Sextractor【1】The size measurement

Usually, we need to run sextractor for several bands with one detection image, which is called dual mode. But when you check the FWHM which is the half light radius for faint galaxies, you may find something different. How do these values come from? Does Sextractor use the same aperture for all bands?

The FWHM of one object in C1226 ( one cluster in CLASH)

The object is not detected in ACS/uvis bands and F775W F814W band, thus the size is only the largest radius of the source region which can be checked in segmentation map.
The object are clearly detected in F105W, so the size measured in F105W could be the most accuracy. The size in other bands are smaller due to the weak detection in these bands.

size measured by myself

Spectrum of this object

images of 16 bands

# photometry for this object

#a1423 nir  833    x    y         ra        dec
f225w           f275w           f336w           f390w        
f435w           f475w           f606w           f625w        
f775w           f814w          f850lp           f105w        
f110w           f125w           f140w           f160w           redshift

c1226   1 1388 2543 2764 186.754096  33.545302
99.0000 99.0000  99.0000 99.0000  29.2562 2.1399  99.0000 99.0000
27.1057 0.1604  27.2714 0.1534  27.2058 0.0910  26.8436 0.1355
27.3479 0.2488  27.2449 0.0892  27.6370 0.4182  26.9311 0.1483
26.5273 0.0706  26.1972 0.0958  26.4933 0.0874  26.9528 0.1281   1.7550

You can see that while the size is as large as the segmentation map, the object will be either not detected or deblend for bad pixels.

size 为何不同
有些波段的流量偏低，甚至无探测，导致sex测量的size不准。没有探测的时候，size将会设置成定值，即segmentation的大小。



Here I add another example:

# photometry for this object

#a1423 nir  833    x    y         ra        dec
f225w           f275w           f336w           f390w        
f435w           f475w           f606w           f625w        
f775w           f814w          f850lp           f105w        
f110w           f125w           f140w           f160w           redshift

c1226   1 2101 2303 2233 186.759314  33.535719  99.0000 99.0000  

30.7607 8.0779  27.8950 0.5042  28.3753 0.4528  27.1087 0.1303  

27.2161 0.1204  27.4034 0.0910  27.6560 0.2317  27.5740 0.2553  

27.9762 0.1456  27.2453 0.2345  27.3148 0.1553  27.2221 0.1081  

26.9002 0.1156  27.0465 0.1132  27.7699 0.2222   1.7550