IRAS Explanatory Supplement
III. The IRAS Mission
C. Design
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- Basic Strategy
- The Second Six Months
- Scan Rate
- Strategy during South Atlantic Anomaly Passage
- Moon and Jupiter Avoidance Strategy
- Strategy of Attitude and Photometric Calibration
- Realization of Survey Strategy
- Half-Orbit Constraint
- Lune Constant
- Hole Recovery Strategy
- Pre-Survey Observations
C.1 Basic Strategy
The basic time interval of the operations and data acquisition was the time between station passes, varying between seven or eight orbits or 10 to 14 hours. During each such period, the survey strategy discussed below was implemented in a series of commands sent to the satellite called a Satellite Observation Plan, hereafter denoted as SOP. There were two SOPs per day and 600 SOPs in the entire mission.
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Figure III.C.1 Lunes are sketched on the celestial sphere. Shaded zone
is not allowed by Sun constraint. larger largest |
The strategy developed to achieve the goals of multiple
survey coverage on various timescales divided the celestial sphere
into units of half overlapping "lunes" in the ecliptic
coordinate system (Lundy 1984).
Lunes were defined as the area
between two ecliptic meridians 30° apart
(Fig. III.C.1).
The lunes were "painted" by survey scans, one after
the other, as they passed through the viewing window of the telescope.
Figure III.C.2 illustrates the shape of a lune
sketched onto
a plane and, with a very exaggerated focal plane width, how it
was covered by different survey scans each at a fixed cone angle
from the Sun. The first scan in a lune was placed so that it
crossed the ecliptic at the lower longitude boundary of the lune.
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Figure III.C.2 Half-overlapping scan swaths (highly exaggerated) fill
a lune with one hours-confirming layer. larger largest |
Successive scans were laid down at increasing ecliptic longitudes,
each one shifted over by 14.23', that is by half the width of
the focal plane minus a safety margin to account for the pointing
limit cycle of the telescope. The overlap ensured that measurements
of the same area of sky were repeated within a few orbits (for
hours-confirmation) and that the "banana effect"
(Section III.B.8) was not too severe.
The criterion for hours-confirmation
was that the hours-confirming scans had to be made within three
SOPs (34-38 hours) of each other.
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Figure III.C.3 A typical days survey coverage (clear regions at
approximately ecliptic longitudes 167° and 347°) is plotted
on a map of the sky in ecliptic coordinates. The two shaded regions
centered on ecliptic longitudes 90° and 270° are forbidden
by the Sun and Earth constraints. larger largest |
Two lunes in opposite hemispheres were observed simultaneously,
one on the ascending side of the orbit and one on the descending
side. Figure III.C.3 shows a typical day's
survey coverage and
the regions forbidden by the constraints. After a lune was filled,
a second lune in the same hemisphere was started; it overlapped
half of the first lune, ensuring that another hour's-confirming
set of scans was repeated after about one to two weeks, thus providing
the required repetition on the time scale of weeks
(Fig. III.C.4).
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Figure III.C.4 The lune coverage scheme is shown for the first two sets
of hours-confirming scans. Each line represents a single hours-confirming
coverage of an entire 30° lune (except short lines at beginning and
end which represent half-lune coverage). larger largest |
The observing conditions were considerably worse during some
orbits than during others. Orbits that crossed the SAA or contained
a station prime pass were interrupted for significant amounts
of time. When a long interruption occurred, the continuation of
a survey scan was sometimes impossible and a small hole in the
coverage resulted. To minimize such events only the nine (out
of 14) orbits per day least affected by the SAA were used for
the survey scans
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Figure III.C.5 A zone of half-circle scans is drawn on the
celestial sphere. larger largest |
The survey strategy aimed for four coverages (two sets of
hours-confirming coverages) in the first six months and two coverages
(a third set of hours-confirming coverages) in the second six
months. Any time left over was used to recover survey observations
lost because of the various constraints, to make the necessary
calibration observations and to carry out additional, non-survey,
pointed observations to attain higher sensitivity or spatial resolution.
