IRAS Explanatory Supplement
III. The IRAS Mission
C. Design

1. Basic Strategy
2. The Second Six Months
3. Scan Rate
4. Strategy during South Atlantic Anomaly Passage
5. Moon and Jupiter Avoidance Strategy
6. Strategy of Attitude and Photometric Calibration
7. Realization of Survey Strategy
8. Half-Orbit Constraint
9. Lune Constant
10. Hole Recovery Strategy
11. Pre-Survey Observations

C.1 Basic Strategy

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.

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.

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).

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

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

C.3 Scan Rate

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

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.

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.

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 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

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 (

C.10 Hole Recovery Strategy

C. 11 Pre-Survey Observations