEL_SCALE 1.0 # size of pixel in arcsec (0=use FITS WCS info)

#------------------------- Star/Galaxy Separation ----------------------------

SEEING_FWHM 1.2 # stellar FWHM in arcsec
STARNNW_NAME default.nnw # Neural-Network_Weight table filename

#------------------------------ Background -----------------------------------

BACK_TYPE AUTO # AUTO or MANUAL
BACK_VALUE 0.0 # Default background value in MANUAL mode
BACK_SIZE 128 # Background mesh: <size> or <width>,<height>
BACK_FILTERSIZE 3 # Background filter: <size> or <width>,<height>

BACKPHOTO_TYPE LOCAL # can be GLOBAL or LOCAL
BACKPHOTO_THICK 24 # thickness of the background LOCAL annulus
BACK_FILTTHRESH 0.0 # Threshold above which the background-
# map filter operates

#------------------------------ Check Image ----------------------------------

CHECKIMAGE_TYPE NONE # can be NONE, BACKGROUND, BACKGROUND_RMS,
# MINIBACKGROUND, MINIBACK_RMS, -BACKGROUND,
# FILTERED, OBJECTS, -OBJECTS, SEGMENTATION,
# or APERTURES
CHECKIMAGE_NAME check.fits # Filename for the check-image

#--------------------- Memory (change with caution!) -------------------------

MEMORY_OBJSTACK 3000 # number of objects in stack
MEMORY_PIXSTACK 300000 # number of pixels in stack
MEMORY_BUFSIZE 1024 # number of lines in buffer

#------------------------------- ASSOCiation ---------------------------------

ASSOC_NAME sky.list # name of the ASCII file to ASSOCiate
ASSOC_DATA 2,3,4 # columns of the data to replicate (0=all)
ASSOC_PARAMS 2,3,4 # columns of xpos,ypos[,mag]
ASSOCCOORD_TYPE PIXEL # ASSOC coordinates: PIXEL or WORLD
ASSOC_RADIUS 2.0 # cross-matching radius (pixels)
ASSOC_TYPE NEAREST # ASSOCiation method: FIRST, NEAREST, MEAN,
# MAG_MEAN, SUM, MAG_SUM, MIN or MAX
ASSOCSELEC_TYPE MATCHED # ASSOC selection type: ALL, MATCHED or -MATCHED

#----------------------------- Miscellaneous ---------------------------------

VERBOSE_TYPE NORMAL # can be QUIET, NORMAL or FULL
HEADER_SUFFIX .head # Filename extension for additional headers
WRITE_XML N # Write XML file (Y/N)?
XML_NAME sex.xml # Filename for XML output
XSL_URL file:///usr/local/share/sextractor/sextractor.xsl
# Filename for XSL style-sheet
NTHREADS 1 # 1 single thread

FITS_UNSIGNED N # Treat FITS integer values as unsigned (Y/N)?
INTERP_MAXXLAG 16 # Max. lag along X for 0-weight interpolation
INTERP_MAXYLAG 16 # Max. lag along Y for 0-weight interpolation
INTERP_TYPE ALL # Interpolation type: NONE, VAR_ONLY or ALL

#--------------------------- Experimental Stuff -----------------------------

PSF_NAME default.psf # File containing the PSF model
PSF_NMAX 1 # Max.number of PSFs fitted simultaneously
PATTERN_TYPE RINGS-HARMONIC # can RINGS-QUADPOLE, RINGS-OCTOPOLE,
# RINGS-HARMONICS or GAUSS-LAGUERRE
SOM_NAME default.som # File containing Self-Organizing Map weights