For the first two weeks of observations (February 10-23, 1983,
SOPs 31-57) half circle scans, which give redundant coverage in
the ecliptic polar areas (Fig. III.C.5), were
used for increased
initial coverage as insurance against an early failure of the
satellite. Subsequently, the more efficient lune method was used
for the rest of the first six months of the survey.
C.2 The Second Six Months
The first sky survey was completed on August 26, l983, with nearly a full second six months of operations expected. The survey was continued, but aimed at a coverage by only one set of hours-confirmed scans and using seven orbits per day. This plan covered many of the large number of small regions that the survey had missed for reasons explained elsewhere in this chapter, without the time consuming necessity of covering each hole separately, and also improved the survey's completeness for weaker sources (signal-to-noise ratio 5-l0). Another important goal was the coverage of a 5° gap left in the first six months' survey (Section III.D.5). This latter goal was never achieved because the helium supply was exhausted earlier than expected. Half circles rather than lunes were used in the second six months because the additional coverage in the ecliptic polar regions in August and September filled in regions that would be inaccessible later due to eclipses and a severe Earth infrared radiation constraint in December and January.C.3 Scan Rate
Using two gyros, the spacecraft clock-angle (X)0 (Fig. III.B.7) was decreased at a rate of (3.85 / cos( 90 -
))' per second
(which resulted in scanning the sky at a rate of 3.85' per second
independent of ), i.e. 10% faster
than the orbital rate of the
satellite of 3.5' per second. This increased rate gave increased
pointing flexibility with respect to the constraints and, in particular,
helped to reduce the effects of the SAA.
C.4 Strategy during South Atlantic Anomaly Passage
Survey observations were interrupted for up to l4 minutes during SAA passages even after selection of the least affected orbits. When the telescope entered the SAA during a survey scan its scanning motion was halted and it remained pointing at the same point on the celestial sphere until it emerged from the SAA when the scan was restarted with an overlap. During the SAA passage, "bias boost" was applied to the 60 and 100 µm detectors until about three minutes before exit. The internal reference sources were flashed just before entry and just after exit at the end and beginning of survey scans (Section III.B.3). Some SAA passages required so much time that the telescope would have violated the Earth infrared constraint (Section III.B.4) after SAA passage. In such cases it was necessary to leave a hole in the sky coverage and try to recover it with specially prepared short scans in the preceding or succeeding pair of SOPs. The failure to recover all of these holes contributed to the variable depth of sky coverage.
C.5 Moon and Jupiter Avoidance Strategy
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Figure III.C.6.1 The first hours-confirmed coverage is overlaid on a
map of the sky in ecliptic coordinates. The scans converge at the ecliptic
poles. Small, equally spaced holes in the ecliptic plane are due to the lunar
avoidance stragegy and represent gaps in the sky coverage used to generate
the extended emission images. The point source survey was not affected by
these gaps. larger largest |
Jupiter was avoided by stopping a scan when it reached within
1° of the planet and side stepping by 1° for 2°
of scan before coming back to resume the original scan; hence
the Jupiter avoidance procedure left a 2° square hole on
the sky which needed to be recovered later in the survey.
Figure III.C.6.1-3 clearly show the three
square holes on the ecliptic
at longitude approximately 250° (RA about 17 hrs). Each
was left by one of the three sets of hours-confirming coverages.
There is no residual Jupiter hole when the effect of all the
coverage is added.
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Figure III.C.6.2 Same as Figure III.C.6.1,
except for the second hours-confirming coverage. larger largest |
To avoid the moon in the same way as Jupiter would have
left extremely large holes which would have been difficult to
fill. The policy adopted, therefore, was to stop the survey on
the moon's side of the orbit for the approximately three days
needed for the moon to pass through this approximately 40°
avoidance region.
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Figure III.C.6.3 Same as Figure III.C.6.1,
except for the third hours-confirming coverage. larger largest |
C.6 Strategy of Attitude and Photometric Calibration
To refine the pointing after the slew to the beginning point of a scan, attitude calibrations were made using visual stars at the beginning and end of each scan. Star sightings near the middle of the scan, if possible, allowed any pointing drifts to be detected and corrected (Section V.B). Long scans proved most suitable as they offered the greatest probability of finding these calibration stars.To check the photometric calibration the internal reference sources were flashed at the beginning and end of every scan, including scans broken by the SAA. The internal reference sources were themselves regularly checked against an astronomical reference source (Section VI.A).
