ISSA Explanatory Supplement
I. Introduction
E. Processing Caveats
Chapter Contents
Table of Contents | Index | Previous Section | Next Section
- Mosaicking
- Saturated Data
- Destriper Anomalies
- Low Spatial Frequency Artifacts
E.1 Mosaicking
All images covering |b|>
50° can be mosaicked
without additional
offset adjustments to an accuracy of about 0.1 MJy sr-1.
This capability is due to the use of a global destriping
algorithm (§ III.C.3.a), which brought all confirming coverages of the sky
to a common background level.
Fields covering the lower latitude sky, |b|<
50°,
were processed differently from the fields in the |b|>
50° sky
(§ III.C.3.b). Fields in the lower latitude sky can be mosaicked with
the same accuracy as those in the |b|>
50° sky except
near the Galactic plane and where the
|b|>
50° sky and |b|<
50° sky join.
At these two locations, ISSA field boundary discrepancies
of 1-2 MJy sr-1
at 60 µm and
3-5 MJy sr-1 at 100 µm are measurable.
An error was found in the algorithm for handling saturated
intensity values. The error eliminated the
wrong detector when saturation occurred and
affects the SkyFlux images as well
as the entire set of ISSA images.
As a result, saturated intensity values were included in the images
while some nonsaturated intensity values were erroneously eliminated.
This effect occurs mainly in the Galactic plane where the 60 and
100 µm detectors saturate, but it is not considered a
significant problem since it affects less than 0.1% of
detector data (§III.D.4.b).
An error was found in the software that derived the parameters for
one of the destriper algorithms implemented for ISSA,
resulting in poor corrections for some scans in the |b|>
50° sky.
This error affects less than 1% of the data. Although destriper problems
were removed in the quality checking process
(§III.D),
some destriper anomalies remain in the |b|>
50°
images. A typical destriper anomaly is visible
at
4h31m46.5s:-63d40m15s (ISSA field 13). The magnitude of the
effect at this location is about 0.5 MJy sr-1 at 12 and 25 µm.
Typically the anomaly is very short, less than 0.5°.
The software was fixed for
processing the |b|<
50° sky.
Residual zodiacal emission effects remain in the ISSA images due to
imperfections in the zodiacal model. This is seen as either sharp
discontinuities or gradients in the ISSA images. Discontinuities
occur where adjacent regions of the sky were observed through a different
part of the zodiacal dust cloud. An example of a discontinuity
occurs at 12 and
25 µm around 60° and 240° ecliptic longitude.
This is referred to as the mission overlap discontinuity.
The 60° point marks the beginning of the descending leg
and the 240° point marks the beginning of the ascending leg of
the HCON-1 and HCON-2 survey. Six months later the descending leg
had progressed to the 240° point and the ascending leg to the
60° point. Thus the same part of the sky was viewed six
months later through a different part of the zodiacal dust cloud.
The peak magnitude of the change in the intensity along this longitude
occurs at -15° ecliptic latitude, where the discrepancy is
enhanced by a geometric effect caused by looking through a zodiacal dust
band at a different time of year. The change in intensity is
roughly 2.0
MJy sr-1
at 12 µm, which is about 7% of the local intensity prior to
zodiacal emission removal. For |b|>
50°,
the worst discrepancy at 12 µm is roughly 5% of local intensity prior
to zodiacal emission removal and about 2% at 25 µm.
In addition to discontinuities, other large angular scale artifacts not
attributable to the Galaxy remain in the images. These are due to
differences between the zodiacal emission and the zodiacal model
used in producing the ISSA
(§ IV.E.3).
Chapter Contents
Table of Contents | Index | Previous Section | Next Section