Sextractor for dummies - METU Astrophysics

64 CHAPTER 8: OUTPUT PARAMETERS
NOTE: If you have FILTERED the image for detection then your ellipticity and
elongation values will be more round than the actual object.
Only the ELONGATION and the ELLIPTICITY parameters do not come with a error. Note
that there is an extensive discussion of these in section 9.1.8 of the manual. 10
8.4.3 Area Parameters
This can be done by the parameters describing the isophotes as fitted by SE or the conclusion of
the neural network classification. To get an idea of the size of an object, reasonably independent of
the brightness, the FWHM or the FLUX RADIUS
SE divides an object up into 7 isophotes above the ANALYSIS THRESH. The areas above the
isophotes is fed to the neural network. These can be put into the catalog with the ISOn parameters.
Name description unit
ISOAREA WORLD Isophotal area above Analysis threshold deg 2
ISOAREAF WORLD Isophotal area (filtered) above Detection deg 2
threshold
ISO0 Isophotal area at level 0 pixel 2
ISO1 Isophotal area at level 1 pixel 2
ISO2 Isophotal area at level 2 pixel 2
ISO3 Isophotal area at level 3 pixel 2
ISO4 Isophotal area at level 4 pixel 2
ISO5 Isophotal area at level 5 pixel 2
ISO6 Isophotal area at level 6 pixel 2
ISO7 Isophotal area at level 7 pixel 2
8.4.4 Full-Width Half Max
Assuming a Gaussian profile for the object, a Full-Width at Half Maximum can be computed. No
object is a Gaussian -especially in the wings- but this does give you a reasonable idea of the PSF
width in the case of a star for instance. It’s a useful thing to have.
Name description unit
FWHM WORLD FWHM assuming a Gaussian core pixel
FWHM WORLD FWHM assuming a Gaussian core deg
VIGNET Pixel data around detection count
VIGNET SHIFT Pixel data around detection corrected for count
shift
THRESHOLDMAX Maximum threshold possible for detection count
10 I’d be saying the exact same thing and typing in that many functions is a pain, even in LaTeX.