C.7 Realization of Survey Strategy
The survey scans resulting from this strategy and the constraints were generated by a computerized scheduling program (Oord et al. 1981). As well as generating the survey scans, the program reported which pieces of sky were not covered due to a combination of constraints, so that scans could be generated separately for inclusion elsewhere in the same or other SOPs (MacDougall 1984). It was not always possible to generate these scans within the constraints and so holes were left.C.8 Half-Orbit Constraint
The scheduling program constrained survey scans to stop at the end of a half-orbit at one of the ecliptic poles. A side effect of this half-orbit design was that on occasions when the satellite had looked back in its orbit to cover a region of sky whose observation had been interrupted by a long SAA passage (Section III.C.4), the scan was cut short by reaching the end of its half-orbit, even though no other constraints had been violated.
C.9 Lune Constraint
Full coverage with the lune method required that while a lune was being painted it had to contain the meridian at 90° from the Sun. Otherwise the ecliptic poles could not be reached and holes would result. The scheduling program used to generate the survey scans also required that
= 90° lie within
the lune. This geometry became a hard constraint on the lune strategy,
and became relevant to the decision that led to the 5° gap
(III.D.5).
C.10 Hole Recovery Strategy
A record was maintained of those regions of the sky that were not covered by the automatically generated scans due to constraint violations and as suitable opportunities arose, attempts were made to fill the holes (Lau and Wolff 1984; Lundy 1984).C. 11 Pre-Survey Observations
Before commencing survey operations numerous checks were required to verify the health and safety of the satellite and to determine the best modes of operation. The cooled aperture cover was kept on the telescope for the first six days to allow sufficient time for contaminants carried up with the satellite to outgas and disperse so that they would not freeze on the cold optics when the cover was ejected. The eight days after cover ejection were used to test those aspects of the instrument that could not be tested with the cover on. This period was followed by a period of repeated surveying on a limited region of sky to verify the survey strategy and the data processing facilities (Section VIII.D; Rowan-Robinson et al., 1984). The scans of this "minisurvey" were hand-tailored for maximum efficiency in coverage. After all these tests had been completed and the problems they revealed had been resolved (MacDougall et al. 1984), the all-sky survey was started. The dates of these and other important events are given in Table III.C.1.
Table III.C.1
Date SOP Event
(All dates are 1983 and given in GMT)
26 Jan
1
Launch 02h 17m
26-31 Jan
1-12
In Orbit Checkout (Cover on)
Outgassing of satellite
31 Jan
12
Cover ejection 19h 37m
31 Jan-8 Feb
12-28
In Orbit Checkout (Cover off)
9 Feb
29
SAA contour A (Fig. III.B.6) usage begins
9-10 Feb
29-30
Minisurvey layer 1 Hand made scans
10 Feb
31
Start first two hours-confirming
coverages using half circles.
11-12 Feb
33-34
Minisurvey layer 2 Hand made scans
13-14 Feb
37-38
Minisurvey layer 3 Hand made scans
15 Feb
41
Minisurvey layer 4a Hand made scans
16 Feb
43
Minisurvey layer 4b Hand made scans
23 Feb
57
Half circle method ended
23 Feb
58
Lune method started
3 Apr
135
Moon avoidance radius lowered
from 25° to 20°
9 May
207
SAA contour B (Fig. III.B.6) usage begins
26 Aug
425
End of first two hours-confirming coverages
26 Aug
426
Start third hours-confirming coverage
26 Aug
426
Moon avoidance radius lowered from 20 to 13°
9 Sep
454
Moon avoidance radius raised from 13 to 20°
18 Nov
593
First eclipse. Fallback to safety mode
21 Nov
600
Survey operations resumed 19h 40m
22 Nov
600
Liquid helium ran out 00h 16m
22 Nov
600
Last survey scan started 03h 34m
23 Nov
603
12 µm detector baselines saturated 09h 30m
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