Sextractor for dummies - METU Astrophysics

38 CHAPTER 7: SE INPUT: THE CONFIGURATION FILE
How ISOCOR works (from the v2.3 manual)
Corrected isophotal magnitudes (MAG ISOCOR) is a quick-and-dirty way for
retrieving the fraction of flux in the wings of the object missed in the isophotal
magnitudes. The latest version of the SE manual (v2.3) does not rate this as
a very good correction, these values have been kept as an output option for
compatibility with SE v2.x and SE v1 12 . The assumtion is that the profiles
of the objects are Gaussian. The fraction of the total flux enclosed within a
particular isophote reads compared to the total flux 13 :
1 − 1 η
ln(1 − η) =
At
I iso
(7.3)
where A is the area and t the threshold related to this isophote. The
relation can not be inverted analytically, but a good approximation to
(error < 10 2 for η > 0.4) can be done with the second-order polynomial of
t:
η ≈ 1 − 0.1961 A.t ( ) A.t 2
− 0.7512
(7.4)
A iso I iso
A total magnitude m tot estimate is then m tot = m iso + 2.5log(η) Clearly this
first correction works best with stars (and maybe starclusters) as these are
the most Gaussian-like, ok with disk galaxies and not-so-ok with ellipticals as
these have broader wings.
Fixed-aperture magnitudes (MAG APER) estimate the flux above the background within a circular
aperture. The diameter of the aperture in pixels
(PHOTOM APERTURES) is supplied by the user (in fact it does not need to be an integer since
each normal pixel is subdivided in 5 × 5 sub-pixels before measuring the flux within the aperture).
If MAG APER is provided as a vector MAG APER[n], at least n apertures must be specified with
PHOTOM APERTURES.
7.4.3 AUTO
SE uses a flexible elliptical aperture around every detected object and measures all the flux inside
that, described in Kron (1980). There are two parameters regulating the elliptical apertures:
PHOT AUTOPARAMS and PHOT AUTOAPERS.
The characteristic radius for the ellipse is:
r 1 = ΣrI(r)
ΣI(r)
Also known as the Kron radius (see section 8.5.1). From the objects second order moments, the
(7.5)