I.E. Processing Caveats

ISSA Explanatory Supplement
I. Introduction
E. Processing Caveats

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  1. Mosaicking
  2. Saturated Data
  3. Destriper Anomalies
  4. 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.

E.2 Saturated Data

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).

E.3 Destriper Anomalies

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.

E.4 Low Spatial Frequency Artifacts

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).

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