ISSA Explanatory Supplement
II. IRAS SKY SURVEY ATLAS OVERVIEW
A. Changes and Improvements in Atlas
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- Improvements in Relative Calibration
- Zodiacal Foreground Removal to Permit Coaddition
- Destripers to Stabilize Detector Baselines
- Oversampling to Improve the Representation of Spatial Information
- Improved Pointing Information
- Particle Radiation Removal
- Known Asteroid Removal from the Coadded Images
- Full-Sized Detectors
The processing that created the ISSA images was designed to correct
several problems that limited the sensitivity and usability of
the SkyFlux images. These problems include the
effects of the photon-induced
responsivity enhancement (Main Supplement §IV.A.8), also known
as hysteresis, which degraded photometric accuracy around bright regions
such as the Galactic plane
at 60 and 100 µm; variations in detector responsivity and
electronic offsets that produced prominent striping in the SkyFlux
images;
and spatial and temporal variations of the observed
zodiacal foreground producing steep, artificial gradients in the
SkyFlux images, which obscured faint sky features and prevented
co-addition of the individual HCON images. Finally, the 2'
pixels of the SkyFlux images just critically sampled
the resolution of the time-ordered detector data for the 12, 25
and 60 µm bands, making interpretation difficult without
further interpolation.
The combination of all improvements reduced the residual
stripes to the level of the intrinsic detector noise and largely
eliminated interference from the zodiacal foreground. The removal
of the zodiacal foreground
emission increased the sensitivity over the SkyFlux images by roughly
a factor of five. The destripers reduced the detector-to-detector
noise by factors of 2-3 at 12 and 25 µm and
1.5-2.0 for 60 and 100 µm. This results in images with
similar noise in the in-scan and cross-scan directions.
Coaddition provides an additional factor of sqrt(3) improvement over
individual HCON images. The co-added images
reveal faint structure at 12, 25 and 60 µm totally invisible
in the SkyFlux images. Details of the quality of the ISSA
images will be found in Chapter IV.
A.1 Improvements in Relative Calibration
When looking at uniform sky, equal-sized detectors within
a band will give different measurements due to variations in
detector baselines and responsivities. With perfect calibration,
these variations are removed and images of the sky appear uniform.
Any imperfections in calibration result in detector-to-detector
striping in the images. Calibration enhancements for ISSA, described in
§III.A.2, reduced the detector-to-detector stripes by roughly
a factor of ten at 12 and 25 µm relative
to the calibration used in the SkyFlux images.
No calibration changes
have affected the IRAS point source calibration.
A.2 Zodiacal Foreground Removal to Permit Coaddition
A foreground predicted by the zodiacal emission model described
in §III.C.2 and Appendix G was removed from the time-ordered
detector data, permitting useful co-addition of the individual HCON
images. The subtraction of the zodiacal model resulted in a
five-fold or better reduction, compared to the non-zodiacal-removed data,
in gradients and artifacts
due to changes
in zodiacal foreground during the IRAS survey. However, some
effects of the zodiacal foreground remain in the data. Since the
zodiacal emission model is not perfect, insufficient foreground
was removed in some places and too much was removed in others.
Residual foreground removal errors for |
|>50°
are 3-5% of the original background,
0.5 MJy sr-1 at 12 µm and 1.0 MJy sr-1
at 25 µm over scales of 10°. For
50°>||>20°,
the residuals are 1.0 MJy sr-1 at 12 µm and
2.0-2.5 MJy sr-1 at 25 µm over scales of 10°.
The zodiacal
emission model assumed a physical dust distribution which did not
include the dust bands. The dust band emission remains
in the data and produces artifacts in the images at low ecliptic
latitudes, the ISSA Reject Set.
A.3 Destripers to Stabilize Detector Baselines
Stripe noise due to residual baseline fluctuations, residual
zodiacal foreground and uncalibrated responsivity variations was
reduced with two destripers. The first destriper globally
compared all of the survey data at 1.2 million points on the sky
(Emerson and Gräves 1988).
At each point, the global destriper attempted to match each detector
to the average of all other detectors in the same wavelength.
This was accomplished by applying a slowly
varying baseline correction to every scan of every detector.
The assumption of global destriping is that the average of all IRAS
measurements, after zodiacal foreground removal, of a particular
point on the sky is the best estimate for the brightness at that
point. The second destriper, known as the local destriper,
used a similar comparison of
each detector to the average of all detectors to make further
baseline adjustments. This destriper used only the data in a single
12.5° field. The local destriper was able to
accomplish about a 10% reduction in cross-scan RMS left after
global destriping.
The two destripers are described in §III.C.3.
A.4 Oversampling to Improve the Representation of Spatial Information
The 1.5' pixel spacing in the ISSA images
improves the sampling interval by 25% over the SkyFlux images and
obviates the need to further smooth the time-ordered data.
A.5 Improved Pointing Information
Improvements in the pointing reconstruction for the IRAS
survey contribute slightly to improved resolution in the ISSA images
(§III.A.1).
A.6 Particle Radiation Removal
Signal processing on board the IRAS satellite attempted to
remove from the IRAS detector data noise spikes due to
high-energy protons and electrons.
However, many small spikes and vestiges of large spikes remained
in the SkyFlux images. A deglitcher removed this noise from the
ISSA images (§III.A.3).
A.7 Known Asteroid Removal from the Coadded Images
Known asteroids listed in the IRAS Asteroid and Comet Survey (Matson
1986) were removed from the data prior to making the
co-added images. This eliminated a major contributor of nonconfirming
sources (§III.C.4) in the co-added images. Asteroids remain in the
individual HCON images.
A.8 Full-Sized Detectors
When flux measurements are converted to surface brightness, point
sources measured by undersized detectors appear too bright. Thus, to
eliminate photometric problems associated with combining
different detector sizes, only the operative, full-sized detectors (Table
II.A.1) were used in making the ISSA images. The SkyFlux images used
3/4-sized as well as full-sized detectors.
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