Each OBJC is passed to the Measure Objects module individually, so all passbands can be measured simultaneously.
Actually measuring the object's properties is simple enough to describe, although some of the algorithms are rather complex.
The first thing to check is if the object's had its wings subtracted; if so we assert that it's already been measured (presumably as a bright object) and return. We should not remeasure any bright objects, as they have had power subtracted from their wings
Then we find a good centre for each
band, a step that's only important if the object being measured is the product
of the deblender. Then a canonical centre is found. If the object
is detected in the band denoted mo_fiber_color
in the
frames parameter file (usually r'), the centre in that band is the
canonical centre. If it isn't detected in that band, the band
with the largest peak counts is used. It would be better to use e.g.
a fibre magnitude, but one is not available at this point.
Once the canonical centre's in hand, we process the object in that band, followed by the other bands in no particular order. For each band, we:
The object can be too large for the method we use, in
which case the OBJECT1_TOO_LARGE
bit is set (the condition
is that no cell may contain more than 65535 pixels, giving an outer
radius of 657 pixels); if the extracted
profile reaches the edge of the frame in any sector, OBJECT1_EDGE
is set.
During the extraction, if the sky level were sufficiently badly estimated,
the central intensity of the object could be negative. If this is so,
set the OBJECT1_BADSKY
and OBJECT1_NOPETRO
bits,
and give up on this band.
This is done as a sum over the cells in the radial profile, rather than as an integral over image. This sacrifices a little statistical efficiency, but makes the PSP's task of estimating the aperture corrections easier (more precisely, it greatly eases the PSP's memory requirements).
The procedure is:
OBJECT1_NOPROFILE
and give up
asinh
of the radius against
the asinh
of the cumulative brightness (a cumulative light
profile is, of course, employed so that these radii are well defined).
OBJECT1_PETROFAINT
bit), and keep the
largest of the remainder. If there are none, set
OBJECT1_NOPETRO
; if there are many, set
OBJECT1_MANYPETRO
.
OBJECT1_NOPETRO_SMALL
and set the Petrosian radius to it's
fallback value.
OBJECT1_MANYPETRO50
and OBJECT1_MANYPETRO90
as appropriate.
OBJECT1_INCOMPLETE_PROFILE
bit.
OBJECT1_NOPROFILE
isn't set, calculate the objects
Q
and U
parameters; see
The Estimation of Objects' Ellipticities for details on algorithms. The
integrals over the object are carried out as described in
Aperture Photometry in Band Limited Data.
In each sector of the extracted cell profile, estimate the first
radius at which the desired surface brightness is reached; this is done
using the cumulative spline technique described above.
If the object's centre is fainter than the desired isophote, set the
OBJECT1_ELLIPFAINT
bit and give up.
Next fit a five-Fourier series (i.e. up to the cos(2 theta) and sin(2 theta) terms) to these radii, and convert the resulting coefficients into a centre, a major and minor axis length, and a position angle. This is currently done by finding the centre of area of the curve defined by the Fourier series, and calling it the centre. The second moments of the curve about this point are then used to determine the axes and orientation of a best-fit ellipse. Finally, adjust the axes so that the area of the ellipse equals the area of the sectors within the isophote. It is not clear that this is a satisfactory procedure; TBD
OBJECT1_EDGE
,
OBJECT1_BLENDED
,
OBJECT1_CHILD
,
OBJECT1_NOPETRO
,
OBJECT1_MANYPETRO
,
OBJECT1_INTERP
,
OBJECT1_CR
,
OBJECT1_SATUR
,
OBJECT1_NOTCHECKED
,
and
OBJECT1_BINNED
. If the OBJC
is blended, the
OBJECT1_BLENDED
bit is set in the OBJECT1
that
we've just processed.
As of May, 1995, we have the ability to create decision trees relatively quickly. However, we have not spent time to try to create trees which yield high accuracy; that will be done later, when we have real test data.