ISSA Explanatory Supplement
Zodiacal History File (ZOHF)
- Production Description
- Analysis Results
- Zodiacal History File Version 3.1
IPAC in 1988. It replaced Version 2.0, which was released in 1986. Version 3.0 incorporates a number of improvements that are outlined below. A subsequent release in 1990, Version 3.1, fixed a problem found in Version 3.0. A statement of the problem and its effect is given below. All references to Version 3.0 in this appendix other than in §H.8 are applicable to Version 3.1.
The major improvements were in the calibration. The baseline calibration was improved and corrections for hysteresis effects were incorporated. The entire IRAS survey was rerun with the improved calibration. Other changes to the ZOHF included a format change, additional calibration improvements, position improvements, a sampling change, and several processing changes. Results of the verification tests are presented. This is not intended to be an exhaustive description of the ZOHF Version 3.0 or its analysis. Only essential information is presented to enable a researcher to use the ZOHF Version 3.0 product.
The ZOHF Version 3.0 incorporated the final calibration of the IRAS data. There are, however, still calibration differences at the few-percent level between observations. In particular, there remains a systematic difference between ascending and descending scans. This systematic problem is discussed in the section on anomalies below.Table H.1. The beam sizes have not changed from those in Version 2.0. Due to elimination of the smallest detectors, they are not the full width of the IRAS focal plane.
|Pixel Size (arcminutes)|
|41||Inu11||12 µm Brightness Density||Jy/sr||E10.4|
|51||Inu2||25 µm Brightness Density||Jy/sr||E10.4|
|61||Inu3||60 µm Brightness Density||Jy/sr||E10.4|
|71||Inu4||100 µm Brightness Density||Jy/sr||E10.4|
Radiation spikes and other electronic glitches were removed by a deglitch processor prior to resampling the data (§III.A.3).
The data used in the ZOHF Version 2.0 were destriped with an algorithm which adjusted the gain and offset of each individual detector in a band to match those of the average of all detectors in that band. This destriper was not used for Version 3.0. This should have little effect since the destriper left the average value of the ZOHF unchanged and did not affect the striping caused by calibration variations between scans.
An error was found in Version 2.0 and corrected in Version 3.0 that advanced the position in-scan by 115" for half of the mission data. Improvements in the satellite pointing reconstruction made to support the IRAS Faint Source Survey 1992 were incorporated in the ZOHF Version 3.0. The impact of these improvements is generally not large relative to the resolution of the ZOHF (§III.A.3).
The sampling interval in the ZOHF Version 3.0 is eight seconds and there is no overlap between adjacent in-scan pixels. Because the ZOHF Version 2.0 was made with overlapping adjacent in-scan pixels, the file size of Version 3.0 is reduced by a factor of two as compared to Version 2.0.§III.A.2 of this Supplement. IPAC to verify the ZOHF Version 3.0 data and characterize it with respect to Version 2.0. Table H.3. The mission mean gain and offset is approximately the value expected from the calibration changes that were implemented for Version 3.0. The mission extremes of gain and offset are caused by attempting to fit a linear transformation to the detector nonlinearities encountered when especially bright sources are covered during a scan.
Mean (1 sigma)
|Difference (")||Version 2 (%)||Version 3.0 (%)|
Figure H.1 Flux ratio at North Ecliptic Pole vs. time
(seconds) from SAA crossing
at 12, 25, 60, and 100 µm. (See text, Anomalies.)
If the IRAS calibration system were working perfectly, the
brightness of the TFPR measured during survey
observations should agree with the TFPR model used during the
daily baseline calibration observations. The discrepancy between
these two values of TFPR brightness gives some measure of the
stability and uncertainty of the baseline. The
difference between the survey observations of the TFPR and the
model is shown in Figure H.1. The scatter is seen to be
approximately 3% at 12 and 25 µm, 4% at 60,
and 8% at 100 µm.
Figure H.1 (cont'd)
We should be able to re-derive from the ZOHF the same
variable part of the TFPR model that we used in the calibration.
Hauser et al.'s check of the variable part of the TFPR
model due to the inclination of the symmetry plane of the
zodiacal dust reproduced that part of the TFPR model to within
the model's internal consistency discussed above. This check is
done by differencing the ends of survey scans that cross both
ecliptic poles. It should be quite accurate and free from the
effects of baseline drift. Derivation of the variability due to
eccentricity from the survey data alone is unreliable because
residual baseline drifts are not eliminated and are large enough
to affect the calculated eccentricity term seriously. Hauser
et al. also found systematic differences between ascending
and descending survey scans, see §H.7 below.
This difference could be caused by a residual hysteresis effect in the DC response of the detector after crossing the South Atlantic Anomaly (SAA). The model implemented in calibration for handling hysteresis after the SAA was derived only for the AC response. The DC response was assumed to vary linearly with the AC response and was obtained by applying a scale factor to the AC response. This assumption may not be correct at the few-percent level.
Figure H.2 Mean flux ratios at North Ecliptic Pole and
deviations vs. time (seconds)
from SAA crossing at 12, 25, 60, and 100 µm.
(See text, Anomalies).
Due to the survey scan strategy, descending
scans dominate the first group of survey scans following an SAA
crossing. These scans have elevated fluxes relative to the next
group of scans, which are further from SAA and are predominantly
ascending. In Figure H.1, the abscissa is the ratio of the measured
flux at the North Ecliptic Pole (NEP)
and the flux calculated from the calibration
model and assigned to the NEP. This is plotted against the time
from the SAA crossing for the 12, 25, 60, and 100 µm bands. If the
calibration were perfect, all measurements would be unity. The
observations fall into groups along the time axis. Figure H.2
shows the mean flux ratio and population standard deviation for
each grouping of scans at 12, 25, 60, and 100 µm.
Figure H.2 (cont'd)
In short, we believe that a large part of the ascending-descending
asymmetry can be attributed to uncorrected calibration drifts.
At this time, we cannot however eliminate the
possibility that some of the asymmetry is a real feature of the
ReferencesIRAS Catalogs and Atlases: Explanatory Supplement 1988, ed. C. A. Beichman, G. Neugebauer, H. J. Habing, P. E. Clegg, and T. J. Chester (Washington, D.C.:GPO).
Moshir, M. et al., 1992, Explanatory Supplement to the IRAS Faint Source Survey Version 2, JPL D-10015 8/92 (Pasadena:JPL).