IRAS Explanatory Supplement
V. Data Reduction
D. Point Source Confirmation
D.3 Band-Merging
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- Overview of Band-Merging
- Band Filling
- Special Considerations Regarding Band-Merging
- Focal Plane Geometry Analysis
D.3.a Overview of Band-Merging
Band-merging was performed in a manner very similar to in-band seconds-confirmation, except that detection buffers were maintained for all bands simultaneously, and detections in different bands were combined. Again a "drop-dead" source was selected as a nucleus to which detections in other bands were attached if they could be associated with it. Up to four attempts were made to obtain some type of flux measure to put in all three other bands for each drop-dead. In the first round a detection from the primary source buffer in each band was sought. These buffers contained sources which were either seconds-confirmed (SC) or were on detectors opposite a failed or noisy redundant detector (NSCF). The drop-dead source was selected as the SC or NSCF source with the earliest time tag. The drop-dead's band determined the order in which the other bands were searched according to Table V.D.4.
Coarse windows in time and in-scan position angle were used
to limit the search for merging candidates. When found, such
candidates were subjected to a fine position test such as that
of in-band seconds-confirmation, an n-
test on the in-scan
position angle (Table V.D.1). In the
cross-scan direction, the nominal
slot extents of the drop-dead and the candidate were required
to overlap.
If more than one candidate was acceptable in a given band the priority was given to SC candidates over NSCF candidates and otherwise the best in-scan match was taken. A bit was set in the confusion status word for the source.
| Drop-Dead Band | Order in which Other Bands Were Searched |
||
|---|---|---|---|
| µm | µm | ||
| 12 | 25 | 60 | 100 |
| 25 | 10 | 60 | 100 |
| 60 | 25 | 100 | 12 |
| 100 | 60 | 25 | 10 |
When a candidate was merged with a drop-dead the position parameters were refined immediately with the same algorithm as that used for in-band seconds-confirmation. This was done before resuming the merging search in the next band so that subsequent candidates would have to be compatible with the position information of all detections merged up to that point.
D.3.b Band Filling
After the attempt to merge sources from the SC/NSCF buffers, if any bands remained empty, the non-seconds-confirmed (NSC) detections were searched for band-filling candidates. These were the detections which failed seconds-confirmation without the alibi of a dead or noisy redundant detector. The tests were identical to those for SC/NSCF sources, except that the threshold used was just under 4.1, which should reject one true case out
of every 20,000.
The NSC detections were used for flux estimates only; their position
information was considered too risky to use for parameter refinement.
If more than one NSC detection was acceptable for filling a given
band the confusion status bit for that band was set, and the best
in-scan position match was kept. After the NSC buffers were searched,
unused NSC detections were passed on to the input file for use
at hours-confirmation.
Any bands which remained empty were filled with low signal-to-noise
detections (Section V.C.6) or with upper limits
based on the noise
histories of the detectors which the source image crossed. If
any detectors were certain to have been crossed then the upper
limit was based on the lowest noise of any such detector otherwise
the highest noise on any detector which might have been crossed
was used.
Three special considerations entered the band-merging problem.
These were deferred in the discussion above in order to minimize
the complexity of the description. The first of these was concerned
with the selection of the drop-dead detection from one of the
SC/NSCF buffers. Rather than selecting the oldest detection in
any of the four bands, a time offset of seven seconds was applied
against detections in the 100 µm band in order to
reduce the amount
of cirrus contamination. This compensated for the fact that the
100 µm band was the first to register observable point source
detections, because of its location at the entrance to the focal
plane. Without the time offset, cirrus detections had been able
to claim nearby point source detections before they could be associated
with their proper partners. Implementing the seven-second delay
forced all multi-band sources to begin band-merging with drop-dead
detections at shorter wavelengths, where cirrus was much less
of a problem.
The second special consideration related to the handing
of SC and NSCF detections of the same source in the same band.
It was not unusual for edge-overlap detectors opposite a noisy
detector to participate in a triple-detection with the noisy detector.
Usually this triple-detection mode was processed without any
problem, but occasionally the three detections would fail the
tests required for acceptance of the triple-detection mode. These
tests attempted to prevent confusion of close sources, as described
above under in-band seconds-confirmation. When these tests failed
it was possible for the source to be carried forward in both the
SC form and the NSCF form. Experience showed that the SC form
was practically always a better representation of the source than
the NSCF form, but half of the time the NSCF had the earlier time
tag and hence had first choice of band-merging candidates. In
order to correct this, a test was added after selection of a drop-dead
to see whether it was NSCF and was followed closely by any
SC source in the same band. In such cases, the SC and NSCF sources
were swapped in the buffer, so that the SC source would be processed
for band-merging first. The time windows used were one second
in the 12, 25, and 60 µm bands, and 1.5 seconds in the 100 µm
band.
The third special consideration involved the use of position
information of NSCF sources in parameter refinement. With three
dead detectors and several more with degraded noise properties,
NSCF sources were too numerous to permit completely ignoring their
position information. On the other hand false alarms on detectors
opposite dead or noisy redundant detectors had to be accepted
as NSCF. The only partially distinguishing characteristic of
false alarms was their tendency toward low detection correlation
coefficients. A compromise was therefore developed which permitted
the use of NSCF position information in parameter refinement provided
that the detection had a signal-to-noise ratio of at least ten
with a correlation coefficient of at least 0.98. The only exception
to this rule was when no SC or qualifying NSCF sources were present,
in which case all NSCF position information was used in parameter
refinement.
The mean in-scan position angle discrepancies for each pair
of redundant detectors in a band were interpreted as errors in
the focal-plane geometry as mapped through the optics onto the
sky. These errors could not not be distinguished from timing errors,
because all data were taken at the survey scan rate, but separation
into components was not necessary for survey analysis purposes.
Whenever detections in different bands were merged and qualified
for position refinement, the in-scan position discrepancies were
computed for all possible band combinations. After sufficient
data were obtained, these mean discrepancies were forced toward
zero by modifying the model of the focal plane geometry.
D.3.c Special Considerations Regarding Band-Merging
D.3.d Focal Plane Geometry Analysis
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